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Vector Group of Transformer

May 23, 2012 8 Comments

15 Votes

Introduction:

Three phase transformer consists of three sets of primary windings, one for each phase, and three sets of secondary windingswound on the same iron core. Separate single-phase transformers can be used and externally interconnected to yield the sameresults as a 3-phase unit.

The primary windings are connected in one of several ways. The two most common configurations are the delta, in which thepolarity end of one winding is connected to the non-polarity end of the next, and the star, in which all three non-polarities (orpolarity) ends are connected together. The secondary windings are connected similarly. This means that a 3-phase transformer

can have its primary and secondary windings connected the same (delta-delta or star-star), or differently (delta-star or star-delta).

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It’s important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primaryand secondary windings are connected the same way. This condition is called “no phase shift.” But when the primary andsecondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary

voltage waveforms by 30 electrical degrees. This is called a 30 degree phase shift. When two transformers are connected inparallel, their phase shifts must be identical; if not, a short circuit will occur when the transformers are energized.”

Basic Idea of Winding:

An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path. The

phase relationship of the two voltages depends upon which ways round the coils are connected. The voltages will eitherbe in-phase or displaced by 180 degWhen 3 coils are used in a 3 phase transformer winding a number of options exist. The coil voltages can be in phase ordisplaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point

(neutral) brought out to an external terminal or not.

Six Ways to wire Star Winding:

Six Ways to wire Delta Winding:



Polarity:

An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path. Thephase relationship of the two voltages depends upon which way round the coils are connected. The voltages will eitherbe in-phase or displaced by 180 deg.When 3 coils are used in a 3 phase transformer winding a number of options exist. The coil voltages can be in phase ordisplaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point(neutral) brought out to an external terminal or not.



When Pair of Coil of Transformer have same direction than voltage induced in both coil are in same direction from one

end to other end.When two coil have opposite winding direction than Voltage induced in both coil are in opposite direction.

Winding connection designations:

First Symbol: for High Voltage: Always capital letters.

D=Delta, S=Star, Z=Interconnected star, N=NeutralSecond Symbol: for Low voltage: Always Small letters.

d=Delta, s=Star, z=Interconnected star, n=Neutral.

Third Symbol: Phase displacement expressed as the clock hour number (1,6,11)Example – Dyn11

Transformer has a delta connected primary winding (D) a star connected secondary (y) with the star point brought out(n) and a phase shift of 30 deg leading (11).

The point of confusion is occurring in notation in a step-up transformer. As the IEC60076-1 standard has stated, thenotation is HV-LV in sequence. For example, a step-up transformer with a delta-connected primary, and star-

connected secondary, is not written as ‘dY11′, but ‘Yd11′. The 11 indicates the LV winding leads the HV by 30degrees.

Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a

vector diagram is given to show the relationship between the primary and other windings.

Vector Group of Transformer:

The three phase transformer windings can be connected several ways. Based on the windings’ connection, the vector

group of the transformer is determined.

The transformer vector group is indicated on the Name Plate of transformer by the manufacturer.The vector group indicates the phase difference between the primary and secondary sides, introduced due to that

particular configuration of transformer windings connection.The Determination of vector group of transformers is very important before connecting two or more transformers in

parallel. If two transformers of different vector groups are connected in parallel then phase difference exist between thesecondary of the transformers and large circulating current flows between the two transformers which is very

detrimental.

Phase Displacement between HV and LV Windings:



The vector for the high voltage winding is taken as the reference vector. Displacement of the vectors of other windingsfrom the reference vector, with anticlockwise rotation, is represented by the use of clock hour figure.

IS: 2026 (Part 1V)-1977 gives 26 sets of connections star-star, star-delta, and star zigzag, delta-delta, delta star,

delta-zigzag, zigzag star, zigzag-delta. Displacement of the low voltage winding vector varies from zero to -330° insteps of -30°, depending on the method of connections.

Hardly any power system adopts such a large variety of connections. Some of the commonly used connections withphase displacement of 0, -300, -180″ and -330° (clock-hour setting 0, 1, 6 and 11).

Symbol for the high voltage winding comes first, followed by the symbols of windings in diminishing sequence ofvoltage. For example a 220/66/11 kV Transformer connected star, star and delta and vectors of 66 and 11 kV

windings having phase displacement of 0° and -330° with the reference (220 kV) vector will be represented As Yy0 –Yd11.

The digits (0, 1, 11 etc) relate to the phase displacement between the HV and LV windings using a clock face notation.

The phasor representing the HV winding is taken as reference and set at 12 o’clock. Phase rotation is always anti-clockwise. (International adopted).

Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a clock, and a circleconsists out of 360°, each hour represents 30°.Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°.

The minute hand is set on 12 o’clock and replaces the line to neutral voltage (sometimes imaginary) of the HV winding.This position is always the reference point.

Example:Digit 0 =0° that the LV phasor is in phase with the HV phasor

Digit 1 =30° lagging (LV lags HV with 30°) because rotation is anti-clockwise.

Digit 11 = 330° lagging or 30° leading (LV leads HV with 30°)Digit 5 = 150° lagging (LV lags HV with 150°)

Digit 6 = 180° lagging (LV lags HV with 180°)When transformers are operated in parallel it is important that any phase shift is the same through each. Paralleling

typically occurs when transformers are located at one site and connected to a common bus bar (banked) or located atdifferent sites with the secondary terminals connected via distribution or transmission circuits consisting of cables and

overhead lines.

Phase Shift (Deg) Connection

0 Yy0 Dd0 Dz0

30 lag Yd1 Dy1 Yz1

60 lag Dd2 Dz2

120 lag Dd4 Dz4

150 lag Yd5 Dy5 Yz5

180 lag Yy6 Dd6 Dz6

150 lead Yd7 Dy7 Yz7



120 lead Dd8 Dz8

60 lead Dd10 Dz10

30 lead Yd11 Dy11 Yz11

The phase-bushings on a three phase transformer are marked either ABC, UVW or 123 (HV-side capital, LV-side

small letters). Two winding, three phase transformers can be divided into four main categories

Group O’clock TC

Group I 0 o’clock, 0° delta/delta, star/star

Group II 6 o’clock, 180° delta/delta, star/star

Group III 1 o’clock, -30° star/delta, delta/star

Group IV 11 o’clock, +30° star/delta, delta/star

Minus indicates LV lagging HV, plus indicates LV leading

HV

Clock Notation: 0

Clock Notation : 1



Clock Notation: 2

Clock Notation: 4

Clock Notation: 5



Clock Notation: 6

Clock Notation: 7

Clock Notation: 11

Points to be consider while Selecting of Vector Group:

Vector Groups are the IEC method of categorizing the primary and secondary winding configurations of 3-phasetransformers. Windings can be connected as delta, star, or interconnected-star (zigzag). Winding polarity is alsoimportant, since reversing the connections across a set of windings affects the phase-shift between primary and

secondary. Vector groups identify the winding connections and polarities of the primary and secondary. From a vectorgroup one can determine the phase-shift between primary and secondary.Transformer vector group depends upon

1. Removing harmonics: Dy connection – y winding nullifies 3rd harmonics, preventing it to be reflected on delta

side.2. Parallel operations: All the transformers should have same vector group & polarity of the winding.



3. Earth fault Relay: A Dd transformer does not have neutral. to restrict the earth faults in such systems, we may

use zig zag wound transformer to create a neutral along with the earth fault relay..4. Type of Non Liner Load: systems having different types of harmonics & non linear Types of loads e.g. furnace

heaters ,VFDS etc for that we may use Dyn11, Dyn21, Dyn31 configuration, wherein, 30 deg. shifts of voltagesnullifies the 3rd harmonics to zero in the supply system.

5. Type of Transformer Application: Generally for Power export transformer i.e. generator side is connected indelta and load side is connected in star. For Power export import transformers i.e. in Transmission PurposeTransformer star star connection may be preferred by some since this avoids a grounding transformer ongenerator side and perhaps save on neutral insulation. Most of systems are running in this configuration. May be

less harmful than operating delta system incorrectly. Yd or Dy connection is standard for all unit connectedgenerators.

6. There are a number of factors associated with transformer connections and may be useful in designing a system,

and the application of the factors therefore determines the best selection of transformers. For example:

For selecting Star Connection:

A star connection presents a neutral. If the transformer also includes a delta winding, that neutral will be stable and canbe grounded to become a reference for the system. A transformer with a star winding that does NOT include a delta

does not present a stable neutral.Star-star transformers are used if there is a requirement to avoid a 30deg phase shift, if there is a desire to construct thethree-phase transformer bank from single-phase transformers, or if the transformer is going to be switched on a single-pole basis (ie, one phase at a time), perhaps using manual switches.

Star-star transformers are typically found in distribution applications, or in large sizes interconnecting high-voltagetransmission systems. Some star-star transformers are equipped with a third winding connected in delta to stabilize theneutral.

For selecting Delta Connection:

A delta connection introduces a 30 electrical degree phase shift.A delta connection ‘traps’ the flow of zero sequence currents.

For selecting Delta-Star Connection:

Delta-star transformers are the most common and most generally useful transformers.

Delta-delta transformers may be chosen if there is no need for a stable neutral, or if there is a requirement to avoid a 30electrical degree phase shift. The most common application of a delta-delta transformer is as tan isolation transformerfor a power converter.

For selecting Zig zag Connection:

The Zig Zag winding reduces voltage unbalance in systems where the load is not equally distributed between phases,and permits neutral current loading with inherently low zero-sequence impedance. It is therefore often used for earthingtransformers.Provision of a neutral earth point or points, where the neutral is referred to earth either directly or through impedance.

Transformers are used to give the neutral point in the majority of systems. The star or interconnected star (Z) windingconfigurations give a neutral location. If for various reasons, only delta windings are used at a particular voltage level ona particular system, a neutral point can still be provided by a purpose-made transformer called a ‘neutral earthing.

For selecting Distribution Transformer:

The first criterion to consider in choosing a vector group for a distribution transformer for a facility is to know whetherwe want a delta-star or star-star. Utilities often prefer star-star transformers, but these require 4-wire input feeders and4-wire output feeders (i.e. incoming and outgoing neutral conductors).



For distribution transformers within a facility, often delta-star are chosen because these transformers do not require 4-wire input; a 3-wire primary feeder circuit suffices to supply a 4-wire secondary circuit. That is because any zerosequence current required by the secondary to supply earth faults or unbalanced loads is supplied by the delta primary

winding, and is not required from the upstream power source. The method of earthing on the secondary is independentof the primary for delta-star transformers.The second criterion to consider is what phase-shift you want between primary and secondary. For example, Dy11 and

Dy5 transformers are both delta-star. If we don’t care about the phase-shift, then either transformer will do the job.Phase-shift is important when we are paralleling sources. We want the phase-shifts of the sources to be identical.If we are paralleling transformers, then you want them to have the same the same vector group. If you are replacing atransformer, use the same vector group for the new transformer, otherwise the existing VTs and CTs used for

protection and metering will not work properly.There is no technical difference between the one vector groups (i.e. Yd1) or another vector group (i.e. Yd11) in termsof performance. The only factor affecting the choice between one or the other is system phasing, ie whether parts of the

network fed from the transformer need to operate in parallel with another source. It also matters if you have an auxiliarytransformer connected to generator terminals. Vector matching at the auxiliary bus bar

Application of Transformer according to Vector Group:

(1) (Dyn11, Dyn1, YNd1, YNd11)

Common for distribution transformers.

Normally Dyn11 vector group using at distribution system. Because Generating Transformer are YNd1 for neutralizingthe load angle between 11 and 1.We can use Dyn1 at distribution system, when we are using Generator Transformer are YNd11.In some industries 6 pulse electric drives are using due to this 5thharmonics will generate if we use Dyn1 it will be

suppress the 5th harmonics.Star point facilitates mixed loading of three phase and single phase consumer connections.The delta winding carry third harmonics and stabilizes star point potential.

A delta-Star connection is used for step-up generating stations. If HV winding is star connected there will be saving incost of insulation.But delta connected HV winding is common in distribution network, for feeding motors and lighting loads from LV side.

(2) Star-Star (Yy0 or Yy6)

Mainly used for large system tie-up Transformer.Most economical connection in HV power system to interconnect between two delta systems and to provide neutral forgrounding both of them.

Tertiary winding stabilizes the neutral conditions. In star connected transformers, load can be connected between lineand neutral, only if (a) the source side transformers is delta connected or (b) the source side is star connected with neutral connected back to the source neutral.

In This Transformers. Insulation cost is highly reduced. Neutral wire can permit mixed loading.Triple harmonics are absent in the lines. These triple harmonic currents cannot flow, unless there is a neutral wire. Thisconnection produces oscillating neutral.

Three phase shell type units have large triple harmonic phase voltage. However three phase core type transformerswork satisfactorily.A tertiary mesh connected winding may be required to stabilize the oscillating neutral due to third harmonics in threephase banks.

(3) Delta – Delta (Dd 0 or Dd 6)



This is an economical connection for large low voltage transformers.Large unbalance of load can be met without difficulty.Delta permits a circulating path for triple harmonics thus attenuates the same.It is possible to operate with one transformer removed in open delta or” V” connection meeting 58 percent of the

balanced load.Three phase units cannot have this facility. Mixed single phase loading is not possible due to the absence of neutral.

(4) Star-Zig-zag or Delta-Zig-zag (Yz or Dz)

These connections are employed where delta connections are weak. Interconnection of phases in zigzag winding effectsa reduction of third harmonic voltages and at the same time permits unbalanced loading.This connection may be used with either delta connected or star connected winding either for step-up or step-downtransformers. In either case, the zigzag winding produces the same angular displacement as a delta winding, and at the

same time provides a neutral for earthing purposes.The amount of copper required from a zigzag winding in 15% more than a corresponding star or delta winding. This isextensively used for earthing transformer.

Due to zigzag connection (interconnection between phases), third harmonic voltages are reduced. It also allowsunbalanced loading. The zigzag connection is employed for LV winding. For a given total voltage per phase, the zigzagside requires 15% more turns as compared to normal phase connection. In cases where delta connections are weakdue to large number of turns and small cross sections, then zigzag star connection is preferred. It is also used in

rectifiers.

(5) Zig- zag/ star (ZY1 or Zy11)

Zigzag connection is obtained by inter connection of phases.4-wire system is possible on both sides. Unbalanced

loading is also possible. Oscillating neutral problem is absent in this connection.This connection requires 15% more turns for the same voltage on the zigzag side and hence costs more. Hence a bankof three single phase transformers cost about 15% more than their 3-phase counterpart. Also, they occupy more space.But the spare capacity cost will be less and single phase units are easier to transport.

Unbalanced operation of the transformer with large zero sequence fundamental mmf content also does not affect itsperformance. Even with Yy type of poly phase connection without neutral connection the oscillating neutral does notoccur with these cores. Finally, three phase cores themselves cost less than three single phase units due to

compactness.

(6) Yd5:

Mainly used for machine and main Transformer in large Power Station and Transmission Substation.

The Neutral point can be loaded with rated Current.

(7) Yz-5

For Distribution Transformer up to 250MVA for local distribution system.The Neutral point can be loaded with rated Current.

Application of Transformer according according to Uses:

Step up Transformer: It should be Yd1 or Yd11.Step down Transformer: It should be Dy1 or Dy11.Grounding purpose Transformer: It should be Yz1 or Dz11.

Distribution Transformer: We can consider vector group of Dzn0 which reduce the 75% of harmonics in secondaryside.



Power Transformer: Vector group is deepen on application for Example : Generating Transformer : Dyn1 , Furnace

Transformer: Ynyn0.

Convert One Group of Transformer to Other Group by ChanningExternal Connection:

(1) Group I: Example: Dd0 (no phase displacement between HV and LV).

The conventional method is to connect the red phase on A/a, Yellow phase on B/b, and the Blue phase on C/c.

Other phase displacements are possible with unconventional connections (for instance red on b, yellow on c and blueon a) By doing some unconventional connections externally on one side of the Transformer, an internal connected Dd0

transformer can be changed either to a Dd4(-120°) or Dd8(+120°) connection. The same is true for internal connected

Dd4 or Dd8 transformers.

(2) Group II: Example: Dd6 (180° displacement between HV and LV).

By doing some unconventional connections externally on one side of the Transformer, an internal connected Dd6

transformer can be changed either to a Dd2(-60°) or Dd10(+60°) connection.

(3) Group III: Example: Dyn1 (-30° displacement between HV and LV).

By doing some unconventional connections externally on one side of the Transformer, an internal connected Dyn1transformer can be changed either to a Dyn5(-150°) or Dyn9(+90°) connection.

(4) Group IV: Example: Dyn11 (+30° displacement between HV and LV).

By doing some unconventional connections externally on one side of the Transformer, an internal connected Dyn11

transformer can be changed either to a Dyn7(+150°) or Dyn3(-90°) connection.

Point to be remembered:

For Group-III & Group-IV: By doing some unconventional connections externally on both sides of the Transformer,

an internal connected Group-III or Group-IV transformer can be changed to any of these two groups.

Thus by doing external changes on both sides of the Transformer an internal connected Dyn1 transformer can bechanged to either a: Dyn3, Dyn5, Dyn7, Dyn9 or Dyn11 transformer, This is just true for star/delta or delta/star

connections.

For Group-I & Group-II: Changes for delta/delta or star/star transformers between Group-I and Group-III can justbe done internally.

Why 30°phase shift occur in star-delta transformer between primary andsecondary?

The phase shift is a natural consequence of the delta connection. The currents entering or leaving the star winding of the

transformer are in phase with the currents in the star windings. Therefore, the currents in the delta windings are also inphase with the currents in the star windings and obviously, the three currents are 120 electrical degrees apart.

But the currents entering or leaving the transformer on the delta side are formed at the point where two of the windings

comprising the delta come together – each of those currents is the phasor sum of the currents in the adjacent windings.When you add together two currents that are 120 electrical degrees apart, the sum is inevitably shifted by 30 degrees.



The Main reason for this phenomenon is that the phase voltage lags line current by 30degrees.consider a delta/startransformer. The phase voltages in three phases of both primary and secondary. you will find that in primary the phase

voltage and line voltages are same, let it be VRY(take one phase).but, the corresponding secondary will have the phasevoltage only in its phase winding as it is star connected. the line voltage of star connected secondary and delta

connected primary won’t have any phase differences between them. so this can be summarized that “the phase shift is

associated with the wave forms of the three phase windings.

Why when Generating Transformer is Yd1 than DistributionTransformer is Dy11:

This is the HV Side or the Switchyard side of the Generator Transformer is connected in Delta and the LV Side or thegenerator side of the GT is connected in Star, with the Star side neutral brought out.

The LV side voltage will “lag” the HV side voltage by 30 degrees.

Thus, in a generating station we create a 30 degrees lagging voltage for transmission, with respect to the generatorvoltage.

As we have created a 30 degrees lagging connection in the generating station, it is advisable to create a 30 degrees

leading connection in distribution so that the user voltage is “in phase” with the generated voltage. And, as thetransmission side is Delta and the user might need three phase, four-wire in the LV side for his single phase loads, the

distribution transformer is chosen as Dyn11.There is magnetic coupling between HT and LT. When the load side (LT) suffers some dip the LT current try to go out

of phase with HT current, so 30 degree phase shift in Dyn-11 keeps the two currents in phase when there is dip.

So the vector group at the generating station is important while selecting distribution Transformer.

Vector Group in Generating-Transmission-Distribution System:

Generating TC is Yd1 transmitted power at 400KV, for 400KV to 220KV Yy is used and by using Yd between e.g.

220 and 66 kV, then Dy from 66 to 11 kV so that their phase shifts can be cancelled out. And for LV (400/230V)

supplies at 50 Hz are usually 3 phase, earthed neutral, so a “Dyn” LV winding is needed. Here GT side -30lag (Yd1)can be nullify +30 by using distribution Transformer of Dy11.

A reason for using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV is that their phase shifts can cancel out

and It is then also possible to parallel a 220/11 kV YY transformer, at 11 kV, with the 66/11 kV (a YY transformeroften has a third, delta, winding to reduce harmonics). If one went Dy11 – Dy11 from 220 to 11 kV, there would be a



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60 degree shift, which is not possible in one transformer. The “standard” transformer groups in distribution avoid thatkind of limitation, as a result of thought and experience leading to lowest cost over many years.

Generator TC is Yd1, Can we use Distribution TC Dy5 instead of Dy11.

With regards to theory, there are no special advantages of Dyn11 over Dyn5.

In Isolation Application: In isolated applications there is no advantage or disadvantage by using Dy5 or Dy11. Ifhowever we wish to interconnect the secondary sides of different Dny transformers, we must have compatible

transformers, and that can be achieved if you have a Dyn11 among a group of Dyn5′s and vice versa.

In Parallel Connection: Practically, the relative places of the phases remain same in Dyn11 compared to Dyn5.If we use Yd1 Transformer on Generating Side and Distribution side Dy11 transformer than -30 lag of generating side

(Yd1) is nullify by +30 Lead at Receiving side Dy11) so no phase difference respect to generating Side and if we are

on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on theLV side will be R- Y -B, but from left to Right.

This will make the Transmission lines have same color (for identification) whether it is input to or output from the

Transformer.If we use Yd1 Transformer on Generating Side and Distribution side Dy5 transformer than -30 lag of generating side

(Yd1) is more lag by -150 Lag at Receiving side (Dy5) so Total phase difference respect to generating Side is 180 deg(-30+-150=-180) and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to

right, the same phases on the LV side will be R- Y -B, but from Right to Left.

This will make the Transmission lines have No same color (for identification) whether it is input to or output from theTransformer.

The difference in output between the Dyn11 and Dny5 and is therefore 180 degrees.

Filed under Uncategorized

Auto Transformer Connection


3 Votes

(7) Auto Transformer Connection:

An Ordinary Transformer consists of two windings called primary winding and secondary winding. These two windings

are magnetically coupled and electrically isolated. But the transformer in which a part of windings is common to bothprimary and secondary is called Auto Transformer.

In Auto Transformer two windings are not only magnetically coupled but also electrically coupled. The input to the

transformer is constant but the output can be varied by varying the tapings.The autotransformer is both the most simple and the most fascinating of the connections involving two windings. It is

used quite extensively in bulk power transmission systems because of its ability to multiply the effective KVA capacity

of a transformer. Autotransformers are also used on radial distribution feeder circuits as voltage regulators. Theconnection is shown in Figure

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The primary and secondary windings of a two winding transformer have induced emf in them due to a common mutual

flux and hence are in phase. The currents drawn by these two windings are out of phase by 180. This prompted theuse of a part of the primary as secondary. This is equivalent to common the secondary turns into primary turns.

The common section need to have a cross sectional area of the conductor to carry (I2−I1) ampere.

Total number of turns between A and C are T1. At point B a connection is taken. Section AB has T2 turns. As thevolts per turn, which is proportional to the flux in the machine, is the same for the whole winding, V1 : V2 = T1 : T2

When the secondary winding delivers a load current of I2 Ampere the demagnetizing ampere turns is I2T2. This will be

countered by a current I1 flowing from the source through the T1 turns such that, I1T1 = I2T2A current of I1 ampere flows through the winding between B and C. The current in the winding between A and B is (I2

− I1) ampere. The cross section of the wire to be selected for AB is proportional to this current assuming a constant

current density for the whole winding. Thus some amount of material saving can be achieved compared to a twowinding transformer. The magnetic circuit is assumed to be identical and hence there is no saving in the same. To

quantify the saving the total quantity of copper used in an auto transformer is expressed as a fraction of that used in a

two winding transformer As copper in auto transformer / copper in two winding transformer =((T1 − T2)I1 + T2(I2 − I1))/T1I1 + T2I2

copper in auto transformer / copper in two winding transformer = 1 –(2T2I1 / (T1I1 + T2I2))But T1I1 = T2I2 so

The Ratio = 1 –(2T2I1 / 2T1I1) = 1 –(T2/T1)

This means that an auto transformer requires the use of lesser quantity of copper given by the ratio of turns.This ratio therefore the savings in copper.

As the space for the second winding need not be there, the window space can be less for an auto transformer, giving

some saving in the lamination weight also. The larger the ratio of the voltages, smaller is the savings. As T2 approachesT1 the savings become significant. Thus auto transformers become ideal choice for close ratio transformations.

The auto transformer shown in Figure is connected as a boosting auto transformer because the series winding boosts

the output voltage. Care must be exercised when discussing ‘‘primary’’ and ‘‘secondary’’ voltages in relationship to

windings in an auto transformer.



In two-winding transformers, the primary voltage is associated with the primary winding, the secondary voltage is

associated with the secondary winding, and the primary voltage is normally considered to be greater than the secondary

voltage. In the case of a boosting autotransformer, however, the primary (or high) voltage is associated with the serieswinding, and the secondary (or low) voltage is associated with the common winding; but the voltage across the

common winding is higher than across the series winding.

Limitation of the autotransformer

One of the limitations of the autotransformer connection is that not all types of three-phase connections are possible.For example, the ∆-Y and Y- ∆ connections are not possible using the autotransformer. The Y-Y connection must

share a common neutral between the high-voltage and low-voltage windings, so the neutrals of the circuits connected to

these windings cannot be isolated.A ∆- ∆ autotransformer connection is theoretically possible; however, this will create a peculiar phase shift. The phase

shift is a function of the ratio of the primary to secondary voltages and it can be calculated from the vector diagram. This

phase shift cannot be changed or eliminated and for this reason, autotransformers are very seldom connected as ∆ – ∆transformers.

Advantages of the autotransformer

There are considerable savings in size and weight.

There are decreased losses for a given KVA capacity.Using an autotransformer connection provides an opportunity for achieving lower series impedances and better

regulation. Its efficiency is more when compared with the conventional one.

Its size is relatively very smaller.

Voltage regulation of autotransformer is much better.Lower cost

Low requirements of excitation current.

Less copper is used in its design and construction.In conventional transformer the voltage step up or step down value is fixed while in autotransformer, we can vary the

output voltage as per out requirements and can smoothly increase or decrease its value as per our requirement.

Disadvantages of the autotransformer:

The autotransformer connection is not available with certain three-phase connections.Higher (and possibly more damaging) short-circuit currents can result from a lower series impedance.

Short circuits can impress voltages significantly higher than operating voltages across the windings of an

autotransformer.For the same voltage surge at the line terminals, the impressed and induced voltages are greater for an autotransformer

than for a two-winding transformer.

Autotransformer consists of a single winding around an iron core, which creates a change in voltage from one end to theother. In other words, the self-inductance of the winding around the core changes the voltage potential, but there is no

isolation of the high and low voltage ends of the winding. So any noise or other voltage anomaly coming in on one side

is passed through to the other. For that reason, Autotransformers are typically only used where there is already somesort of filtering or conditioning ahead of it, as in electronic applications, or the downstream device is unaffected by those

anomalies, such as an AC motor during starting

Applications:

Used in both Synchronous motors and induction motors.



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Used in electrical apparatus testing labs since the voltage can be smoothly and continuously varied.They find application as boosters in AC feeders to increase the voltage levels.

Used in HV Substation due to following reasons.

1. If we use normal transformer the size of transformer will be very high which leads to heavy weight, more copper and

high cost.

2. The tertiary winding used in Auto transformer balances single phase unbalanced loads connected to secondary and itdoes not pass on these unbalanced currents to Primary side. Hence Harmonics and voltage unbalance are eliminated.

3. Tertiary winding in the Auto Transformer balances amp turns so that Auto transformer achieves magnetic separation

like two winding transformers.


Scott-T Connection of Transformer

May 6, 2012 Leave a comment

3 Votes

(6) Scott-T Connection of Transformer:

Transforming 3 Phase to 2 Phase:

There are two main reasons for the need to transform from three phases to two phases,

1. To give a supply to an existing two phase system from a three phase supply.

2. To supply two phase furnace transformers from a three phase source.

Two-phase systems can have 3-wire, 4-wire, or 5-wire circuits. It is needed to be considering that a two-phase system

is not 2/3 of a three-phase system. Balanced three-wire, two-phase circuits have two phase wires, both carrying

approximately the same amount of current, with a neutral wire carrying 1.414 times the currents in the phase wires. Thephase-to-neutral voltages are 90° out of phase with each other.

Two phase 4-wire circuits are essentially just two ungrounded single-phase circuits that are electrically 90° out of phase

with each other. Two phase 5-wire circuits have four phase wires plus a neutral; the four phase wires are 90° out ofphase with each other.


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The easiest way to transform three-phase voltages into two-phase voltages is with two conventional single-phase

transformers. The first transformer is connected phase-to-neutral on the primary (three-phase) side and the secondtransformer is connected between the other two phases on the primary side.

The secondary windings of the two transformers are then connected to the two-phase circuit. The phase-to-neutral

primary voltage is 90° out of phase with the phase-to-phase primary voltage, producing a two-phase voltage across thesecondary windings. This simple connection, called the T connection, is shown in Figure

The main advantage of the T connection is that it uses transformers with standard primary and secondary voltages. The

disadvantage of the T connection is that a balanced two-phase load still produces unbalanced three-phase currents; i.e.,the phase currents in the three-phase system do not have equal magnitudes, their phase angles are not 120° apart, and

there is a considerable amount of neutral current that must be returned to the source.

The Scott Connection of Transformer:

A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase power from athree-phase source or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the

source.Scott T Transformers require a three phase power input and provide two equal single phase outputs called Main and

Teaser. The MAIN and Teaser outputs are 90 degrees out of phase. The MAIN and the Teaser outputs must not be

connected in parallel or in series as it creates a vector current imbalance on the primary side.MAIN and Teaser outputs are on separate cores. An external jumper is also required to connect the primary side of

the MAIN and Teaser sections.

The schematic of a typical Scott T Transformer is shown below:

Scott T Transformer is built with two single phase transformers of equal power rating. The MAIN and Teaser sections

can be enclosed in a floor mount enclosure with MAIN on the bottom and Teaser on top with a connecting jumpercable. They can also be placed side by side in separate enclosures.

Assuming the desired voltage is the same on the two and three phase sides, the Scott-T transformer connection consists

of a center-tapped 1:1 ratio main transformer, T1, and an 86.6% (0.5√3) ratio teaser transformer, T2. The center-tapped side of T1 is connected between two of the phases on the three-phase side. Its center tap then connects to one

end of the lower turn count side of T2, the other end connects to the remaining phase. The other side of thetransformers then connects directly to the two pairs of a two-phase four-wire system.



The Scott-T transformer connection may be also used in a back to back T to T arrangement for a three-phase to 3

phase connection. This is a cost saving in the smaller kVA transformers due to the 2 coil T connected to a secondary 2

coil T in-lieu of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the Neutraltap is part way up on the secondary teaser transformer . The voltage stability of this T to T arrangement as compared to

the traditional 3 coil primary to three-coil secondary transformer is questioned

Key Point:

If the main transformer has a turn’s ratio of 1: 1, then the teaser transformer requires a turn’s ratio of 0.866: 1 forbalanced operation. The principle of operation of the Scott connection can be most easily seen by first applying a

current to the teaser secondary windings, and then applying a current to the main secondary winding, calculating the

primary currents separately and superimposing the results.

Load connected between phaseY1 and phase Y2 of the secondary:

Secondary current from the teaser winding into phase X1 =1.0 <90°

Secondary current from the teaser winding into phase X2 =-1.0< 90°

Primary current from H3 phase into the teaser winding= 1.1547< 90°Primary current from H2 phase into the main winding= 0.5774 <90°

Primary current from H1 phase into the main winding= -0.5774< 90°

The reason that the primary current from H3 phase into the teaser winding is 1.1547 due to 0.866: 1 turn’s ratio of theteaser, transforming 1/0.866= 1.1547 times the secondary current. This current must split in half at the center tap of the

main primary winding because both halves of the main primary winding are wound on the same core and the total

ampere-turns of the main winding must equal zero.

Load connected between phase X2 and phase X1 of the secondary:

Secondary current from the main winding into phase X2 =1.0< 0°

Secondary current from the main winding into phase X4= -1.0 <0°Primary current from H2 phase into the main winding =1.0 <0°



Primary current from H1 phase into the main winding=- 1.0 <0°Primary current from H3 phase into the teaser winding= 0

Superimpose the two sets of primary currents:

I H3= 1.1547 <90° +0= 1.1547 <90°I H2 =0.5774 <90° +1.0< 0°= 1.1547 < 30°

I H1 =0.5774 <90°+ 1.0 <0°=1.1547 <210°

Notice that the primary three-phase currents are balanced; i.e., the phase currents have the same magnitude and theirphase angles are 120° apart. The apparent power supplied by the main transformer is greater than the apparent power

supplied by the teaser transformer. This is easily verified by observing that the primary currents in both transformers

have the same magnitude; however, the primary voltage of the teaser transformer is only 86.6% as great as the primaryvoltage of the main transformer. Therefore, the teaser transforms only 86.6% of the apparent power

transformed by the main.We also observe that while the total real power delivered to the two phase load is equal to the total real power supplied

from the three-phase system, the total apparent power transformed by both transformers is greater than the total

apparent power delivered to the two-phase load.The apparent power transformed by the teaser is 0.866 X IH1= 1.0 and the apparent power transformed by the main

is 1.0X IH2 =1.1547 for a total of 2.1547 of apparent power transformed.

The additional 0.1547 per unit of apparent power is due to parasitic reactive power owing between the two halves ofthe primary winding in the main transformer.

Single-phase transformers used in the Scott connection are specialty items that are virtually impossible to buy ‘‘off the

shelf ’’ nowadays. In an emergency, standard distribution transformers can be used

Advantages of the Scott T Connection:

If desired, a three phase, two phase, or single phase load may be supplied simultaneously

The neutral points can be available for grounding or loading purposes

Disadvantages when used for 3 Phase Loading

This type of asymmetrical connection (3 phases, 2 coils), reconstructs three phases from 2 windings. This can causeunequal voltage drops in the windings, resulting in potentially unbalanced voltages to be applied to the load.

The transformation ratio of the coils and the voltage obtained may be slightly unbalanced due to manufacturing variances

of the interconnected coils.This design’s neutral has to be solidly grounded. If it is not grounded solidly, the secondary voltages could become

unstable.Since this design will have a low impedance, special care will have to be taken on the primary protection fault current

capacity. This could be an issue if the system was designed for a Delta-Star connection.

The inherent single phase construction and characteristics of this connection produces a comparatively bulky andheavier transformer when compared with a normal three phase transformer of the same rating.

Application:

For Industrial Furnace Transformer.

For Traction Purpose: The power is obtained from the 220 kV or 132 kV or 110 kV or 66 kV, three-phase,effectively earthed transmission network of the State Electricity Board, through single-phase transformers or Scott

connected transformer installed at the Traction Substation. The primary winding of the single-phase transformer is

connected to two phases of the transmission network or Where Scott-connected transformer is used, the primarywindings are connected to the three phases of the transmission network.

The single-phase transformers at a Traction Substation are connected to the same two phases of the transmission

network (referred as single-phase connection), or alternatively to different pairs of phases- the three single phase



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transformers forming a delta-connection on the primary side. Out of three single-phase transformers, one transformer

feeds the overhead equipment (OHE) on one side of the Traction Substation, another feeds the OHE on the other sideof the Traction Substation, and the third remains as standby. Thus the two single-phase transformers which feed the

OHE constitute an open-delta connection (alternatively, referred as V-connection) on the three-phase transformers

network. The Scott-connected transformer and V-connected single-phase transformers are effective in reducing voltageimbalance on the transmission network. The spacing between adjacent substations is normally between 70 and 100 km.


Zig-zag Connection of Transformer

May 4, 2012 1 Comment

5 Votes

(5) The Zigzag Connection:

The zigzag connection is also called the interconnected star connection. This connection has some of the features of the

Y and the ∆ connections, combining the advantages of both.The zigzag transformer contains six coils on three cores. The first coil on each core is connected contrariwise to the

second coil on the next core. The second coils are then all tied together to form the neutral and the phases are

connected to the primary coils. Each phase, therefore, couples with each other phase and the voltages cancel out. Assuch, there would be negligible current through the neutral pole and it can be connected to ground

One coil is the outer coil and the other is the inner coil. Each coil has the same number of windings turns (Turnsratio=1:1) but they are wound in opposite directions. The coils are connected as follows:

The outer coil of phase a1-a is connected to the inner coil of phase c2-N.

The outer coil of phase b1-b is connected to the inner coil of phase a2-N.The outer coil of phase c1-c is connected to the inner coil of phase b2-N.

The inner coils are connected together to form the neutral and our tied to ground

The outer coils are connected to phases a1,b1,c1 of the existing delta system.

If three currents, equal in magnitude and phase, are applied to the three terminals, the ampere-turns of the a2-N

winding cancel the ampere-turns of the b1-b winding, the ampere-turns of the b2-N winding cancel the ampere

turns of the c1-c winding, and the ampere-turns of the c2-N winding cancel the ampere turns of the a1-a winding.Therefore, the transformer allows the three in-phase currents to easily flow to neutral.


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If three currents, equal in magnitude but 120° out of phase with each other, are applied to the three terminals, the

ampere-turns in the windings cannot cancel and the transformer restricts the current flow to the negligible level of

magnetizing current. Therefore, the zigzag winding provides an easy path for in-phase currents but does not allow theflow of currents that are 120°out of phase with each other.

Under normal system operation the outer and inner coil winding’s magnetic flux will cancel each other and only

negligible current will flow in the in the neutral of the zig –zag transformer.During a phase to ground fault the zig-zag transformer’s coils magnetic flux are no longer equal in the faulted line. This

allows zero sequence.

If one phase, or more, faults to earth, the voltage applied to each phase of the transformer is no longer in balance; fluxesin the windings no longer oppose. (Using symmetrical components, this is Ia0 = Ib0 = Ic0.) Zero sequence (earth fault)

current exists between the transformers’ neutral to the faulting phase. Hence, the purpose of a zigzag transformer is toprovide a return path for earth faults on delta connected systems. With negligible current in the neutral under normal

conditions, engineers typically elect to under size the transformer; a short time rating is applied. Ensure the impedance isnot too low for the desired fault limiting. Impedance can be added after the secondary’s are summed (the 3Io path)

The neutral formed by the zigzag connection is very stable. Therefore, this type of transformer, or in some cases an autotransformer, lends itself very well for establishing a neutral for an ungrounded 3 phase system.

Many times this type of transformer or auto transformer will carry a fairly large rating, yet physically be relatively small.

This particularly applies in connection with grounding applications. The reason for this small size in relation to thenameplate KVA rating is due to the fact that many types of grounding auto transformers are rated for 2 seconds. This is

based on the time to operate an over current protection device such as a breaker. Zigzag transformers used to be

employed to enable size reductions in drive motor systems due to the stable wave form they present. Other means arenow more common, such as 6 phase star.

Advantages of Zig-Zag Transformer:

The ∆ -zigzag connection provides the same advantages as the ∆-Y connection.

Less Costly for grounding Purpose: It is typically the least costly than Y-D and Scott Transformer.Third harmonic suppression: The zigzag connection in power systems to trap triple harmonic (3rd, 9th, 15th, etc.)

currents. Here, We install zigzag units near loads that produce large triple harmonic currents. The windings trap the

harmonic currents and prevent them from traveling upstream, where they can produce undesirable effects.Ground current isolation: If we need a neutral for grounding or for supplying single-phase line to neutral loads when

working with a 3-wire, ungrounded power system, a zigzag connection may be the better solution. Due to itscomposition, a zigzag transformer is more effective for grounding purposes because it has less internal winding

impedance going to the ground than when using a Star type transformer.

No Phase Displacement: There is no phase angle displacement between the primary and the secondary circuits withthis connection; therefore, the ∆-zigzag connection can be used in the same manner as Y-Y and ∆- ∆ transformers

without introducing any phase shifts in the circuits.

Application:

An Earthing Reference: Occasionally engineers use a combination of YD and zigzag windings to achieve a vectorphase shift. For example, an electrical network may have a transmission network of 220 kV/66 kV star/star

transformers, with 66 kV/11 kV delta/star for the high voltage distribution network. If a transformation is required

directly between the 220 kV/11 kV network the most obvious option is to use 220 kV/11 kV star/delta. The problemis that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the secondary side of

the 220 kV/11 kV transformer provides the required earth reference point.

As a Grounding Transformer:The ability to provide a path for in-phase currents enables us to use the zigzagconnection as a grounding bank, which is one of the main applications for this connection.

We rarely use zigzag configurations for typical industrial or commercial use, because they are more expensive toconstruct than conventional Star connected transformers. But zigzag connections are useful in special applications where



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conventional transformer connections aren’t effective.D or Y / Zig-zag are used in unbalanced low voltage system – mostly with single phase appliances


Star-Delta Connection of Transformer

May 3, 2012 1 Comment

2 Votes

(4) Star-Delta Connection:

In this type of connection, then primary is connected in star fashion while the secondary is connected in delta fashion asshown in the Fig.

The voltages on primary and secondary sides can be represented on the phasor diagram as shown in the Fig.

Key point:


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As Primary in Star connectedLine voltage on Primary side = √3 X Phase voltage on Primary side. So

Phase voltage on Primary side = Line voltage on Primary side / √3Now Transformation Ration (K) = Secondary Phase Voltage / Primary Phase VoltageSecondary Phase Voltage = K X Primary Phase Voltage.

As Secondary in delta connected:Line voltage on Secondary side = Phase voltage on Secondary side.Secondary Phase Voltage = K X Primary Phase Voltage. =K X (Line voltage on Primary side / √3)

Secondary Phase Voltage = (K/√3 ) X Line voltage on Primary side.There is s +30 Degree or -30 Degree Phase Shift between Secondary Phase Voltage to Primary PhaseVoltage

Advantages of Star Delta Connection:

The primary side is star connected. Hence fewer numbers of turns are required. This makes the connection economicalfor large high voltage step down power transformers.The neutral available on the primary can be earthed to avoid distortion.

The neutral point allows both types of loads (single phase or three phases) to be met.Large unbalanced loads can be handled satisfactory. The Y-D connection has no problem with third harmonic components due to circulating currents inD. It is also more

stable to unbalanced loads since the D partially redistributes any imbalance that occurs.The delta connected winding carries third harmonic current due to which potential of neutral point is stabilized. Somesaving in cost of insulation is achieved if HV side is star connected. But in practice the HV side is normally connected indelta so that the three phase loads like motors and single phase loads like lighting loads can be supplied by LV side

using three phase four wire system.As Grounding Transformer: In Power System Mostly grounded Y- ∆ transformer is used for no other purpose thanto provide a good ground source in ungrounded Delta system. Take, for example, a distribution system supplied by ∆

connected (i.e., un-grounded) power source. If it is required to connect phase-to-ground loads to this system agrounding bank is connected to the system, as shown in Figure

This system a grounding bank is connected to the system, as shown in Figure. Note that the connected winding is not

connected to any external circuit in Figure.With a load current equal to 3 times i, each phase of the grounded Y winding provides the same current i, with the -connected secondary winding of the grounding bank providing the ampere-turns required to cancel the ampere-turns of

the primary winding. Note that the grounding bank does not supply any real power to the load; it is there merely toprovide a ground path. All the power required by the load is supplied by two phases of the ungrounded supply

Disadvantages of Star-Delta Connection:



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In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate this

connection in parallel with star-star or delta-delta connected transformer.

One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primaryvoltage. This can cause problems when paralleling 3-phase transformers since transformers secondary voltages must bein-phase to be paralleled. Therefore, we must pay attention to these shifts.

If secondary of this transformer should be paralleled with secondary of another transformer without phase shift, therewould be a problem

Application:

It is commonly employed for power supply transformers.

This type of connection is commonly employed at the substation end of the transmission line. The main use with thisconnection is to step down the voltage. The neutral available on the primary side is grounded. It can be seen that thereis phase difference of 30° between primary and secondary line voltages.

Commonly used in a step-down transformer, Y connection on the HV side reduces insulation costs the neutral point onthe HV side can be grounded, stable with respect to unbalanced loads. As for example, at the end of a transmissionline. The neutral of the primary winding is earthed. In this system, line voltage ratio is 1/√3 Times of transformer turn-

ratio and secondary voltage lags behind primary voltage by 30°. Also third harmonic currents flow in the to give asinusoidal flux.


Delta-Star Connection of Transformer


2 Votes

(3) Delta-Star Connection of Transformer

In this type of connection, the primary connected in delta fashion while the secondary current is connected in star.


Likell



The main use of this connection is to step up the voltage i.e. at the begining of high tension transmission system. It can

be noted that there is a phase shift of 30° between primary line voltage and secondary line voltage as leading.

Key point:

As primary in delta connected:Line voltage on primary side = Phase voltage on Primary side.Now Transformation Ration (K) = Secondary Phase Voltage / Primary Phase Voltage

Secondary Phase Voltage = K X Primary Phase Voltage.As Secondary in Star connectedLine voltage on Secondary side = √3 X Phase voltage on Secondary side. So,

Line voltage on Secondary side = √3 X K X Primary Phase Voltage.Line voltage on Secondary side = √3 X K X Primary Line Voltage.There is s +30 Degree or -30 Degree Phase Shift between Secondary Phase Voltage to Primary Phase

Voltage

Advantages of Delta-Star Connection:

Cross section area of winding is less at Primary side: On primary side due to delta connection winding cross-section required is less.

Used at Three phase four wire System: On secondary side, neutral is available, due to which it can be used for 3-phase, 4 wire supply system.No distortion of Secondary Voltage: No distortion due to third harmonic components.

Handled large unbalanced Load: Large unbalanced loads can be handled without any difficulty.Grounding Isolation between Primary and Secondary: Assuming that the neutral of the Y-connected secondarycircuit is grounded, a load connected phase-to-neutral or a phase-to-ground fault produces two equal and opposite

currents in two phases in the primary circuit without any neutral ground current in the primary circuit. Therefore, incontrast with the Y-Y connection, phase-to-ground faults or current unbalance in the secondary circuit will not affectground protective relaying applied to the primary circuit. This feature enables proper coordination of protective devices

and is a very important design consideration.The neutral of the Y grounded is sometimes referred to as a grounding bank, because it provides a local source ofground current at the secondary that is isolated from the primary circuit.

Harmonic Suppression: The magnetizing current must contain odd harmonics for the induced voltages to be sinusoidaland the third harmonic is the dominant harmonic component. In a three-phase system the third harmonic currents of allthree phases are in phase with each other because they are zero-sequence currents. In the Y-Y connection, the only

path for third harmonic current is through the neutral. In the ∆ -Y connection, however, the third harmonic currents,



being equal in amplitude and in phase with each other, are able to circulate around the path formed by the ∆ connectedwinding. The same thing is true for the other zero-sequence harmonics.Grounding Bank: It provides a local source of ground current at the secondary that is isolated from the primarycircuit. For suppose an ungrounded generator supplies a simple radial system through ∆-Y transformer with grounded

Neutral at secondary as shown Figure. The generator can supply a single-phase-to-neutral load through the -groundedY transformer.Let us refer to the low-voltage generator side of the transformer as the secondary and the high-voltage load side of the

transformer as the primary. Note that each primary winding is magnetically coupled to a secondary winding Themagnetically coupled windings are drawn in parallel to each other.

Through the second transformer law, the phase-to-ground load current in the primary circuit is reflected as a current in

the A-C secondary winding. No other currents are required to flow in the A-C or B-C windings on the generator sideof the transformer in order to balance ampere-turns.Easy Relaying of Ground Protection: Protective relaying is MUCH easier on a delta-wye transformer becauseground faults on the secondary side are isolated from the primary, making coordination much easier. If there is

upstream relaying on a delta-wye transformer, any zero-sequence current can be assumed to be from a primary groundfault, allowing very sensitive ground fault protection. On a wye-wye, a low-side ground fault causes primary groundfault current, making coordination more difficult. Actually, ground fault protection is one of the primary advantages of

delta-wye units.

Disadvantages of Delta-Star Connection:

In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate thisconnection in parallel with star-star or delta-delta connected transformer.

One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primaryvoltage. This can cause problems when paralleling 3-phase transformers since transformers secondary voltages must bein-phase to be paralleled. Therefore, we must pay attention to these shifts.

If secondary of this transformer should be paralleled with secondary of another transformer without phase shift, therewould be a problem.

Applications:

Commonly used in a step-up transformer:As for example, at the beginning of a HT transmission line. In this case

neutral point is stable and will not float in case of unbalanced loading. There is no distortion of flux because existence ofa Δ -connection allows a path for the third-harmonic components. The line voltage ratio is √3 times of transformer turn-ratio and the secondary voltage leads the primary one by 30°. In recent years, this arrangement has become very

popular for distribution system as it provides 3- Ø, 4-wire system.Commonly used in commercial, industrial, and high-density residential locations: To supply three-phasedistribution systems. An example would be a distribution transformer with a delta primary, running on three 11kVphases with no neutral or earth required, and a star (or wye) secondary providing a 3-phase supply at 400 V, with the

domestic voltage of 230 available between each phase and an earthed neutral point.Used as Generator Transformer:The ∆-Y transformer connection is used universally for connecting generators totransmission systems because of two very important reasons. First of all, generators are usually equipped with sensitive

ground fault relay protection. The ∆-Y transformer is a source of ground currents for loads and faults on the



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transmission system, yet the generator ground fault protection is completely isolated from ground currents on theprimary side of the transformer. Second, rotating machines can literally be


Delta-Delta Connection of Transformer

May 1, 2012 Leave a comment

2 Votes

(2) Delta-Delta Connection:

In this type of connection, both the three phase primary and secondary windings are connected in delta as shown in theFig.

The voltages on primary and secondary sides can be shown on the phasor diagram.

This connection proves to be economical for large low voltage transformers as it increases number of turns per phase.


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Key point:

Primary Side Line Voltage = Secondary Side Line Voltage.Primary Side Phase Voltage= Secondary Side Phase Voltage.

No phase shift between primary and secondary voltages

Advantage of Delta-Delta Connection:

Sinusoidal Voltage at Secondary: In order to get secondary voltage as sinusoidal, the magnetizing current oftransformer must contain a third harmonic component. The delta connection provides a closed path for circulation of

third harmonic component of current. The flux remains sinusoidal which results in sinusoidal voltages.Suitable for Unbalanced Load: Even if the load is unbalanced the three phase voltages remains constant. Thus itsuitable for unbalanced loading also.

Carry 58% Load if One Transfer is Faulty in Transformer Bank: If there is bank of single phase transformersconnected in delta-delta fashion and if one of the transformers is disabled then the supply can be continued withremaining tow transformers of course with reduced efficiency.No Distortion in Secondary Voltage: there is no any phase displacement between primary and secondary voltages.

There is no distortion of flux as the third harmonic component of magnetizing current can flow in the delta connectedprimary windings without flowing in the line wires .there is no distortion in the secondary voltages.Economical for Low Voltage: Due to delta connection, phase voltage is same as line voltage hence winding have

more number of turns. But phase current is (1/√3) times the line current. Hence the cross-section of the windings is veryless. This makes the connection economical for low voltages transformers.

Reduce Cross section of Conductor: The conductor is required of smaller Cross section as the phase current is 1/√3

times of the line current. It increases number of turns per phase and reduces the necessary cross sectional area ofconductors thus insulation problem is not present.Absent of Third Harmonic Voltage: Due to closed delta, third harmonic voltages are absent.

The absence of star or neutral point proves to be advantageous in some cases.

Disadvantage of Delta-Delta Connection:

Due to the absence of neutral point it is not suitable for three phase four wire system.More insulation is required and the voltage appearing between windings and core will be equal to full line voltage in

case of earth fault on one phase.

Application:

Suitable for large, low voltage transformers.This Type of Connection is normally uncommon but used in some industrial facilities to reduce impact of SLG faults on

the primary systemIt is generally used in systems where it need to be carry large currents on low voltages and especially when continuity ofservice is to be maintained even though one of the phases develops fault.



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Star-Star Connection of Transformer

April 30, 2012 4 Comments

2 Votes

Transformer Connection:

The windings of three phase transformers may be connected in by Y or Δ in the same manner as for three single phasetransformers. Since the secondary’s may be connected either in Y or Δ regardless of which connection is used on the

primaries, there must be four ways of connecting the windings of a 3-phase transformer for transformation of 3-phasevoltages, namely Y-y,Δ -Δ, Y-Δ, and Δ -y. The inter-connections are made inside of the case so that only the terminal leadsneed to be brought outside the case

1. Star – Star Transformer (Yy0 or Yy6)

2. Delta – Delta Transformer (Dd0 or Dd6)3. Delta – Star Transformer (Dy)4. Star – Delta Transformer Yd) (Grounding Transformer).

5. Zig-zag Transformer (Yz, Dz) (Grounding Transformer)6. Scott (“T” Type) Transformer (Grounding Transformer).

(1) Star-Star(Y-y)Connection:

In Primary Winding Each Phase is120°electrical degrees out of phase with the other two phases.In Secondary Winding Each Phase is120°electrical degrees out of phase with the other two phases.Each primary winding is magnetically linked to one secondary winding through a common core leg. Sets of windings

that are magnetically linked are drawn parallel to each other in the vector diagram. In the Y-Y connection, each primaryand secondary winding is connected to a neutral point.The neutral point may or may not be brought out to an external physical connection and the neutral may or may not be

grounded.



Transformer magnetizing currents are not purely sinusoidal, even if the exciting voltages are sinusoidal. The magnetizingcurrents have significant quantities of odd-harmonic components. If three identical transformers are connected to each

phase and are excited by 60 Hz voltages of equal magnitude, the 60 Hz fundamental components of the excitingcurrents cancel out each other at the neutral. This is because the 60 Hz fundamental currents of A, B, and C phase are120° out of phase with one another and the vector sum of these currents is zero.The third, ninth, fifteenth and other so-called zero-sequence harmonic currents are in phase with each other; therefore,

these components do not cancel out each other at the neutral but add in phase with one another to produce a zero-sequence neutral current, provided there is a path for the neutral current to flow.Due to the nonlinear shape of the B-H curve, odd-harmonic magnetizing currents are required to support sinusoidal

induced voltages. If some of the magnetizing current harmonics are not present, then the induced voltages cannot besinusoidal.Y-Y Connection with Grounded Neutral :

Figure Show the situation where the primary neutral is returned to the voltage source in a four-wire three-phase circuit.Each of the magnetizing currents labeled IR, IY, and IB contain the 60 Hz fundamental current and all of the oddharmonic currents necessary to support sinusoidal induced voltages.

The zero-sequence magnetizing currents combine to form the neutral current IN, which returns these odd harmonics to

the voltage source. Assuming that the primary voltage is sinusoidal, the induced voltages VR , VY , and VB (in both theprimary and secondary) are sinusoidal as well.The connection of primary neutral to the neutral of generator has an add advantage that it eliminates distortion in the

secondary phase voltages. If the flux in the core has sinusoidal waveform then it will give sinusoidal waveform for thevoltage. But due to characteristic of iron, a sinusoidal waveform of flux requires a third harmonic component in theexciting current. As the frequency of this component is thrice the frequency of circuit at any given constant. It will try to

flow either towards or away from the neutral point in the transformer windings. With isolated neutral, the triplefrequency current cannot flow so the flux in the core will not be a sine wave and the voltages are distorted. If primaryneutral is connected to generator neutral the triple frequency currents get the path to solve the difficulty. The alternative

way of overcoming with this difficulty is the use of tertiary winding of low KVA rating. These windings are connected indelta and provide a circuit in which triple frequency currents can flow. Thus sinusoidal voltage on primary will givesinusoidal voltage on secondary side.

This situation changes if the neutrals of both sets of the primary and secondary windings are not grounded.Y-Y Connection without Grounded Neutral: If the neutrals of both the primary and the secondary are open-circuited and so there is no path for the zero-sequence harmonic currents to flow and the induced voltages will not be

sinusoidal.



V’R, V’Y, and V’B will not be sinusoidal. This results in distortions of the secondary voltages. The resulting voltage

distortion is equivalent to a Y-Y transformer with zero-sequence currents allowed to flow in the primary neutral with animaginary superimposed primary winding carrying only the zero-sequence currents 180° out of phase with the normalzero-sequence currents.

Analysis of the voltages induced by the ‘‘primary windings’’ is greatly complicated by the fact that the core is highlynonlinear so that each of the individual zero-sequence harmonics currents carried by the phantom primary windings willinduce even higher-order harmonic voltages as well.Fourier analysis can be used to arrive at an approximation of the secondary voltages with an open primary neutral.

Taking one phase at a time, the normal magnetizing current for a sinusoidal exciting voltage is plotted from the B-Hcurve of the transformer. The normal magnetizing current is converted to a Fourier series and then it is reconstructed byremoving all of the zero-sequence harmonics. The resulting exciting current will have a shape different from the normal

exciting current, which is then used to construct an induced voltage using the B-H curve in there verse manner that wasused to construct the original exciting current. This process is rather laborious, so suffice it to say that if a Y-Ytransformer does not have a neutral path for zero-sequence exciting currents, there will be harmonic voltages induced in

the secondary even if the exciting voltage is purely sinusoidal.

Advantage of Y-Y Connection:

No Phase Displacement: The primary and secondary circuits are in phase; i.e., there are no phase angledisplacements introduced by the Y-Y connection. This is an important advantage when transformers are used to

interconnect systems of different voltages in a cascading manner. For example, suppose there are four systemsoperating at 800, 440, 220, and 66 kV that need to be interconnected. Substations can be constructed using Y-Ytransformer connections to interconnect any two of these voltages. The 800 kV systems can be tied with the 66 kV

systems through a single 800 to 66 kV transformation or through a series of cascading transformations at 440,220 and66 kV.Required Few Turns for winding: Due to star connection, phase voltages is (1/√3) times the line voltage. Hence less

number of turns is required. Also the stress on insulation is less. This makes the connection economical for small highvoltage purposes.Required Less Insulation Level: If the neutral end of a Y-connected winding is grounded, then there is an

opportunity to use reduced levels of insulation at the neutral end of the winding. A winding that is connected across thephases requires full insulation throughout the winding.Handle Heavy Load: Due to star connection, phase current is same as line current. Hence windings have to carry high

currents. This makes cross section of the windings high. Thus the windings are mechanically strong and windings canbear heavy loads and short circuit current.Use for Three phases Four Wires System:As neutral is available, suitable for three phases four wire system.

Eliminate Distortion in Secondary Phase Voltage: The connection of primary neutral to the neutral of generatoreliminates distortion in the secondary phase voltages by giving path to triple frequency currents toward to generator.Sinusoidal voltage on secondary side: Neutral give path to flow Triple frequency current to flow Generator side thussinusoidal voltage on primary will give sinusoidal voltage on secondary side.

Used as Auto Transformer: A Y-Y transformer may be constructed as an autotransformer, with the possibility ofgreat cost savings compared to the two-winding transformer construction.



Better Protective Relaying: The protective relay settings will be protecting better on the line to ground faults whenthe Y-Y transformer connections with solidly grounded neutrals are applied.

Disadvantage of Y-Y Connection:

The Third harmonic issue: The voltages in any phase of a Y-Y transformer are 1200 apart from the voltages in any

other phase. However, the third-harmonic components of each phase will be in phase with each other. Nonlinearities inthe transformer core always lead to generation of third harmonic. These components will add up resulting in large (canbe even larger than the fundamental component) third harmonic component.Overvoltage at Lighting Load: The presence of third (and other zero-sequence) harmonics at an ungrounded neutral

can cause overvoltage conditions at light load. When constructing a Y-Y transformer using single-phase transformersconnected in a bank, the measured line-to-neutral voltages are not 57.7% of the system phase-to-phase voltage at noload but are about 68% and diminish very rapidly as the bank is loaded. The effective values of voltages at different

frequencies combine by taking the square root of the sum of the voltages squared. With sinusoidal phase-to-phasevoltage, the third-harmonic component of the phase-to-neutral voltage is about 60%.Voltage drop at Unbalance Load: There can be a large voltage drop for unbalanced phase-to-neutral loads. This is

caused by the fact that phase-to-phase loads cause a voltage drop through the leakage reactance of the transformerwhereas phase-to-neutral loads cause a voltage drop through the magnetizing reactance, which is 100 to 1000 timeslarger than the leakage reactance.

Overheated Transformer Tank: Under certain circumstances, a Y-Y connected three-phase trans- can producesevere tank overheating that can quickly destroy the transformer. This usually occurs with an open phase on the primarycircuit and load on the secondary.

Over Excitation of Core in Fault Condition: If a phase-to-ground fault occurs on the primary circuit with theprimary neutral grounded, then the phase-to-neutral voltage on the un faulted phases increases to 173% of the normalvoltage. This would almost certainly result in over excitation of the core, with greatly increased magnetizing currents and

core lossesIf the neutrals of the primary and secondary are both brought out, then a phase-to-ground fault on the secondary circuitcauses neutral fault current to flow in the primary circuit. Ground protection relaying in the neutral of the primary

circuit may then operate for faults on the secondary circuitNeutral Shifting: If the load on the secondary side unbalanced then the performance of this connection is notsatisfactory then the shifting of neutral point is possible. To prevent this, star point of the primary is required to be

connected to the star point of the generator.Distortion of Secondary voltage: Even though the star or neutral point of the primary is earthed, the third harmonicpresent in the alternator voltage may appear on the secondary side. This causes distortion in the secondary phasevoltages.

Over Voltage at Light Load: The presence of third (and other zero-sequence) harmonics at an ungrounded neutralcan cause overvoltage conditions at light load.Difficulty in coordination of Ground Protection: In Y-Y Transformer, a low-side ground fault causes primary

ground fault current, making coordination more difficult.Increase Healthy Phase Voltage under Phase to ground Fault: If a phase-to-ground fault occurs on the primarycircuit with the primary neutral grounded, then the phase-to-neutral voltage on the UN faulted phase’s increases to

173% of the normal voltage. If the neutrals of the primary and secondary are both brought out, then a phase-to-groundfault on the secondary circuit causes neutral fault current to flow in the primary circuit.Trip the T/C in Line-Ground Fault: All harmonics will propagate through the transformer, zero-sequence current

path is continuous through the transformer, one line-to-ground fault will trip the transformer.Suitable for Core Type Transformer: The third harmonic voltage and current is absent in such type of connectionwith three phase wire system. or shell type of three phase units, the third harmonic phase voltage may be high. This type

of connection is more suitable for core type transformers.

Application:



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This Type of Transformer is rarely used due to problems with unbalanced loads.It is economical for small high voltage transformers as the number of turns per phase and the amount of insulationrequired is less.


Calculate % Voltage Regulation of Small Distribution Line.

April 7, 2012 4 Comments

5 Votes

FREE DOWNLOAD.

Calculate % Voltage Regulation of Small Distribution LineCalculate % Voltage drop of Individual Load.Calculate % Voltage drop of Various ACSR, AAAC, AAC conductor overhead Line.

Calculate Receiving end Voltage of Distribution Line.Calculate Total % Voltage Regulation of Distribution Line


Insulation Resistance (IR) Values

March 23, 2012 10 Comments

11 Votes

Introduction:

The measurement of insulation resistance is a common routine test performed on all types of electrical wires and cables. As a

production test, this test is often used as a customer acceptance test, with minimum insulation resistance per unit length often

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specified by the customer. The results obtained from IR Test are not intended to be useful in finding localized defects in theinsulation as in a true HIPOT test, but rather give information on the quality of the bulk material used as the insulation.

Even when not required by the end customer, many wire and cable manufacturers use the insulation resistance test to track

their insulation manufacturing processes, and spot developing problems before process variables drift outside of allowed limits.

Selection of IR Testers (Megger):

Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available.The recommended ratings of the insulation testers are given below:

Voltage Level IR Tester

650V 500V DC

1.1KV 1KV DC

3.3KV 2.5KV DC

66Kv and Above 5KV DC

Test Voltage for Meggering:

When AC Voltage is used, The Rule of Thumb is Test Voltage (A.C) = (2X Name Plate Voltage) +1000.When DC Voltage is used (Most used in All Megger), Test Voltage (D.C) = (2X Name Plate Voltage).

Equipment / Cable

Rating

DC Test Voltage

24V To 50V 50V To 100V

50V To 100V 100V To 250V

100V To 240V 250V To 500V

440V To 550V 500V To 1000V

2400V 1000V To 2500V

4100V 1000V To 5000V

Measurement Range of Megger:

Test voltage MeasurementRange

250V DC 0MΩ to 250GΩ

500V DC 0MΩ to 500GΩ

1KV DC 0MΩ to 1TΩ

2.5KV DC 0MΩ to 2.5TΩ

5KV DC 0MΩ to 5TΩ

Precaution while Meggering:

Before Meggering:

Make sure that all connections in the test circuit are tight.Test the megger before use, whether it gives INFINITY value when not connected, and ZERO when the two terminalsare connected together and the handle is rotated.

During Meggering:



Make sure when testing for earth, that the far end of the conductor is not touching, otherwise the test will show faultyinsulation when such is not actually the case.

Make sure that the earth used when testing for earth and open circuits is a good one otherwise the test will give wronginformationSpare conductors should not be meggered when other working conductors of the same cable are connected to the

respective circuits.

After completion of cable Meggering:

Ensure that all conductors have been reconnected properly.Test the functions of Points, Tracks & Signals connected through the cable for their correct response.

In case of signals, aspect should be verified personally.In case of points, verify positions at site. Check whether any polarity of any feed taken through the cable has gotearthed inadvertently.

Safety Requirements for Meggering:

All equipment under test MUST be disconnected and isolated.Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order tobe absolutely safe for the person conducting the test.

Never use Megger in an explosive atmosphere.Make sure all switches are blocked out and cable ends marked properly for safety.Cable ends to be isolated shall be disconnected from the supply and protected from contact to supply, or ground, or

accidental contact.Erection of safety barriers with warning signs, and an open communication channel between testing personnel.Do not megger when humidity is more than 70 %.Good Insulation: Megger reading increases first then remain constant.

Bad Insulation: Megger reading increases first and then decreases.Expected IR value gets on Temp. 20 to 30 decree centigrade.If above temperature reduces by 10 degree centigrade, IR values will increased by two times.

If above temperature increased by 70 degree centigrade IR values decreases by 700 times.

How to use Megger:

Meggers is equipped with three connection Line Terminal (L), Earth Terminal (E) and Guard Terminal (G).

Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The “Guard”terminal is provided for special testing situations where one resistance must be isolated from another. Let’sus check one situation where the insulation resistance is to be tested in a two-wire cable.

To measure insulation resistance from a conductor to the outside of the cable, we need to connect the “Line” lead of themegger to one of the conductors and connect the “Earth” lead of the megger to a wire wrapped around the sheath ofthe cable.



In this configuration the Megger should read the resistance between one conductor and the outside sheath.We want to measure Resistance between Conductor- 2To Sheaths but Actually Megger measure resistance in parallelwith the series combination of conductor-to-conductor resistance (Rc1-c2) and the first conductor to the sheath (Rc1-s).

If we don’t care about this fact, we can proceed with the test as configured. If we desire to measure only the resistance

between the second conductor and the sheath (Rc2-s), then we need to use the megger’s “Guard” terminal.

Connecting the “Guard” terminal to the first conductor places the two conductors at almost equal potential.

With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no currentbetween the two conductors. Consequently, the Megger’s resistance indication will be based exclusively on the currentthrough the second conductor’s insulation, through the cable sheath, and to the wire wrapped around, not the current

leaking through the first conductor’s insulation.The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, itallows you to be selective in evaluating certain specific components in a large piece of electrical equipment. Forexample consider a two core cable with a sheath. As the diagram below shows there are three resistances to be

considered.

If we measure between core B and sheath without a connection to the guard terminal some current will pass from B to

A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cablecores will be at very nearly the same potential and thus the shunting effect is eliminated.

(1) IR Values For Electrical Apparatus & Systems:

(PEARL Standard / NETA MTS-1997 Table 10.1)



Max.Voltage

Rating OfEquipment

Megger Size Min.IR

Value

250 Volts 500 Volts 25 MΩ

600 Volts 1,000 Volts 100 MΩ

5 KV 2,500 Volts 1,000 MΩ

8 KV 2,500 Volts 2,000 MΩ

15 KV 2,500 Volts 5,000 MΩ

25 KV 5,000 Volts 20,000 MΩ

35 KV 15,000 Volts 100,000 MΩ

46 KV 15,000 Volts 100,000 MΩ

69 KV 15,000 Volts 100,000 MΩ

One Meg ohm Rule for IR Value for Equipment:

Based upon equipment rating:< 1K V = 1 MΩ minimum

>1KV = 1 MΩ /1KV

As per IE Rules-1956:

At a pressure of 1000 V applied between each live conductor and earth for a period of one minute the insulationresistance of HV installations shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards.

Medium and Low Voltage Installations- At a pressure of 500 V applied between each live conductor and earth for aperiod of one minute, the insulation resistance of medium and low voltage installations shall be at least 1 Mega ohm oras specified by the Bureau of Indian Standards] from time to time.

As per CBIP specifications the acceptable values are 2 Mega ohms per KV

(2) IR Value for Transformer:

Insulation resistance tests are made to determine insulation resistance from individual windings to ground or betweenindividual windings. Insulation resistance tests are commonly measured directly in megohms or may be calculated from



measurements of applied voltage and leakage current.The recommended practice in measuring insulation resistance is to always ground the tank (and the core). Short circuiteach winding of the transformer at the bushing terminals. Resistance measurements are then made between eachwinding and all other windings grounded.

Windings are never left floating for insulation resistance measurements. Solidly grounded winding must have the ground

removed in order to measure the insulation resistance of the winding grounded. If the ground cannot be removed, as inthe case of some windings with solidly grounded neutrals, the insulation resistance of the winding cannot be measured.Treat it as part of the grounded section of the circuit.

We need to test winding to winding and winding to ground ( E ).For three phase transformers, We need to test winding( L1,L2,L3 ) with substitute Earthing for Delta transformer or winding ( L1,L2,L3 ) with earthing ( E ) and neutral ( N )for wye transformers.

IR Value for Transformer

(Ref: A Guide to Transformer Maintenance by. JJ. Kelly. S.DMyer)

Transformer Formula

1 Phase Transformer IR Value (MΩ) = C X E /(√KVA)

3 Phase Transformer (Star) IR Value (MΩ) = C X E (P-n) /(√KVA)

3 Phase Transformer(Delta)

IR Value (MΩ) = C X E (P-P) /(√KVA)

Where C= 1.5 for Oil filled T/C with Oil Tank, 30 for Oil filled

T/C without Oil Tank or Dry Type T/C.

Temperature correction Factor (Base 20°C):

Temperature correctionFactor

OC OF Correction Factor

0 32 0.25

5 41 0.36



10 50 0.50

15 59 0.720

20 68 1.00

30 86 1.98

40 104 3.95

50 122 7.85

Example: For 1600KVA, 20KV/400V,Three Phase Transformer

IR Value at HV Side= (1.5 x 20000) / √ 1600 =16000 / 40 = 750 MΩ at 200C

IR Value at LV Side = (1.5 x 400 ) / √ 1600= 320 / 40 = 15 MΩ at 200C

IR Value at 300C =15X1.98= 29.7 MΩ

Insulation Resistance of Transformer Coil

Transformer

Coil Voltage

Megger Size

Min.IR Value

Liquid Filled T/C

Min.IRValue Dry

Type T/C

0 – 600 V 1KV 100 MΩ 500 MΩ

600 V To 5KV 2.5KV 1,000 MΩ 5,000 MΩ

5KV To 15KV 5KV 5,000 MΩ 25,000 MΩ

15KV To 69KV 5KV 10,000 MΩ 50,000 MΩ

IR Value of Transformers:

Voltage Test Voltage

(DC) LVside

Test Voltage

(DC) HVside

Min IR

Value

415V 500V 2.5KV 100MΩ

Up to 6.6KV 500V 2.5KV 200MΩ

6.6KV to 11KV 500V 2.5KV 400MΩ

11KV to 33KV 1000V 5KV 500MΩ



33KV to 66KV 1000V 5KV 600MΩ

66KV to 132KV 1000V 5KV 600MΩ

132KV to 220KV1000V 5KV 650MΩ

Steps for measuring the IR of Transformer:

Shut down the transformer and disconnect the jumpers and lightning arrestors.

Discharge the winding capacitance.Thoroughly clean all bushingsShort circuit the windings.

Guard the terminals to eliminate surface leakage over terminal bushings.Record the temperature.Connect the test leads (avoid joints).Apply the test voltage and note the reading. The IR. Value at 60 seconds after application of the test voltage is referred

to as the Insulation Resistance of the transformer at the test temperature.The transformer Neutral bushing is to be disconnected from earth during the test.All LV surge diverter earth connections are to be disconnected during the test.

Due to the inductive characteristics of transformers, the insulation resistance reading shall not be taken until the testcurrent stabilizes.Avoid meggering when the transformer is under vacuum.

Test Connections of Transformer for IR Test (Not Less than 200 MΩ):

Two winding transformer:

1. (HV + LV) – GND2. HV – (LV + GND)

3. LV – (HV + GND)

Three winding transformer:

1. HV – (LV + TV + GND)2. LV – (HV + TV + GND)

3. (HV + LV + TV) – GND4. TV – (HV + LV + GND)

Auto transformer (two winding):

1. (HV + LV) – GND

Auto Transformer (three winding):

1. (HV + LV) – (TV + GND)2. (HV + LV + TV) – GND3. TV – (HV + LV + GND)

For any installation, the insulation resistance measured shall not be less than:

HV – Earth 200 M ΩLV – Earth 100 M ΩHV – LV 200 M Ω



Factors affecting on IR value of Transformer

The IR value of transformers are influenced by

surface condition of the terminal bushing

quality of oilquality of winding insulationtemperature of oil

duration of application and value of test voltage

(3) IR Value for Tap Changer:

IR between HV and LV as well as windings to earth. Minimum IR value for Tap changer is 1000 ohm per volt service voltage

(4) IR Value for Electric motor:

For electric motor, we used a insulation tester to measure the resistance of motor winding with earthing ( E ).

For rated voltage below 1KV, measured with a 500VDC Megger.For rated voltage above 1KV, measured with a 1000VDC Megger.

In accordance with IEEE 43, clause 9.3, the following formula should be applied.Min IR Value (For Rotating Machine) =(Rated voltage (v) /1000) + 1

As per IEEE 43 Standard 1974,2000

IR Value in MΩ

IR (Min) = kV+1 For most windings made before about

1970, all field windings, and others notdescribed below

IR (Min) = 100 MΩ For most dc armature and ac windingsbuilt after about 1970 (form wound coils)

IR (Min) = 5 MΩ For most machines with random -wound

stator coils and form-wound coils ratedbelow 1kV

Example-1: For 11KV, Three Phase Motor.

IR Value =11+1=12 MΩ but as per IEEE43 It should be 100 MΩExample-2: For 415V,Three Phase MotorIR Value =0.415+1=1.41 MΩ but as per IEEE43 It should be 5 MΩ.

As per IS 732 Min IR Value of Motor=(20XVoltage(p-p/(1000+2XKW))



IR Value of Motor as per NETA ATS 2007. Section 7.15.1

Motor Name Plate(V)

Test Voltage Min IR Value

250V 500V DC 25 MΩ

600V 1000V DC 100MΩ

1000V 1000V DC 100MΩ

2500V 1000V DC 500MΩ

5000V 2500V DC 1000MΩ

8000V 2500V DC 2000MΩ

15000V 2500V DC 5000MΩ

25000V 5000V DC 20000MΩ

34500V 15000V DC 100000MΩ

IR Value of Submersible Motor:

IR Value of Submersible Motor

Motor Out off Well (Without Cable) IR Value

New Motor 20 MΩ

A used motor which can be reinstalled 10 MΩ

Motor Installed in Well (WithCable)

New Motor 2 MΩ

A used motor which can be reinstalled 0.5 MΩ

(5) IR Value for Electrical cable and wiring:

For insulation testing, we need to disconnect from panel or equipment and keep them isolated from power supply. Thewiring and cables need to test for each other ( phase to phase ) with a ground ( E ) cable. The Insulated Power CableEngineers Association (IPCEA) provides the formula to determine minimum insulation resistance values.

R = K x Log 10 (D/d)

R =IR Value in MΩs per 1000 feet (305 meters) of cable.K =Insulation material constant.( Varnished Cambric=2460, Thermoplastic Polyethlene=50000,Composite

Polyethylene=30000)D =Outside diameter of conductor insulation for single conductor wire and cable( D = d + 2c + 2b diameter of single conductor cable )

d – Diameter of conductorc – Thickness of conductor insulationb – Thickness of jacket insulation

HV test on new XLPE cable (As per ETSA Standard)

Application Test Voltage Min IR Value

New cables – Sheath 1KV DC 100 MΩ

New cables – Insulation 10KV DC 1000 MΩ



After repairs – Sheath 1KV DC 10 MΩ

After repairs – Insulation 5KV DC 1000MΩ

11kV and 33kV Cables between Cores and Earth (As per ETSA Standard)

Application Test Voltage Min IR Value

11KV New cables – Sheath 5KV DC 1000 MΩ

11KV After repairs – Sheath 5KV DC 100 MΩ

33KV no TF’s connected 5KV DC 1000 MΩ

33KV with TF’s connected. 5KV DC 15MΩ

IR Value Measurement (Conductors to conductor (Cross Insulation))

The first conductor for which cross insulation is being measured shall be connected to Line terminal of the megger. Theremaining conductors looped together (with the help of crocodile clips) i. e. Conductor 2 and onwards, are connectedto Earth terminal of megger. Conductors at the other end are left free.

Now rotate the handle of megger or press push button of megger. The reading of meter will show the cross Insulationbetween conductor 1 and rest of the conductors. Insulation reading shall be recorded.Now connect next conductor to Line terminal of the megger & connect the remaining conductors to earth terminal of

the megger and take measurements.

IR Value Measurement (Conductor to Earth Insulation)

Connect conductor under test to the Line terminal of the megger.Connect earth terminal of the megger to the earth.

Rotate the handle of megger or press push button of megger. The reading of meter will show the insulation resistance ofthe conductors. Insulation reading shall be recorded after applying the test voltage for about a minute till a steadyreading is obtained.

IR Value Measurements:

If during periodical testing, insulation resistance of cable is found between 5 and 1 MΩ /km at buried temperature, thesubject cable should be programmed for replacement.If insulation resistance of the cable is found between 1000 and 100 KΩ /km, at buried temperature, the subject cableshould be replaced urgently within a year.

If the insulation resistance of the cable is found less than 100 kilo ohm/km., the subject cable must be replacedimmediately on emergency basis.

(6) IR Value for Transmission / Distribution Line:

Equipment. Megger Size Min IR Value

S/S .Equipments 5 KV 5000MΩ



EHVLines. 5 KV 10MΩ

H.T. Lines. 1 KV 5MΩ

LT / Service Lines. 0.5 KV 5MΩ

(7) IR Value for Panel Bus:

IR Value for Panel = 2 x KV rating of the panel.Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 MΩ.

(8) IR Value for Substation Equipment:

Generally meggering Values of Substation Equipments are.

.Typical IR Value of S/S Equipments

Equipment Megger Size IR Value(Min)

Circuit Breaker

(Phase-Earth) 5KV,10 KV 1000 MΩ

(Phase-Phase) 5KV,10 KV 1000 MΩ

Control Circuit 0.5KV 50 MΩ

CT/PT

(Pri-Earth) 5KV,10 KV 1000 MΩ

(Sec-Phase) 5KV,10 KV 50 MΩ


Isolator

(Phase-Earth) 5KV,10 KV 1000 MΩ

(Phase-Phase) 5KV,10 KV 1000 MΩ


L.A (Phase-Earth) 5KV,10 KV 1000 MΩ

Electrical Motor (Phase-Earth) 0.5KV 50 MΩ

LT Switchgear (Phase-Earth) 0.5KV 100 MΩ



LT Transformer (Phase-Earth) 0.5KV 100 MΩ

IR Value of S/S Equipments As per DEP Standard

Equipment MeggeringIR Value at

CommissioningTime (MΩ)

IR Value atMaintenanceTime(MΩ)

Switchgear

HV Bus 200 MΩ 100 MΩ

LV Bus 20 MΩ 10 MΩ

LV wiring 5 MΩ 0.5 MΩ

Cable(min 100 Meter) HV & LV (10XKV) / KM (KV) / KM

Motor & Generator Phase-Earth 10(KV+1) 2(KV+1)

Transformer Oilimmersed

HV & LV 75 MΩ 30 MΩ

Transformer Dry Type

HV 100 MΩ 25 MΩ

LV 10 MΩ 2 MΩ

FixedEquipments/Tools

Phase-Earth 5KΩ / Volt 1KΩ / Volt

Movable Equipments Phase-Earth 5 MΩ 1MΩ

Distribution Equipments Phase-Earth 5 MΩ 1MΩ

Circuit BreakerMain Circuit 2 MΩ / KV

Control Circuit 5MΩ

D.C Circuit-Earth 40MΩ



Relay

LT Circuit-Earth 50MΩ

LT-D.C Circuit 40MΩ

LT-LT 70MΩ

(9) IR Value for Domestic /Industrial Wiring:

A low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakagecurrent. This cause deterioration of the insulation, as well as involving a waste of energy which would increase the

running costs of the installation.The resistance between Phase-Phase-Neutral-Earth must never be less than 0.5 M Ohms for the usual supplyvoltages.In addition to the leakage current due to insulation resistance, there is a further current leakage in the reactance of theinsulation, because it acts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but wewish to measure the resistance of the insulation, so DC Voltage is used to prevent reactance from being included

in the measurement.

1 Phase Wiring:

The IR test between Phase-Natural to earth must be carried out on the complete installation with the main switch off,with phase and neutral connected together, with lamps and other equipment disconnected, but with fuses in, circuitbreakers closed and all circuit switches closed.Where two-way switching is wired, only one of the two stripper wires will be tested. To test the other, both two-wayswitches should be operated and the system retested. If desired, the installation can be tested as a whole, when a value

of at least 0.5 M Ohms should be achieved.

3 Phase Wiring:

In the case of a very large installation where there are many earth paths in parallel, the reading would be expected to belower. If this happens, the installation should be subdivided and retested, when each part must meet the minimum

requirement.



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The IR tests must be carried out between Phase-Phase-Neutral-Earth with a minimum acceptable value for each test of0.5 M Ohms.

IR Testing for Low voltage

circuit voltage Test voltage IR Value(Min)

Extra Low Voltage 250V DC 0.25MΩ

Up to 500 V except for above 500 V DC 0.5MΩ

500 V To 1KV 1000 V DC 1.0MΩ

Min IR Value = 50 MΩ / No of Electrical outlet. (All Electrical Points with fitting & Plugs).

Min IR Value = 100 MΩ / No of Electrical outlet. (All Electrical Points without fitting & Plugs).

Required Precautions:

Electronic equipment like electronic fluorescent starter switches, touch switches, dimmer switches, power controllers,delay timers could be damaged by the application of the high test voltage should be disconnected.Capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will result.Where any equipment is disconnected for testing purposes, it must be subjected to its own insulation test, using avoltage which is not likely to result in damage. The result must conform with that specified in the British Standardconcerned, or be at least 0.5 M Ohms if there is no Standard.


Star-Delta Starter


8 Votes

Introduction:


Likell



Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbanceof voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors arestarted at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. Twomethods are used for reduction of starting voltage are star delta starting and auto transformer stating.

Working Principal of Star-Delta Starter:

This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physicallyreconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected instar configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three.After a period of time the winding are reconfigured as delta and the motor runs normally.

Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the

start current applied to the motor during start as a means of reducing the disturbances and interference on the electricalsupply.Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greaterthan 5HP (4KW). The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical reduced voltagestarters that can be applied.The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are

smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. Thecurrents through the winding are 1/root 3 (58%) of the current in the line.There are two contactors that are close during run, often referred to as the main contractor and the delta contactor.These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that onlycarries star current while the motor is connected in star. The current in star is one third of the current in delta, so thiscontactor can be AC3 rated at one third (33%) of the motor rating.

Star-delta Starter Consists following units:

1) Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).

2) Time relay (pull-in delayed) 1 No.



3) Three-pole thermal over current release 1No.

4) Fuse elements or automatic cut-outs for the main circuit 3 Nos.

5) Fuse element or automatic cut-out for the control circuit 1No.

Power Circuit of Star Delta Starter:

The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit.The main contactor connects the reference source voltage R, Y, B to the primary terminal of the motor U1, V1, W1.In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed initially, and then after a period oftime, the star contactor is opened, and then the delta contactor (KM2) is closed. The control of the contactors is by thetimer (K1T) built into the starter. The Star and Delta are electrically interlocked and preferably mechanically interlockedas well. In effect, there are four states:

The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequenceduring the initial run of the motor from standstill. This provides one third of DOL current to the motor, thus reducing thehigh inrush current inherent with large capacity motors at startup.Controlling the interchanging star connection and delta connection of an AC induction motor is achieved by means of astar delta or wye delta control circuit. The control circuit consists of push button switches, auxiliary contacts and a

timer.

Control Circuit of Star-Delta Starter (Open Transition):



The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil(KT) circuit.When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.When Star Auxiliary Contactor (1)( which is placed on Main Contactor coil circuit )became NO to NC it’s complete

The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s Main andAuxiliary Contactor Change its Position from NO To NC. This sequence happens in a friction of time.After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected inparallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coilactivated which eventually maintains the control circuit active even after releasing the ON push button switch.When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until TimeDelay Auxiliary contact KT (3) become NC to NO.

Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will changeits position from NC to NO and at the Same Time Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Positionfrom NO To NC so Delta coil energized and Delta Main Contactor becomes NO To NC. Now Motor terminalconnection change from star to delta connection.A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star anddelta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star

and delta contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactorcoil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be activewhile the delta contactor coil is active.The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switchbreak the control circuit and the motor when necessary. The thermal overload contact is a protective device whichautomatically opens the STOP Control circuit in case when motor overload current is detected by the thermal overload

relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor isdetected by the thermal overload relay.At some point during starting it is necessary to change from a star connected winding to a delta connected winding.Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.

What is Open or Closed Transition Starting

(1) Open Transition Starters.



Discuss mention above is called open transition switching because there is an open state between the star state and thedelta state.In open transition the power is disconnected from the motor while the winding are reconfigured via external switching.When a motor is driven by the supply, either at full speed or at part speed, there is a rotating magnetic field in the stator.This field is rotating at line frequency. The flux from the stator field induces a current in the rotor and this in turn results

in a rotor magnetic field.When the motor is disconnected from the supply (open transition) there is a spinning rotor within the stator and therotor has a magnetic field. Due to the low impedance of the rotor circuit, the time constant is quite long and the action ofthe spinning rotor field within the stator is that of a generator which generates voltage at a frequency determined by thespeed of the rotor. When the motor is reconnected to the supply, it is reclosing onto an unsynchronized generator andthis result in a very high current and torque transient. The magnitude of the transient is dependent on the

phase relationship between the generated voltage and the line voltage at the point of closure can be muchhigher than DOL current and torque and can result in electrical and mechanical damage.Open transition starting is the easiest to implement in terms or cost and circuitry and if the timing of the changeover isgood, this method can work well. In practice though it is difficult to set the necessary timing to operate correctly anddisconnection/reconnection of the supply can cause significant voltage/current transients.In Open transition there are Four states:

1. OFF State: All Contactors are open.

2. Star State: The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. Themotor is connected in star and will produce one third of DOL torque at one third of DOL current.

3. Open State: This type of operation is called open transition switching because there is an open state between the starstate and the delta state. The Main contractor is closed and the Delta and Star contactors are open. There is voltage onone end of the motor windings, but the other end is open so no current can flow. The motor has a spinning rotor andbehaves like a generator.

4. Delta State: The Main and the Delta contactors are closed. The Star contactor is open. The motor is connected to fullline voltage and full power and torque are available

(2) Closed Transition Star/Delta Starter.

There is a technique to reduce the magnitude of the switching transients. This requires the use of a fourth contactor anda set of three resistors. The resistors must be sized such that considerable current is able to flow in the motor windingswhile they are in circuit.The auxiliary contactor and resistors are connected across the delta contactor. In operation, just before the star

contactor opens, the auxiliary contactor closes resulting in current flow via the resistors into the star connection. Oncethe star contactor opens, current is able to flow round through the motor windings to the supply via the resistors. Theseresistors are then shorted by the delta contactor. If the resistance of the resistors is too high, they will not swamp thevoltage generated by the motor and will serve no purpose.In closed transition the power is maintained to the motor at all time. This is achieved by introducing resistors totake up the current flow during the winding changeover. A fourth contractor is required to place the resistor in circuit

before opening the star contactor and then removing the resistors once the delta contactor is closed. These resistorsneed to be sized to carry the motor current. In addition to requiring more switching devices, the control circuit is morecomplicated due to the need to carry out resistor switchingIn Close transition there are Four states:

1. OFF State. All Contactors are open2. Star State. The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The

motor is connected in star and will produce one third of DOL torque at one third of DOL current.

3. Star Transition State. The motor is connected in star and the resistors are connected across the delta contactor viathe aux [KM4] contactor.

4. Closed Transition State. The Main [KM3] contactor is closed and the Delta [KM2] and Star [KM1] contactors are



open. Current flows through the motor windings and the transition resistors via KM4.

5. Delta State. The Main and the Delta contactors are closed. The transition resistors are shorted out. The Starcontactor is open. The motor is connected to full line voltage and full power and torque are available.

Effect of Transient in Starter (Open Transient starter)

It is Important the pause between star contactor switch off and Delta contactor switch is on correct. This is because

Star contactor must be reliably disconnected before Delta contactor is activated. It is also important that the switchover pause is not too long.For 415v Star Connection voltage is effectively reduced to 58% or 240v. The equivalent of 33% that is obtained withDirect Online (DOL) starting.If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected inDelta mode.

When connected to Delta configuration the phase voltage increases by a ratio of V3 or 173%. The phase currentsincrease by the same ratio. The line current increases three times its value in star connection.During transition period of switchover the motor must be free running with little deceleration. While this is happening“Coasting” it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to orsubtract from the applied line voltage. This is known as transient current. Only lasting a few milliseconds it causesvoltage surges and spikes. Known as a changeover transient.

Size of each part of Star-Delta starter

(1) Size of Over Load Relay:

For a star-delta starter there is a possibility to place the overload protection in two positions, in the line or in thewindings.Overload Relay in Line:

In the line is the same as just putting the overload before the motor as with a DOL starter.The rating of Overload (In Line) = FLC of Motor.Disadvantage: If the overload is set to FLC, then it is not protecting the motor while it is in delta (setting is x1.732 toohigh).Overload Relay in Winding:In the windings means that the overload is placed after the point where the wiring to the contactors are split into main

and delta. The overload then always measures the current inside the windings.The setting of Overload Relay (In Winding) =0.58 X FLC (line current).Disadvantage: We must use separate short circuit and overload protections.

(2) Size of Main and Delta Contractor:

There are two contactors that are close during run, often referred to as the main contractor and the delta contactor.These are AC3 rated at 58% of the current rating of the motor.Size of Main Contactor= IFL x 0.58

(3) Size of Star Contractor:

The third contactor is the star contactor and that only carries star current while the motor is connected in star. Thecurrent in star is 1/ √3= (58%) of the current in delta, so this contactor can be AC3 rated at one third (33%) of themotor rating.Size of Star Contactor= IFL x 0.33

Motor Starting Characteristics of Star-Delta Starter:



Available starting current: 33% Full Load Current.Peak starting current: 1.3 to 2.6 Full Load Current.Peak starting torque: 33% Full Load Torque.

Advantages of Star-Delta starter:

The operation of the star-delta method is simple and ruggedIt is relatively cheap compared to other reduced voltage methods.Good Torque/Current Performance.It draws 2 times starting current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter:

Low Starting Torque (Torque = (Square of Voltage) is also reduce).Break In Supply – Possible TransientsSix Terminal Motor Required (Delta Connected).

It requires 2 set of cables from starter to motor.It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at thetime of starting than very heavy transients and stresses are produced while changing from star to delta connections, andbecause of these transients and stresses many electrical and mechanical break-down occurs.In this method of starting initially motor is connected in star and then after change over the motor is connected in delta.The delta of motor is formed in starter and not on motor terminals.

High transmission and current peaks: When starting up pumps and fans for example, the load torque is low at thebeginning of the start and increases with the square of the speed. When reaching approx. 80-85 % of the motor ratedspeed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch overto delta position is necessary, and this will very often result in high transmission and current peaks. In some cases thecurrent peak can reach a value that is even bigger than for a D.O.L start.Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta

starter.Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from themotor are configured in a star or delta electrical connection. The initial connection should be in the star pattern thatresults in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 ofthe current at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque .The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as

low as 50% of nominal speed which make transient Sparks.

Features of star-delta starting

For low- to high-power three-phase motors.Reduced starting current

Six connection cablesReduced starting torqueCurrent peak on changeover from star to deltaMechanical load on changeover from star to delta

Application of Star-Delta Starter:

The star-delta method is usually only applied to low to medium voltage and light starting Torque motors.The received starting current is about 30 % of the starting current during direct on line start and the starting torque isreduced to about 25 % of the torque available at a D.O.L start. This starting method only works when the application



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is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate themotor up to speed before switching over to the delta position.


Direct On Line Starter


4 Votes

Introduction:

Different starting methods are employed for starting induction motors because Induction Motor draws more starting

current during starting. To prevent damage to the windings due to the high starting current flow, we employ differenttypes of starters.The simplest form of motor starter for the induction motor is the Direct On Line starter. The DOL starter consist aMCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can beopened by the thermal overload relay under fault conditions.Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor

is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor isoperating.

Principle of DOL:

To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush

current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked RotorCurrent of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed,typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and theterminal voltage, and is totally independent of the motor load.The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high

starting current, but not the magnitude of the starting current.Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor willreach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the startcycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motormust be replaced with a motor which can develop a higher starting torque.The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motoraccelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the

acceleration torque and the load inertia.DOL starting have a maximum start current and maximum start torque. This may cause an electrical problemwith the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users ofthe supply line, always experience a voltage drop when starting a motor. But if this motor is not a high power one itdoes not affect much.


Likell



Parts of DOL Starters:

(1) Contactors & Coil.

Magnetic contactors are electromagnetically operated switches that provide a safe and convenient means for connecting

and interrupting branch circuits.Magnetic motor controllers use electromagnetic energy for closing switches. The electromagnet consists of a coil ofwire placed on an iron core. When a current flow through the coil, the iron of the magnet becomes magnetized,attracting an iron bar called the armature. An interruption of the current flow through the coil of wire causes thearmature to drop out due to the presence of an air gap in the magnetic circuit.

Line-voltage magnetic motor starters are electromechanical devices that provide a safe, convenient, and economicalmeans of starting and stopping motors, and have the advantage of being controlled remotely. The great bulk of motor

controllers sold are of this type.Contactors are mainly used to control machinery which uses electric motors. It consists of a coil which connects to avoltage source. Very often for Single phase Motors, 230V coils are used and for three phase motors, 415V coils areused. The contactor has three main NO contacts and lesser power rated contacts named as Auxiliary Contacts [NOand NC] used for the control circuit. A contact is conducting metal parts which completes or interrupt an electricalcircuit.NO-normally open

NC-normally closed

(2) Over Load Relay (Overload protection).

Overload protection for an electric motor is necessary to prevent burnout and to ensure maximum operating life.Under any condition of overload, a motor draws excessive current that causes overheating. Since motor windinginsulation deteriorates due to overheating, there are established limits on motor operating temperatures to protect amotor from overheating. Overload relays are employed on a motor control to limit the amount of current drawn.The overload relay does not provide short circuit protection. This is the function of over current protective

equipment like fuses and circuit breakers, generally located in the disconnecting switch enclosure.The ideal and easiest way for overload protection for a motor is an element with current-sensing properties very similarto the heating curve of the motor which would act to open the motor circuit when full-load current is exceeded. Theoperation of the protective device should be such that the motor is allowed to carry harmless over-loads but is quicklyremoved from the line when an overload has persisted too long.Normally fuses are not designed to provide overload protection. Fuse is protecting against short circuits (over current

protection). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishingbetween this temporary and harmless inrush current and a damaging overload. Selection of Fuse is depend on motorfull-load current, would “blow” every time the motor is started. On the other hand, if a fuse were chosen large enoughto pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occurlater.



The overload relay is the heart of motor protection. It has inverse-trip-time characteristics, permitting it to hold in duringthe accelerating period (when inrush current is drawn), yet providing protection on small overloads above the full-loadcurrent when the motor is running. Overload relays are renewable and can withstand repeated trip and reset cycleswithout need of replacement. Overload relays cannot, however, take the place of over current protection equipment.

The overload relay consists of a current-sensing unit connected in the line to the motor, plus a mechanism, actuated bythe sensing unit, which serves, directly or indirectly, to break the circuit.Overload relays can be classified as being thermal, magnetic, or electronic.

1. Thermal Relay: As the name implies, thermal overload relays rely on the rising temperatures caused by the overload

current to trip the overload mechanism. Thermal overload relays can be further subdivided into two types: melting alloyand bimetallic.

2. Magnetic Relay: Magnetic overload relays react only to current excesses and are not affected by temperature.3. Electronic Relay: Electronic or solid-state overload relays, provide the combination of high-speed trip, adjustability,

and ease of installation. They can be ideal in many precise applications.

Wiring of DOL Starter:

(1) Main Contact:

Contactor is connecting among Supply Voltage, Relay Coil and Thermal Overload Relay.L1 of Contactor Connect (NO) to R Phase through MCCBL2 of Contactor Connect (NO) to Y Phase through MCCBL3 of Contactor Connect (NO) to B Phase through MCCB.

NO Contact (-||-):(13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is connected to Common wireof Start/Stop Button.NC Contact (-|/|-):(95-96) is a normally closed NC contact (opens when the thermal overloads trip if associated with the overload block)

(2) Relay Coil Connection:

A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermal over Load Relay’s NCConnection (95).

(3) Thermal Overload Relay Connection:

T1,T2,T3 are connect to Thermal Overload RelayOverload Relay is Connecting between Main Contactor and MotorNC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and Common Connection ofStart/Stop Button.



Wiring Diagram of DOL Starter:

Working of DOL Starter:

The main heart of DOL starter is Relay Coil. Normally it gets one phase constant from incoming supply Voltage(A1).when Coil gets second Phase relay coil energizes and Magnet of Contactor produce electromagnetic field and dueto this Plunger of Contactor will move and Main Contactor of starter will closed and Auxiliary will change its positionNO become NC and NC become (shown Red Line in Diagram) .Pushing Start Button:When We Push the start Button Relay Coil will get second phase from Supply Phase-Main contactor(5)-Auxiliary

Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2).Now Coil energizes and Magnetic field produce byMagnet and Plunger of Contactor move. Main Contactor closes and Motor gets supply at the same time Auxiliarycontact become (53-54) from NO to NC .Release Start Button:Relay coil gets supply even though we release Start button. When We release Start Push Button Relay Coil gets Supplyphase from Main contactor (5)-Auxiliary contactor (53) – Auxiliary contactor (54)-Stop Button-96-95-Relay coil

(shown Red / Blue Lines in Diagram).In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95.Pushing Stop Button:When we push Stop Button Control circuit of Starter will be break at stop button and Supply of Relay coil is broken,Plunger moves and close contact of Main Contactor becomes Open, Supply of Motor is disconnected.



Motor Starting Characteristics on DOL Starter:

Available starting current: 100%.Peak starting current: 6 to 8 Full Load Current.Peak starting torque: 100%

Advantages of DOL Starter:

1. Most Economical and Cheapest Starter2. Simple to establish, operate and maintain3. Simple Control Circuitry4. Easy to understand and trouble‐shoot.5. It provides 100% torque at the time of starting.6. Only one set of cable is required from starter to motor.

7. Motor is connected in delta at motor terminals.

Disadvantages of DOL Starter:

1. It does not reduce the starting current of the motor.2. High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).

3. Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.4. Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current affecting other

customers connected to the same lines and therefore not suitable for higher size squirrel cage motors5. High starting Torque: Unnecessary high starting torque, even when not required by the load, thereby increased

mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connectedequipments, etc. leading to premature failure and plant downtimes.

Features of DOL starting

For low- and medium-power three-phase motorsThree connection lines (circuit layout: star or delta)High starting torque

Very high mechanical loadHigh current peaksVoltage dipsSimple switching devices



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DOL is Suitable for:

A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in thesupply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility forthis reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10kW.DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts.

DOL is not suitable for:

The peak starting current would result in a serious voltage drop on the supply systemThe equipment being driven cannot tolerate the effects of very high peak torque loadingsThe safety or comfort of those using the equipment may be compromised by sudden starting as, for example, withescalators and lifts.


Effects of High Voltage Transmission Lines on Humans and Plants

February 17, 2012 7 Comments

7 Votes

Introduction:

By increasing population of the world, towns are expanding, many buildings construct near high voltage overhead powertransmission lines. The increase of power demand has increased the need for transmitting huge amount of power over longdistances. Large transmission lines configurations with high voltage and current levels generate large values of electric andmagnetic fields stresses which affect the human being and the nearby objects located at ground surfaces. This needs to beinvestigating the effects of electromagnetic fields near the transmission lines on human health.

The electricity system produces extremely low frequency electromagnetic field which comes under Non ionizing radiations

which can cause health effects. Apart from human effect, the electrostatic coupling & electromagnetic interference of highvoltage transmission lines have impact on plants and telecommunication equipments mainly operating in frequency range belowUHF.

IS Power Line EMF safe? This is the controversy Discussion directly eludes on Government Regulation policy and PowerCompany. There are lots of supporting documents and research paper in favor and criticize this arguments.


Likell



What is The Electric and Magnetic fields:

Electric and magnetic fields, often referred to as electromagnetic fields or EMF, occur naturally and as a result of thePower generation, Power Transmission, Power distribution and use of electric power.

EMF is fields of force and is created by electric voltage and current. They occur around electrical devices or wheneverpower lines are energized.Electric fields are due to voltage so they are present in electrical appliances and cords whenever the electric cord toan appliance is plugged into an outlet (even if the appliance is turned off).Electric fields (E) exist whenever a (+) or (-) electrical charge is present. They exert forces on other charges within thefield. Any electrical wire that is charged will produce an electric field (i.e. Electric field produces charging of bodies,

discharge currents, biological effects and sparks). This field exists even when there is no current flowing. The higher thevoltage, the stronger is electric field at any given distance from the wire.The strength of the electric field is typically measured in volts per meter (V/m) or in kilovolts per meter (kV/m). Electricfields are weakened by objects like trees, buildings, and vehicles. Burying power lines can eliminate human exposure toelectric fields from this source.Magnetic fields result from the motion of the electric charge or current, such as when there is current flowing througha power line or when an appliance is plugged in and turned on. Appliances which are plugged in but not turned on do

not produce magnetic fields.Magnetic field lines run in circles around the conductor (i.e. produces magnetic induction on objects and inducedcurrents inside human and animal (or any other conducting) bodies causing possible health effects and a multitude ofinterference problems). The higher the current, the greater the strength of the magnetic field.Magnetic fields are typically measured in tesla (T) or more commonly, in gauss (G) and milli gauss (mG). One teslaequals 10,000 gauss and one gauss equals 1,000 milli gauss.

The strength of an EMF decreases significantly with increasing distance from the source.The Strength of an electric field is proportional to the voltage of the source. Thus, the electric fields beneath high voltagetransmission lines far exceed those below the lower voltage distribution lines. The magnetic field strength, by contrast, isproportional to the current in the lines, so that a low voltage distribution line with a high current load may produce amagnetic field that is as high as those produced by some high voltage transmission lines.In fact, electric distribution systems account for a far higher proportion of the population’s exposure to magnetic fields

than the larger and more visible high voltage transmission lines.Electrical field: the part of the EMF that can easily be shielded.Magnetic field: part of the EMF that can penetrate stone, steel and human flesh. In fact, when it comes to magneticfields, human flesh and bone has the same penetrability as air!Both fields are invisible and perfectly silent: People who live in an area with electric power, some level of artificialEMF is surrounding them.

The magnetic field strength produced from a transmission line is proportional to: load current, phase to phase spacing,and the inverse square of the distance from the line.Many previous works studied the effect of different parameters on the produced magnetic field such as: the distancefrom the line, the conductor height, line shielding and transmission line configuration and compaction.

Electric and Magnetic Field (EMF) Effects

Extremely high voltages in EHV lines cause electrostatic effects, where as short circuit currents & line loading currentsare responsible for electromagnetic effects. The effect of these electrostatic fields is seen prominent with living things likehumans, plants, animals along with vehicles, fences & buried pipes under & close to these lines.

1) EMF Effects Human beings:

The human body is a composed of some biological materials like blood, bone, brain, lungs, muscle, skin etc. Thepermeability of human body is equals to permeability of air but within a human body has different electromagnetic values



at a certain frequency for different material.The human body contains free electric charges (largely in ion-rich fluids such as blood and lymph) that move inresponse to forces exerted by charges on and currents flowing in nearby power lines. The processes that produce thesebody currents are called electric and magnetic induction.In electric induction, charges on a power line attract or repel free charges within the body. Since body fluids are goodconductors of electricity, charges in the body move to its surface under the influence of this electric force. For example,

a positively charged overhead transmission line induces negative charges to flow to the surfaces on the upper part of thebody. Since the charge on power lines alternates from positive to negative many times each second, the chargesinduced on the body surface alternate also. Negative charges induced on the upper part of the body one instant flowinto the lower part of the body the next instant. Thus, power-frequency electric fields induce currents in the body(Eddy Current) as well as charges on its surface.

The currents induced in the body by magnetic fields are greatestnear the periphery of the body and smallest at the center of the body.

It is believed that, the magnetic field might induce a voltage in the tissue of human body which causes a current to flowthrough it due to its conductivity of around them.The magnetic field has influence on tissues in the human body. These influences may be beneficial or harmful dependingupon its nature.The magnitude of surface charge and internal body currents that are induced by any given source of power-frequencyfields depends on many factors. These include the magnitude of the charges and currents in the source, the distance of

the body from the source, the presence of other objects that might shield or concentrate the field, and body posture,shape, and orientation. For this reason the surface charges and currents which a given field induces are verydifferent for different Human and animals.When a person who is isolated from ground by some insulating material comes in close proximity to an overheadtransmission line, an electrostatic field is set in the body of human being, having a resistance of about 2000 ohms.When the same person touches a grounded object, it will discharge through his body causing a large amount of

discharge current to flow through the body. Discharge currents from 50-60 Hz electromagnetic fields are weaker thannatural currents in the body, such as those from the electrical activity of the brain and heart.For human beings the limit for undisturbed field is 15 kV/m, R.M.S., to experience possible shock. When designing atransmission lines this limit is not crossed, in addition to this proper care has been taken in order to keep minimumclearance between transmission lines.According to research and publications put out by the World Health Organization(WHO), EMF such as those frompower lines, can also cause:

Short term Health Problem

1. Headaches.2. Fatigue



3. Anxiety4. Insomnia

5. Prickling and/or burning skin6. Rashes7. Muscle pain

Long term Health Problem:

Following serious health Problems may be arise due to EMF effects on human Body.

(1) Risk of damaging DNA.

Our body acts like an energy wave broadcaster and receiver, incorporating and responding to EMFs. In fact, scientificresearch has demonstrated that every cell in your body may have its own EMF, helping to regulate important functionsand keep you healthy.Strong, artificial EMFs like those from power lines can scramble and interfere with your body’s natural EMF, harming

everything from your sleep cycles and stress levels to your immune response and DNA!

(2) Risk of Cancer

After hundreds of international studies, the evidence linking EMFs to cancers and other health problems is loud andclear. High Voltage power lines are the most obvious and dangerous culprits, but the same EMFs exist in graduallydecreasing levels all along the grid, from substations to transformers to homes.

(3) Risk of Leukemia:

Researchers found that children living within 650 feet of power lines had a 70% greater risk for leukemia than childrenliving 2,000 feet away or more.(As per British Medical Journal, June, 2005).

(4) Risk of Neurodegenerative disease:

“Several studies have identified occupational exposure to extremely low-frequency electromagnetic fields (EMF) as apotential risk factor for neuro degenerative disease.”(As per Epidemiology, 2003 Jul; 14(4):413-9).

(5) Risk of Miscarriage:

There is “strong prospective evidence that prenatal maximum magnetic field exposure above a certain level (possiblyaround 16 mG) may be associated with miscarriage risk.” (As per Epidemiology, 2002 Jan; 13(1):9-20)

2) EMF Effects on Animals

Many researchers are studying the effect of Electrostatic field on animals. In order to do so they keeps the cages ofanimals under high Electrostatic field of about 30 kV/m. The results of these Experiments are shocking as animals (arekept below high Electrostatic field their body acquires a charge & when they try to drink water, a spark usually jumpsfrom their nose to the grounded Pipe) like hens are unable to pick up grain because of chattering of their beaks whichalso affects their growth.

3) EMF Effects on Plant Life

Most of the areas in agricultural and forest lands where high power transmission lines pass. The voltage level of highpower transmission Lines are 400KV, 230KV, 110KV, 66KV etc. The electromagnetic field from high powertransmission lines affects the growth of plants.



Gradually increases or decreases and reaches to maximum current or minimum current and thereafter it starts to falldown to lowest current or raises to maximum current or a constant current. Again the current, it evinces with littlefluctuations till the next day morning.Current in Power transmission lines varies according to Load (it depending upon the amount of electricity consumed bythe consumers). Hence the effect of EMF (due to current flowing in the power lines) upon the growth of plants under

the high power transmission lines remains unaltered throughout the year.From various practically study it was found that the response of the crop to EMF from 110 KV and 230 KV Powerlines showed variations among themselves. Based on the results the growth characteristics like shoot length, root length,leaf area, leaf fresh weight, specific leaf weight, shoot/root ratio, total biomass content and total water content of thefour crop plants were reduced significantly over the control plants.Similar trend were observed in the biochemical characteristics like chlorophyll.

Reduced growth and physiological parameter was primarily due to the effect of reduced cell division and cellenlargement. Further the growth was stunted which may be due to poor action of hormones responsible for cell divisionand cell enlargement.The bio-chemical changes produced in this plant due to EMF stress quite obvious and it affects the production leadingto economic loss.It is concluded that the reduced growth parameter shown in the crop plants would indicates that the EMF has exerted a

stress on that plants and this EMF stress was quite obvious and it affects the production leading to economic loss. Sofurther research activities are needed to safe guard plants from EMF stress.

4) EMF Effects on Vehicles parked near Line

When a vehicle is parked under high voltage transmission line an electrostatic field is developed in it. When a person

who is grounded touches it a discharge current flows through the human being. In order to avoid this parking lots arelocated below the transmission lines the recommended clearance is 17 m for 345 kV and 20 m for 400 kV lines.

5) EMF Effects on Pipe Line/Fence/Cables:

A fence, irrigation pipe, pipeline, electrical distribution line forms a conducting loops when it is grounded at both ends.

The earth forms the other portion of the loop. The magnetic field from a transmission line can induce a current to flow insuch a loop if it is oriented parallel to the line. If only one end of the fence is grounded, then an induced voltage appearsacross the open end of the loop. The possibility for a shock exists if a person closes the loop at the open end bycontacting both the ground and the conductor.For fences, buried cables, and pipe lines proper care has been taken to prevent them from charging due to Electrostaticfield. When using pipelines which are more than 3 km in length & 15 cm in Diameter they must be buried at least 30

laterally from the line center.

6) EMF Effects on Maintenance Worker:

For providing continuous and uninterrupted supply of electric power to consumers maintenance operations of powerlines are often performed with systems energized or live.

This is live line maintenance or hot line maintenance. The electric fields and magnetic fields associated with these powerlines may affect the health of live line workers. Its electric field and current densities affect the health of humans andcause several diseases by affecting majority parts of the human body. These electric field and current densities affectshumans of all stages and causes short term diseases in them and sometimes death also.

Contradiction of EMF Effect on Human Health:

There are two reasons why electromagnetic fields associated with power systems could pose no threat to human health.First, The EMF from power lines and appliances are of extremely low frequency and low energy. They are non-ionizing



and are markedly different in frequency from ionizing radiation such as X-rays and gamma rays. As a comparison,

transmission lines have a low frequency of 60Hz while television transmitters have higher frequencies in the 55 to 890MHZ range. Microwaves have even higher frequencies, 1,000 MHZ and above. Ionizing radiation, such as X-rays andgamma rays, has frequencies above 1015 Hz. The energy from higher-frequency fields is absorbed more readily bybiological material. Microwaves can be absorbed by water in body tissues and cause heating which can be harmful,depending upon the degree of heating that occurs. X-rays have so much energy that they can ionize (form chargedparticles) and break up molecules of genetic material (DNA) and no genetic material, leading to cell death or mutation.

In contrast, extremely low frequency EMF does not have enough energy to heat body tissues or cause ionization.Second, all cells in the body maintain large natural electric fields across their outer membranes. These naturallyoccurring fields are at least 100 times more intense than those that can be induced by exposure to common power-frequency fields. However, despite the low energy of power-frequency fields and the very small perturbations that theymake to the natural fields within the body.When an external agent such as an ELF fields lightly perturbs a process in the cell, other processes may compensate forit so that there is no overall disturbance to the organism. Some perturbations may be within the ranges of disturbances

that a system can experience and still function properly.During Research on health effects of electric and magnetic fields, it has come forward that electric field intensityexposure of about 1-10 mv/m in tissue interact with cells but not proved to be harmful. But strong fields cause harmfuleffects when their magnitude exceeds stimulation thresholds for neural tissues (central nervous system and brain),muscle and heart

Surface CurrentDensity(mA/m2)

Health Effect

<1 Absence of any established effects.

1 To 10 Minor biological effects.

10 To 100 Well established effects(a) Visual effect.

(b) Possible nervous system effect

100 To 1000 Changes in central nervous System

>1000 Ventricular Fibrillation (Heart Condition 0. Healthhazards.

In India it is stipulated that electric field intensity should not exceed 4.16 kV/m and magnetic field intensity should notexceed 100μT in public areas.Even when effect is demonstrated consistently on the cellular level in laboratory experiments, it is hard to predictwhether and how they will affect the whole organism. Processes at the individual cell level are integrated throughcomplex mechanisms in the animal.

Mitigation of EMF Effect of Transmission Line:

1) Line shielding:

There are two basic 60-Hz magnetic field mitigation (reduction) methods: passive and active.Passive magnetic field mitigation includes rigid magnetic shielding with ferromagnetic and highly conductive materials,and the use of passive shield wires installed near transmission lines that generate opposing cancellation fields from

electromagnetic induction.Active magnetic field mitigation uses electronic feedback to sense a varying 60-Hz magnetic field, then generates aproportionally opposing (nulling) cancellation field within a defined area (room or building) surrounded by cancellationcoils. Ideally, when the two opposing 180-degree out- of-phase magnetic fields of equal magnitude intersect, the



resultant magnetic field is completely cancelled (nullified). This technology has been successfully applied in bothresidential and commercial environments to mitigate magnetic fields from overhead transmission and distribution lines,and underground residential distribution (URD) lines.

2) Line Configuration and Compaction

Line compaction means that, bringing the conductors close together keeping the minimum (safe) phase-to-phase

spacing constant. Keeping all the parameters the same and the only variable is the phase-to- phase spacing. Themagnetic field is proportional to the dimensions of the phase-to-phase spacing.Other studies showed that, increasing the distance between phases by increasing the height of the central phaseconductor above the level of the other phase conductors leads to the reduction of the peak value of the magnetic field.Reducing the phase-to-phase distance, leads to the decrease of the magnetic field. This reduction between phases islimited by the electrical insulation level between phases.

(A) For single circuit lines, compaction causes a great reduction to the maximum magnetic field values. This reduction ofmagnetic field allows for lower conductor heights above the ground. This leads to transmit the same power on shortertowers. This gives a great reduction of the tower cost.(B) For double circuit lines, some studies showed that, the use of optimum phase arrangement causes a drasticreduction to the maximum magnetic field values for both conventional and compact lines i.e. with vertical conductor

3) Grounding:

Induced currents are always present in electric fields under transmission lines and will be present. However, there must

be a policy to ground metal objects, such as fences, that are located on the right-of-way. The grounding eliminatesthese objects as sources of induced current and voltage shocks. Multiple grounding points are used to provideredundant paths for induced current flow and mitigate nuisance shocks.

4) Providing Right of Way(R.O.W):

Overhead transmission systems required strips of land to be designed as right-of-ways (R.O.W.). These strips of landare usually evaluated to decrease the effects of the energized line including magnetic and electric field effects.

5) Maintaining Proper Clearance:

Unlike fences or buildings, mobile objects such as vehicles and farm machinery cannot be grounded permanently.

Limiting the possibility of induced currents from such objects to persons is accomplished by maintaining properclearances for above-ground conductors tend to limit field strengths to levels that do not represent a hazard or nuisance. Limiting access area by increasing conductor clearances in areas where large vehicles could be present.

Conclusion:

Based on the review and analysis and other research projects it is of the opinion that there is no conclusive and convincingevidence that exposure to extremely low frequency EMF emanated from nearby high voltage Transmission lines is causallyassociated with an increased incidence of cancer or other detrimental health effects in humans. Even if it is assumed that thereis an increased risk of cancer as implied in some epidemiological studies, the empirical relative risk appears to be fairly small inmagnitude and the observed association appears to be tenuous. Although the possibility is still remain about the verse effect onhealth by EMF.

References:

SSGBCOE&T, Electronics and Communication Engineering-Girish Kulkarni1, Dr.W.Z.GandharePharmacology, School of Medicine, Chung-Ang University, Seoul, Korea-Sung-Hyuk Yim, Ji-Hoon Jeong.Electrical Engineering Department, Shoubra, Benha University, Cairo, Egypt- Nagat Mohamed Kamel Abdel-Gawad.



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Madurai Kamaraj University-S. Somasekaran.Electrical Engineering Department at King Fahd University of Petroleum & Minerals- J. M. Bakhashwain, M. H.Shwehdi, U. M. Johar and A. A. AL-Naim.Dept. of Electrical Engineering. College of Engineering – University of Tikrit-Iraq- Ghanim Thiab Hasan, Kamil JaduAli, Mahmood Ali Ahmed.


Dark and Bright side of CFL Bulbs (Is it dangerous to our Health?)


5 Votes

Introduction:

We are becoming more conscious about climate change and many governments in the world are lookingfor different ways to reduce greenhouse gases and to reduce consumption of fossil fuels. One of the simplest solution for this is

aggressively adopting CFL which is phasing out energy inefficient light. Compact fluorescent lights (CFL), heavily promotedfor their energy saving properties and quickly pushing traditional incandescent bulbs out of the market. They are nowinexpensive, payback in electricity savings is nearly immediate, and there is that side benefit of reducing power plant emissions.CFL bulbs use approximately 75% less energy than incandescent light bulbs and last longer. At first glance this seems like agood way to conserve energy and to protect our environment. However,

Many environmentally conscious people think they are doing a great thing by using compact fluorescent light bulbs – CFLs.We see them advertised everywhere, even our most trusted environmental news sources tells us we should be using them.

Production of traditional incandescent light bulbs may be phased out completely by the year 2014.

Unfortunately most people are unaware of and not many are talking about the fact that although CFL bulbs reduce energy andgreenhouse gases, they put our health at an even greater risk than incandescent bulbs. They are energy efficient but notenvironmentally friendly. There are a number of serious problems associated with CFL bulbs that need to be considered andcorrected. These include mercury content, emission of UV radiation, emission of radio frequency radiation, and generation ofdirty electricity. There is the additional concern that these lights are making some people ill. This includes those who suffer

from migraines, skin problems, epilepsy, and electrical sensitivity.

Governments are mandating CFL use and banning incandescent light bulbs. Media, industry, and governments have “screwed”the benefits of CFL bulbs into the deepest sockets of our mind. We are neutrally try to highlight dark and bright side of CFLby some fact and supporting arguments.


Likell



Bright Side of CFL:

There are some advantages of CFL over incandescent light bulbs

(1) Compact fluorescent lamps are four time more efficient than traditional light bulbs. (13 Watt CFL would give off as much

light as a 60 Watt incandescent).

(2) CFL bulbs use approximately 75% less energy than incandescent light bulbs.

(3) CFL has long life compared to incandescent light bulbs.

(4) Compact fluorescent light bulbs are easily shrinking power bill and carbon footprint.

(5) CFL reduces greenhouse gas emissions and other pollutants created by fossil-fuel power plants.

(6) Price so CFL is less compared to incandescent light bulbs.

Dark Side of CFL (Health problems created by CFL)

CFL’s save energy. Saving energy is good for the planet and may retard global warming. Sounds good although CFL havesome seriously negative effects.

(1) Mercury emissions by CFL:

One of the negatives side of CFLs that it contain mercury so it must be disposed of properly in order to preventcontamination of our environment, our landfills and our water supplyMercury is an essential ingredient for most energy efficient lighting products, including CFLs. It is the mercury thatexcites phosphors in a CFL, causing them to glow and give light. When electric current passes through mercury vapor,the mercury emits ultraviolet energy. When this ultraviolet energy passes through the phosphor coating, it produceslight very efficiently. Because mercury is consumed during lamp operation, a certain amount is necessary to producelight and achieve long lamp life.

Mercury can be added to the CFL in two ways. Some manufacturers use liquid mercury, which is less expensive andmore difficult to accurately dose. Uses amalgam, a small “pill” which is a solid state form of mercury and otherelements. Amalgam is much easier and more accurate to dose. This is the only manufacturer using 100 percent amalgamin its CFL products.Airborne mercury poses a very low risk of exposure. However, when mercury emissions deposit into lakes and oceans,they can transform into a highly toxic form that builds up in fish. Fish consumption is the most common pathway for

human exposure to mercury. Pregnant women and young children are most vulnerable to the effects of this type ofmercury exposure. However, the most people are not exposed to harmful levels of mercury through fish consumption.Mercury is an element found naturally in the environment. Mercury emissions in the air can come from both natural andman-made sources. Utility power plants (mainly coal-fired) are the primary man-made source, as mercury that naturallyexists in coal is released into the air when coal is burned to make electricity. Coal-fired power generation accounts forroughly 40% of the mercury emissions.

Health problems associated with mercury depend on how much has entered your body, how it entered your body, howlong you have been exposed to it, and how your body responds to the mercury. Children are more susceptible tomercury poisoning than adults. Exposure to small amounts of mercury over a long period, and brief contact with highlevels of mercury may cause adverse health effects. Symptoms depend on the length or level of exposure.Mercury is a powerful neurotoxin that can cause serious damage to the all the tissues and organs in the body as well asthe central nervous system and endocrine system and it disrupts functioning of crucial neurotransmitters in the brain. It is

one of the most toxic substances on the planet and has been linked to a variety of serious health conditions like autism,memory problems, infertility, depression, thyroid disorders, Alzheimer’s, adrenal disorders, anxiety,



Parkinson’s and MS to name a few. It is especially toxic to children, pregnant women and small pets.While the mercury is contained in the light bulb there is no risk, however if you drop the bulb on the floor of your home,then you are exposed to dangerous mercury vapors. Many are reporting that it is quite easy to break CFL light bulbs asyou are screwing it in the socket. Additionally, when we toss them in the garbage and they are picked up by thegarbage company, they are getting broken all over the city and in the landfills. This means that our air and soil is being

contaminated with mercury across our cities.

Arguments to oppose this Drawback

The amount of mercury in the most popular and widely used CFLs is minimal, ranging between 2.3 mg and 3.5 mg.That is lower than other CFLs on the market, which generally contain approximately 5 mg, roughly the equivalent of thetip of a ballpoint pen.By comparison, older home thermometers contain 500 milligrams of mercury and many manual thermostats contain upto 3000 milligrams. It would take between 100 and 665 CFLs to equal those amounts.

CFLs are safe to use in your home. No mercury is released when the bulbs are in use and they pose no danger to youor your family when used properly.CFLs are responsible for less mercury than standard incandescent light bulbs, and actually work to prevent mercuryfrom entering our air, where it most affects our health. The highest source of mercury in our air comes from burningfossil fuels such as coal, the most common fuel used to produce electricity. A CFL uses 75% less energy than anincandescent light bulb and lasts up to 13 times longer. 70% of power plants are coal fired and thus burn fossilfuel to produce energy. These power plants will emit 10 mg of mercury to produce the electricity to run an incandescent

bulb compared to only 2.4 mg of mercury to run a CFL for the same time. Coal-fired power generation accounts forroughly 40% of the mercury emissions.

(2) Compact fluorescent bulbs and Migraine

In the past, some people reported headaches or eye strain when using fluorescent lighting. Some could see a flicker in

the lighting, caused by lower frequencies and magnetic ballasts. The newer CFLs use higher frequencies and electronicballasts, which mean the human eye, cannot detect any change in the light frequency. There is also less of a ‘hum’ in thenewer lights. The ‘hum’ in older lights may have caused headaches.The flickering of fluorescent bulbs is a known migraine trigger. Compact fluorescent bulbs have made great strides inreducing the flickering that is common in this class of light bulbs. Despite this, many individuals are finding that compactfluorescent bulbs cause migraine headaches. CFL bulb manufacturers have denied any link between the bulbs and

increased headache problems. Currently, there is little research to support the link between migraine and CFL use;however, personal, anecdotal evidence demonstrates that many migraines cannot tolerate the new lights. Migraine is notjust a headache. Migraine disease is a neurological condition that not only causes pain but can impact motor function,sensory function, vision, memory, and speechIndividuals who have problems with fluorescent bulbs can try the following tips to lessen the impact of a CFL onmigraine disease:

Arguments to oppose this Drawback:

1. Use the newest compact fluorescent bulbs available.2. Sit as far from the bulbs as possible3. If flickering is interfering with TV or computer monitor use, try repositioning the light to see if the flickering effect on the

screen lessens.4. Try eye glasses or contacts that block out UV radiation.5. Use halogen or LED lighting

6. Try double walled bulbs or a light diffuser.

(3) Compact fluorescent bulbs and Lupus



Compact fluorescent bulbs can produce more ultraviolet light and have a different light spectrum than incandescentbulbs. This makes compact fluorescent bulbs problematic for people with Lupus or other light sensitive skin conditions.

Individuals with light sensitivity should monitor the effect of compact fluorescent bulbs on their health.

Arguments to eliminate this Drawback:

1. Keep at least 1 foot between yourself and the compact fluorescent bulb.2. Try a light cover or diffuser over the light.3. Investigate the amount of ultraviolet light produced by different brands of CFLs.4. Use halogen or LED lighting.

(4) Ultra violet light emissions from CFL

Ultra violet light is responsible for skin cancer. It can also be a problem for individuals with ultra violet sensitiveconditions such as Lupus. One would think that staying inside would keep a person safe from this harmful radiation.


1. This is not completely true. Fluorescent lights put off UV light. While this exposure is much smaller than that of sunlight,it is important to keep it in mind. The current guideline limit is 30 J m-2 for the eye and skin, which is equivalent to a

constant irradiance of 1 mW m-2 effective for 30,000 seconds or 8 hours, a normal working day. At close proximity (2cm or ¾ inch), the exposure limit would be exceeded in less than 10 minutes by about 20% of the CFLs tested.

2. About half of the CFLs exceeded the exposure limit at this distance after 30 minutes. If the distance is increased toabout 8″ only around 8% of the CFL bulbs exceed this limit.

3. Also, encapsulated bulbs that have a globe of glass around the CFL itself emit less UV radiation than the traditionalbulbs.

4. Do not use compact fluorescent bulbs for close up work or lighting.5. Purchase double walled CFLs that are encapsulated.

(5) Spectral distributions by CFL:

Natural daylight provides the only true full spectrum lighting. Incandescent light is closer in spectral distribution to natural

daylight; fluorescent light is far different which accounts for its negative effects on the human body. There arethousands, of well documented scientific photo biological studies indicating the negative effects of fluorescent lighting.The effects of different light sources on the body have been researched at a long list of prestigious institutions includingMIT, and Harvard University. The latest research is being done on how different colors of light (spectral distributions)affect the body’s circadian rhythms. Researchers used to think of the eye as the main organ for vision but because ofthe recent discovery of additional nerve connections, it is now understood that light mediates and controls a number of

biochemical processes in the human body, including the production of important hormones through control of thelight/dark cycle (circadian rhythms) – the body’s biological clock.Fluorescent light gives off a very much distorted spectrum which is very different from the natural daylight in which ourbodies have evolved. Fluorescent light disrupts our circadian rhythms – our body’s regulator mechanism – and in doingso studies have shown negative health effects from minor annoyances such as headaches, eyestrain, fatigue, and weightgain, to serious effects such as insomnia and sleep disturbances, an increased risk of cancer, and a suppressed immune

system.

(6) Emission of UV Radiation by CFL:

Fluorescent light bulbs contain mercury, which emits UV radiation when it is electrically excited. This UV radiation theninteracts with the chemicals on the inside of the bulb to generate light. According to Philippe Laroche, Media Relations

Officer for Health Canada, compact fluorescent light bulbs, unlike tube fluorescent bulbs, do not have prismatic



diffusers to filter UV radiation. “Therefore, there may be skin sensitivity issues, especially in people with certain skindiseases.”Interestingly, the British Dermatological Association has spoken out against CFL bulbs because their patients haveadverse reactions to them. They are asking the UK government to allow people with skin problems to continue using

incandescent light bulbs once the ban for energy inefficient bulbs becomes law.


1. Not all CFL are the same. GE produces a low-UV bulb called Safe-T-Guard (registered Trade mark) for dark rooms.So the technology to produce safer bulbs is available and should be required for all bulbs.

(7) Emission of Radio Frequency Radiation by CFL:

CFLs emit radio frequency radiation at levels that may interfere with various types of wireless technology.


1. GE has started to put on General Electric acknowledges this and notice on the back of product packaging for all GEelectronically ballasted CFLs:“This product complies with Part 18 of the FCC Rules, but may cause interferenceto radios, televisions, wireless telephones, and remote controls. Avoid placing this product near these devices. Ifinterference occurs, move the product away from the device or plug either into a different outlet. Do not installthis product near maritime safety equipment or other critical navigation or communication equipment

operating between 0.45-30MHz. “

(8) Poor Power Quality Produced by CFL:

CFL is affecting Quality of Electrical Power. There is a deviation in the magnitude and frequency of the sinusoidalwaveform.

Fluorescent lamps will only run on alternating current. They also need a pulse of high voltage and heated filaments ateither end to start the electrical discharge that lights them. After that, the current must be limited externally, otherwisetoo much would flow and they would burn out. In a traditional fluorescent strip light, this is accomplished by the starterswitch and the choke (a coil of wire wound around an iron core). Once started, the current flows through the tube as asmooth sine wave at mains frequency, which is 50Hz (cycles per second).This makes the light flash on and off with eachhalf cycle (i.e. 100 or 120 times a second) and some people, such as epileptics and migraine sufferers find thisdisturbing.

CFLs produce transients that contribute to poor power quality on electrical wires. According to General Electric (GE)their typical electronically-ballasted CFL operate in the 24-100 kHz frequency range. This range is within the radiofrequency band of the electromagnetic spectrum and is classified as Intermediate Frequency (IF) by the World HealthOrganization. There is concern about electromagnetic interference (EMI) associated with IF and recently studies haveshown that IFs are biologically active and can have adverse health effects.


1. Not all CFL are the same some generate more dirty electricity than others. In a recent study the values for dirty

electricity ranged from 47 to 1450 GS units compared with a background value (with lights off) between 54-58 GSunits. Clearly technology exists to produce CFL that do not generate dirty electricity.

2. However, almost all CFLs use electronic control gear. This usually incorporates a switched-mode power supply in thebase of the lamp itself. It rectifies the AC from the mains to convert it to DC and then chops it electronically into aseries of sharp rectangular alternating pulses, which then light the lamp. However, the new frequency, which is usuallyabout 40 kHz (40,000 cycles per second), is so high and the gaps between pulses are so short that the relatively slow

response of the phosphors can fill them easily. Consequently, these lamps do not flash.



Required Vigilant Awareness while using of CFL:

Although CFLs are considered safe to use, here are some steps you can take to further protect you and your family:

Always handle CFLs carefully when installing and removing them.Buy CFLs that are marked low UV.Buy CFLs that have a glass cover already added, which will help further filter out UV radiation.Use additional glass, plastic or fabric materials in your lighting fixtures to act as UV filters.Increase the distance you are from the CFL, as this will reduce the level of UV exposure.

All ENERGY STAR® qualified CFLs have less than 5 milligrams of mercury (some manufacturers are able to produceCFLs that have only about 1 milligram of mercury). Avoid purchasing non-ENERGY STAR® CFLs, as they may havemuch higher levels of mercury in them.As of September 2008, all ENERGY STAR® qualified CFLs are required to list their mercury content on thepackaging. This information is not required on non-ENERGY STAR® CFL packaging.A CFL is a sealed unit, and no mercury is released when it is in use or as long as it is intact. Some mercury is released

when a bulb breaks, and appropriate clean-up guidance should be followed.If the bulb breaks, make sure to clean it up properly. Also, check your local regulations to make sure that you won’tbreak any laws while disposing of the bulb.Look for recycling programs online, through local stores, or through the light bulb manufacturers. Make an informedchoice. If CFLs concern you or if you have health problems do to them, switch to a LED or incandescent bulb

Disposal of CFL Bulbs:

If CFL breaks- carefully sweep up all the fragments, wipe the area with a wet towel, and dispose of all fragments,including the used towel, in a sealed plastic bag.Follow all disposal instructions. If possible, open windows to allow the room to ventilate. Do NOT use a vacuum.Place all fragments in a sealed plastic bag and follow disposal instructions.Due to the mercury in fluorescent bulbs, they require special disposal methods. When these bulbs are sent to a

traditional landfill, the bulbs often will break and will then emit mercury gas that is harmful to workers and to theenvironment. The health threat to workers is especially large at transfer stations where large quantities of light bulbs maybe crushed in a single location. Due to the dangers associated with mercury “ten states and multiple local jurisdictionsprohibit the disposal of mercury containing products, including CFLs and other mercury containing lamps, in solidwaste.”Require CFLs to go through special CFL recycling programs or for individuals to dispose of CFLs at hazardous waste

collection centers.

CONCLUSION

If we can afford the discomfort of higher electrical bills, it is OK to go back to incandescent. The Earth will be fine, it just goesthrough cyclical warming and cooling’s, and we humans might not have as much impact on it as we give ourselves credit for.

The heat generated by incandescent is not always wasted either. In colder months the heat reduces the amount of energydrawn from household heating. In the next some year the prices of LED lighting will start to come down, and new LED lightingfixtures will be introduced. The CFLs will begin to be phased out, leaving behind a long term problem of mercury disposal,remediation, and a so far untold toll on human health

Instead of promoting compact fluorescent light bulbs governments around the world should be insisting thatmanufactures produces light bulbs that are electromagnetically clean and contain no toxic chemicals. Some ofthese are available (LED) but are not yet affordable. With a growing number of people developing electro hypersensitivity we

have a serious emerging and newly identified health risk that is likely to get worse until regulations restricting our exposure toelectromagnetic pollutants are enforced. Also, with improper disposal of these bulbs we are creating a mercury-time bomb.



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Since everyone uses light bulbs and since the energy inefficient incandescent light bulbs are being phased out in many countriesby this is an area that requires immediate attention. “Try a CFL, but use and dispose it very carefully”


Analysis the Truth behind Household Power Savers


3 Votes

Introduction:

A House hold power saving devices has recently received a lot of attention from both consumers and manufacturers. It isgenerally used in residential homes to save energy and to reduce electricity bills. It is a small device which is to be plugged inany of the AC sockets in the house (Mostly near Energy Meter). Moreover, some of the companies claim that their powersavers save up to 40% of the energy.

Many people believe that the claims made by the power saver manufacturing companies arefalse. Almost all people who buy power savers do it to reduce their electricity bills. Many people who have used these power

savers said that they could reduce their electricity bills with the devices; however the reduction was not as much as they hadexpected. Moreover, they could not figure out if the reduction in electricity bills was due to the power savers or because oftheir efforts to reduce their electrical usage. There have been several serious discussions about the genuineness of the device.In This Note, We will try to find the real truth behind these power savers which claim to save as much as 40% of energy.

Working Principle of Power Saver as per Manufacture:

A Power Saver is a device which plugs in to power socket. Apparently just by keeping the device connected it willimmediately reduce your power consumption. Typical claims are savings between 25% and 40%.It is known that the electricity that comes to our homes is not stable in nature. There are many fluctuations, raise andfalls, and surges/Spikes in this current. This unstable current cannot be used by any of the household appliances.Moreover, the fluctuating current wastes the electric current from the circuit by converting electrical energy into heat

energy. This heat energy not only gets wasted to the atmosphere, but also harms the appliances and wiring circuit.


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Power Saver stores the electricity inside of it using a system of capacitorsand they release it in a smoother wayto normal without the spikes. The systems also automatically remove carbon from the circuit which also encourages asmoother electrical flow. This means that we will have less power spikes. More of the electricity flowing around circuitcan be used to power appliances than before.

Basically it is claimed that Power savers work on the principle of surge protection technology. Power saverswork on straightening this unstable electric current to provide a smooth and constant output. The fluctuation in

voltage is unpredictable and cannot be controlled. However, the power savers utilize current fluctuation toprovide a usable power by acting like a filter and allowing only smooth current to pass through the circuit. Powersavers use capacitors for this purpose. When there is a surge of current in the circuit, the capacitor of the powersaver stores the excess current and releases it when there is a sudden drop. Thus only smooth output currentcomes out of the device.Moreover, a power saver also removes any type of carbon in the system, which facilitates further smoother flow.

The main advantage of power savers is not that they provide a backup system in times of low current, butthat it protects the household appliances. It is known that a sudden rise in the power can destroy the electricalappliance. Thus, the power saver not only protects the appliance but also increases its life. Moreover, they alsoreduce the energy consumption and thus the electricity bills.The amount of power saved by a power saver depends on the number of appliances on the circuit. Also, thesystem takes at least a week to adapt itself fully to the circuit, before it starts showing its peak performance. The

maximum amount of voltage savings will be seen in areas where in the current fluctuation is the highest.

House Hold Power Saver Scam Review:

Power Factor Correction for residential customers (home owners) is a scam? At most, each unit is worth as aninvestment. Power factor correction does make sense for some commercial / industrial customers.

Many Companies promoting and advertise that their Power Saver unit are able to save domestic residential powerconsumption by employing an “active power factor correction” method on the supply line. The concept seems prettyimpressive as the concept is true and legally accepted. But practically, we will find that it’s not feasible.To support above statement First we need to understand three terms.

1. Type of Electrical Load of House,2. Basic Power Terminology (KW, KVA, KVAR).3. Electrical Tariff method of Electricity Company for Household Consumer and Industrial Consumer.

There are basically two kinds of load that exists in every house: one that is resistive like incandescent lamps, heaters etc.and the other that’s capacitive or inductive like ACs, refrigerators, computers, etc.The power factor of a Resistive Load like toaster or ordinary incandescent light bulb is 1 (one). Devices with coils orcapacitors (like pumps, fans and florescent light bulb ballasts)-Reactive Load have power factors less than one. Whenthe power factor is less than 1, the current and voltage are out of phase. This is due to energy being stored and releasedinto inductors (motor coil) or capacitors on every AC cycle (usually 50 or 60 times per second).

There are three terms need to be understand when dealing with alternating (AC) power.

1. First Term is kilowatt (kW) and it represents Real power. Real power can perform work. Utility meters on the sideof House measure this quantity (Real Power) and Power Company charge for it.

2. The second term is reactive power, measured in KVAR. Unlike kW, it cannot perform work. Residential customersdo not pay for KVAR, and utility meters on houses do not record it too.



3. The third term is apparent power, referred to as KVA. By use of multi meters we can measure current and voltage andthen multiply the readings together we get apparent power in VA.

Power Factor = Real Power (Watts) /Apparent Power (VA),Therefore, Real Power (Watts) = Apparent Power × PF = Voltage × Ampere × PF.Ideally a PF = 1, or unity, for an appliance defines a clean and a desired power consumption mostly HouseholdEquipments (The dissipated output power becomes equal to the applied input power).In the above formula we can see that if PF is less than 1, the amperes (current consumption) of the appliances increase,and vice verse.With AC Resistive Load, the voltage is always in phase with the current and constitutes an ideal power factor equal to

1. However, with inductive or capacitive loads, the current waveform lags behind the voltage waveform and is not intandem. This happens due to the inherent properties of these devices to store and release energy with the changing ACwaveform, and this causes an overall distorted wave form, lowering the net PF of the appliance.Manufacture claim that the above problem may be solved by installing a well-calculated inductor/capacitor network andswitching it automatically and appropriately to correct these fluctuations. A power saver unit is designed exactly for thispurpose. This correction is able to bring the level of PF very close to unity, thus improving the apparent power to a

great extent. An improved apparent power would mean less CURRENT consumption by all the domesticappliances. So far everything looks fine, but what’s the use of the above correction? The Utility Bill Which We payis never based on Apparent Power (KVA) but it is based on Real Power (KW). The utility bill that we pay isnever for the Apparent Power- it’s for the Real Power.By Reducing Current Consumption Does Not Reduce Power Bills of Household Consumer.

Study of Power Saver in Domestic Load

Let us try to study Household’s Reactive-Resistive Electrical Load and Voltage Spike Characteristic by example.

(1) Power Saver in Reactive Load of Home:

Let’s take One Example for reactive Load: A refrigerator having a rated Real Power of 100 watts at 220 V AC has aPF = 0.6. So Power=Volt X Ampere X P.F becomes 100 = 220 × A × 0.6 Therefore, A = 0.75 AmpereNow suppose after Installing Power Saver if the PF is brought to about 0.9, the above result will now show as: 100 =220 × A × 0.9 And A = 0.5 AmpereIn the second expression we clearly show that a reduction in current consumption by the refrigerator, but interestingly inboth the above cases, the Real Power remains the same, i.e. the refrigerator continues to consume 100 watts, and

therefore the utility bill remains the same. This simply proves that although the PF correction done by an energy savermay decrease the Amperage of the appliances, it can never bring down their power consumption and the electricBill amount.Reactive power is not a problem for a Reactive Load of Home appliances like A.C, Freeze, motor for its operation. Itis a problem for the electric utility company when they charge for KW only. If two customers both use the sameamount of real energy but one has a power factor of 0.5, then that customer also draws double the current. This

increased current requires the Power Company to use larger transformers, wiring and related equipment. To recoverthese costs Power Company charged a Penalty to industrial customers for their Low power factors and give thembenefits if they improve their Power Factor in. Residential customers (homes) are never charged extra for theirreactive Power.



(2) Power Saver in Resistive Load of Home:

Since a resistive load does not carry a PF so there is not any issue regarding filtering of Voltage and Current, So Power= Voltage X Current.

(3) In Voltage Spike/Fluctuation condition of Household Appliances:

In above discussion simply proves that as long as the voltage and the current are constant, the consumed power willalso be constant. However, if there’s any rise in the input voltage because of a fluctuation, then as explained above your

appliances will be forced to consume a proportionate amount of power. This becomes more apparent because current,being a function of voltage, also rises proportionately. However, this rise in the power consumption will be negligiblysmall; the following simple math will prove this.Consider a bulb consuming 100 watts of power at 220 volts. This simply means at 240 volts it will use up about 109watts of power. The rise is just of around 9% and since such fluctuations are pretty seldom, this value may befurthermore reduced to less than 1%, and that is negligible.

Thus the above discussions convincingly prove that energy savers can never work and the concept is not practicallyfeasible.

What happens when Power Saver is installed?

The Fig shows the result of using Power Saver. The air conditioner (which has a large compressor motor) is still

consuming reactive power but it is being supplied by a nearby capacitor (which is what is in those “KVAR” boxes). Ifyou were to mount it at the air conditioner and switch it on with the air conditioner plus you sized the capacitorperfectly, then there would be no reactive power on the line going back to the fuse panel. If the wire between your fusepanels is very long and undersized, reducing the current would result in it running cooler and having a higher voltage atthe air conditioner. These savings due to cooler wiring is minimal.

A further complication is that if you install the “KVAR” unit at the fuse panel, it does nothing for the heat losses exceptfor the two feet of huge wire between the fuse panel and the utility meter. Many KVAR units are marketed as boxesthat you install at a single location. If your power factor box is too large, then it will be providing reactive power forsomething else, perhaps your neighbor.

Conclusion:

Power factor correction devices improve power quality but do not generally improve energy efficiency (meaning theywould not reduce your energy bill). There are several reasons why their energy efficiency claims could beexaggerated.First, residential customers are not charged for KVA- hour usage, but by kilowatt-hour usage. This means that any



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savings in energy demand will not directly result in lowering a residential user’s utility bill.

Second, the only potential for real power savings would occur if the product were only put near in the circuit while areactive load (such as a motor) were running, and taken out of the circuit when the motor is not running. This isimpractical, given that there are several motors in a typical home that can come on at any time (refrigerator, airconditioner, HVAC blower, vacuum cleaner, etc.), but the Power Saver itself is intended for permanent, unattendedconnection near the house breaker panel. And certainly not in the way the manufacturers recommend that they beinstalled, that is, permanently connecting them at the main panel. Doing that drags the power factor capacitive when the

inductive motors are off and could create some real problems with ringing voltages.The KVAR needs to be sized perfectly to balance the inductive loads. Since our motors cycle on and off and we don’tuse the air conditioner in the winter, there is no way to get it sized properly unless we have something to monitor the lineand switch it on and off capacity (capacitors) as necessary.Adding a capacitor can increase the line voltage to dangerous levels because it interacts with the incoming powertransmission lines.Adding a capacitor to a line that has harmonic frequencies (created by some electronic equipment) on it can result in

unwanted resonance and high currents.For commercial facilities, power factor correction will rarely be cost-effective based on energy savings alone. The bulkof cost savings power factor correction can offer is in the form of avoided utility charges for low power factor.Energy savings are usually below 1% and always below 3% of load, the higher percentage occurring where motors area large fraction of the overall load of a facility. Energy savings alone do not make an installation cost effective.


Types of Neutral Earthing in Power Distribution

January 21, 2012 7 Comments

3 Votes

Types of Neutral Earthing in Power Distribution:

Introduction:

In the early power systems were mainly Neutral ungrounded due to the fact that the first ground fault did not require thetripping of the system. An unscheduled shutdown on the first ground fault was particularly undesirable for continuous processindustries. These power systems required ground detection systems, but locating the fault often proved difficult. Although

achieving the initial goal, the ungrounded system provided no control of transient over-voltages.

A capacitive coupling exists between the system conductors and ground in a typical distribution system. As a result, this seriesresonant L-C circuit can create over-voltages well in excess of line-to-line voltage when subjected to repetitive re-strikes ofone phase to ground. This in turn, reduces insulation life resulting in possible equipment failure.

Neutral grounding systems are similar to fuses in that they do nothing until something in the system goes wrong. Then, likefuses, they protect personnel and equipment from damage. Damage comes from two factors, how long the fault lasts and howlarge the fault current is. Ground relays trip breakers and limit how long a fault lasts and Neutral grounding resistors limit how

large the fault current is.


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Importance of Neutral Grounding:

There are many neutral grounding options available for both Low and Medium voltage power systems. The neutral points oftransformers, generators and rotating machinery to the earth ground network provides a reference point of zero volts. Thisprotective measure offers many advantages over an ungrounded system, like,

1. Reduced magnitude of transient over voltages2. Simplified ground fault location3. Improved system and equipment fault protection4. Reduced maintenance time and expense5. Greater safety for personnel6. Improved lightning protection

7. Reduction in frequency of faults.

Method of Neutral Earthing:

There are five methods for Neutral earthing.

1. Unearthed Neutral System2. Solid Neutral Earthed System.

3. Resistance Neutral Earthing System.1. Low Resistance Earthing.2. High Resistance Earthing.

4. Resonant Neutral Earthing System.5. Earthing Transformer Earthing.

(1) Ungrounded Neutral Systems:

In ungrounded system there is no internal connection between the conductors and earth. However, as system, acapacitive coupling exists between the system conductors and the adjacent grounded surfaces. Consequently, the“ungrounded system” is, in reality, a “capacitive grounded system” by virtue of the distributed capacitance.Under normal operating conditions, this distributed capacitance causes no problems. In fact, it is beneficial because it

establishes, in effect, a neutral point for the system; As a result, the phase conductors are stressed at only line-to-neutralvoltage above ground.But problems can rise in ground fault conditions. A ground fault on one line results in full line-to-line voltage appearingthroughout the system. Thus, a voltage 1.73 times the normal voltage is present on all insulation in the system. Thissituation can often cause failures in older motors and transformers, due to insulation breakdown.

Advantage:



1. After the first ground fault, assuming it remains as a single fault, the circuit may continue in operation, permittingcontinued production until a convenient shut down for maintenance can be scheduled.

Disadvantages:

1. The interaction between the faulted system and its distributed capacitance may cause transient over-voltages (severaltimes normal) to appear from line to ground during normal switching of a circuit having a line-to ground fault (short).These over voltages may cause insulation failures at points other than the original fault.

2. A second fault on another phase may occur before the first fault can be cleared. This can result in very high line-to-line

fault currents, equipment damage and disruption of both circuits.3. The cost of equipment damage.4. Complicate for locating fault(s), involving a tedious process of trial and error: first isolating the correct feeder, then the

branch, and finally, the equipment at fault. The result is unnecessarily lengthy and expensive down downtime.

(2) Solidly Neutral Grounded Systems:

Solidly grounded systems are usually used in low voltage applications at 600 volts or less.In solidly grounded system, the neutral point is connected to earth.Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the ungrounded system andprovided path for the ground fault current is in the range of 25 to 100% of the system three phase fault current.However, if the reactance of the generator or transformer is too great, the problem of transient over voltages will not be

solved.While solidly grounded systems are an improvement over ungrounded systems, and speed up the location of faults, theylack the current limiting ability of resistance grounding and the extra protection this provides.To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must be extend from thesource to the furthest point of the system within the same raceway or conduit. Its purpose is to maintain very lowimpedance to ground faults so that a relatively high fault current will flow thus insuring that circuit breakers or fuses willclear the fault quickly and therefore minimize damage. It also greatly reduces the shock hazard to personnel

If the system is not solidly grounded, the neutral point of the system would “float” with respect to ground as a functionof load subjecting the line-to-neutral loads to voltage unbalances and instability.The single-phase earth fault current in a solidly earthed system may exceed the three phase fault current. The magnitudeof the current depends on the fault location and the fault resistance. One way to reduce the earth fault current is to leavesome of the transformer neutrals unearthed.Advantage:

1. The main advantage of solidly earthed systems is low over voltages, which makes the earthing design common at high

voltage levels (HV).

Disadvantage:



1. This system involves all the drawbacks and hazards of high earth fault current: maximum damage and disturbances.2. There is no service continuity on the faulty feeder.3. The danger for personnel is high during the fault since the touch voltages created are high.

Applications:

1. Distributed neutral conductor.2. 3-phase + neutral distribution.3. Use of the neutral conductor as a protective conductor with systematic earthing at each transmission pole.4. Used when the short-circuit power of the source is low.

(3) Resistance earthed systems:

Resistance grounding has been used in three-phase industrial applications for many years and it resolves many of theproblems associated with solidly grounded and ungrounded systems.Resistance Grounding Systems limits the phase-to-ground fault currents. The reasons for limiting the Phase to groundFault current by resistance grounding are:

1. To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers, cables, and rotating

machines.2. To reduce mechanical stresses in circuits/Equipments carrying fault currents.3. To reduce electrical-shock hazards to personnel caused by stray ground fault.4. To reduce the arc blast or flash hazard.5. To reduce the momentary line-voltage dip.6. To secure control of the transient over-voltages while at the same time.7. To improve the detection of the earth fault in a power system.

Grounding Resistors are generally connected between ground and neutral of transformers, generators and groundingtransformers to limit maximum fault current as per Ohms Law to a value which will not damage theequipment in the power system and allow sufficient flow of fault current to detect and operate Earth protective relaysto clear the fault. Although it is possible to limit fault currents with high resistance Neutral grounding Resistors, earthshort circuit currents can be extremely reduced. As a result of this fact, protection devices may not sense the fault.Therefore, it is the most common application to limit single phase fault currents with low resistance Neutral Grounding

Resistors to approximately rated current of transformer and / or generator.In addition, limiting fault currents to predetermined maximum values permits the designer to selectively coordinate theoperation of protective devices, which minimizes system disruption and allows for quick location of the fault.There are two categories of resistance grounding:

(1) Low resistance Grounding.

(2) High resistance Grounding.

Ground fault current flowing through either type of resistor when a single phase faults to ground will increase the phase-to-ground voltage of the remaining two phases. As a result, conductor insulation and surge arrestor ratings must

be based on line-to-line voltage. This temporary increase in phase-to-ground voltage should also be consideredwhen selecting two and three pole breakers installed on resistance grounded low voltage systems.The increase in phase-to-ground voltage associated with ground fault currents also precludes the connection of line-to-neutral loads directly to the system. If line-to neutral loads (such as 277V lighting) are present, they must be served by asolidly grounded system. This can be achieved with an isolation transformer that has a three-phase delta primary and athree-phase, four-wire, wye secondary



Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated with phase-to-phasefaults, but both systems significantly reduce or essentially eliminate the arc-flash hazards associated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses and reduce thermal damage to electricalequipment, circuits, and apparatus carrying faulted current.

The difference between Low Resistance Grounding and High Resistance Grounding is a matter of perception and,therefore, is not well defined. Generally speaking high-resistance grounding refers to a system in which theNGR let-through current is less than 50 to 100 A. Low resistance grounding indicates that NGR currentwould be above 100 A.A better distinction between the two levels might be alarm only and tripping. An alarm-only system continues to operatewith a single ground fault on the system for an unspecified amount of time. In a tripping system a ground fault is

automatically removed by protective relaying and circuit interrupting devices. Alarm-only systems usually limit NGRcurrent to 10 A or less.Rating of The Neutral grounding resistor:

1. 1. Voltage: Line-to-neutral voltage of the system to which it is connected.2. 2. Initial Current: The initial current which will flow through the resistor with rated voltage applied.3. 3. Time: The “on time” for which the resistor can operate without exceeding the allowable temperature rise.

(A).Low Resistance Grounded:

Low Resistance Grounding is used for large electrical systems where there is a high investment in capital equipment orprolonged loss of service of equipment has a significant economic impact and it is not commonly used in low voltagesystems because the limited ground fault current is too low to reliably operate breaker trip units or fuses. This makessystem selectivity hard to achieve. Moreover, low resistance grounded systems are not suitable for 4-wire loads and

hence have not been used in commercial market applicationsA resistor is connected from the system neutral point to ground and generally sized to permit only 200A to 1200 ampsof ground fault current to flow. Enough current must flow such that protective devices can detect the faulted circuit andtrip it off-line but not so much current as to create major damage at the fault point.

Since the grounding impedance is in the form of resistance, any transient over voltages are quickly damped out and the



whole transient overvoltage phenomena is no longer applicable. Although theoretically possible to be applied in lowvoltage systems (e.g. 480V),significant amount of the system voltage dropped across the grounding resistor, there is not

enough voltage across the arc forcing current to flow, for the fault to be reliably detected. For this reason, lowresistance grounding is not used for low voltage systems (under 1000 volts line to-line).Advantages:

1. Limits phase-to-ground currents to 200-400A.2. Reduces arcing current and, to some extent, limits arc-flash hazards associated with phase-to-ground arcing current

conditions only.

3. May limit the mechanical damage and thermal damage to shorted transformer and rotating machinery windings.

Disadvantages:

1. Does not prevent operation of over current devices.2. Does not require a ground fault detection system.3. May be utilized on medium or high voltage systems.4. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must

be served through an isolation transformer.

Used: Up to 400 amps for 10 sec are commonly found on medium voltage systems.

(B).High Resistance Grounded:

High resistance grounding is almost identical to low resistance grounding except that the ground fault current magnitudeis typically limited to 10 amperes or less. High resistance grounding accomplishes two things.The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable damage

is done at the fault point. This means that the faulted circuit need not be tripped off-line when the fault first occurs.Means that once a fault does occur, we do not know where the fault is located. In this respect, it performs just like anungrounded system.The second point is it can control the transient overvoltage phenomenon present on ungrounded systems ifengineered properly.Under earth fault conditions, the resistance must dominate over the system charging capacitance but not to the point of

permitting excessive current to flow and thereby excluding continuous operation

High Resistance Grounding (HRG) systems limit the fault current when one phase of the system shorts or arcs toground, but at lower levels than low resistance systems.In the event that a ground fault condition exists, the HRG typically limits the current to 5-10A.

HRG’s are continuous current rated, so the description of a particular unit does not include a time rating. UnlikeNGR’s, ground fault current flowing through a HRG is usually not of significant magnitude to result in the operation ofan over current device. Since the ground fault current is not interrupted, a ground fault detection system must beinstalled.These systems include a bypass contactor tapped across a portion of the resistor that pulses (periodically opens andcloses). When the contactor is open, ground fault current flows through the entire resistor. When the contactor is closeda portion of the resistor is bypassed resulting in slightly lower resistance and slightly higher ground fault current.



To avoid transient over-voltages, an HRG resistor must be sized so that the amount of ground fault currentthe unit will allow to flow exceeds the electrical system’s charging current. As a rule of thumb, charging current isestimated at 1A per 2000KVA of system capacity for low voltage systems and 2A per 2000KVA of system capacityat 4.16kV.

These estimated charging currents increase if surge suppressors are present. Each set of suppressors installed on a lowvoltage system results in approximately 0.5A of additional charging current and each set of suppressors installed on a4.16kV system adds 1.5A of additional charging current. A system with 3000KVA of capacity at 480 volts would have an estimated charging current of 1.5A.Add one set ofsurge suppressors and the total charging current increases by 0.5A to 2.0A. A standard 5A resistor could be used onthis system. Most resistor manufacturers publish detailed estimation tables that can be used to more closely estimate anelectrical system’s charging current.

Advantages:

1. Enables high impedance fault detection in systems with weak capacitive connection to earth2. Some phase-to-earth faults are self-cleared.3. The neutral point resistance can be chosen to limit the possible over voltage transients to 2.5 times the fundamental

frequency maximum voltage.4. Limits phase-to-ground currents to 5-10A.

5. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase-to-ground arcing currentconditions only.

6. Will eliminate the mechanical damage and may limit thermal damage to shorted transformer and rotating machinerywindings.

7. Prevents operation of over current devices until the fault can be located (when only one phase faults to ground).8. May be utilized on low voltage systems or medium voltage systems up to 5kV. IEEE Standard 141-1993 states that

“high resistance grounding should be restricted to 5kV class or lower systems with charging currents of about 5.5A orless and should not be attempted on 15kV systems, unless proper grounding relaying is employed”.

9. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads mustbe served through an isolation transformer.

Disadvantages:

1. Generates extensive earth fault currents when combined with strong or moderate capacitive connection to earth Costinvolved.

2. Requires a ground fault detection system to notify the facility engineer that a ground fault condition has occurred.

(4) Resonant earthed system:

Adding inductive reactance from the system neutral point to ground is an easy method of limiting the available groundfault from something near the maximum 3 phase short circuit capacity (thousands of amperes) to a relatively low value(200 to 800 amperes).

To limit the reactive part of the earth fault current in a power system a neutral point reactor can be connected betweenthe transformer neutral and the station earthing system.A system in which at least one of the neutrals is connected to earth through an

1. Inductive reactance.2. Petersen coil / Arc Suppression Coil / Earth Fault Neutralizer.

The current generated by the reactance during an earth fault approximately compensates the capacitive component ofthe single phase earth fault current, is called a resonant earthed system.

The system is hardly ever exactly tuned, i.e. the reactive current does not exactly equal the capacitive earth fault currentof the system.



A system in which the inductive current is slightly larger than the capacitive earth fault current is over compensated. A

system in which the induced earth fault current is slightly smaller than the capacitive earth fault current is undercompensated

However, experience indicated that this inductive reactance to ground resonates with the system shunt capacitance to

ground under arcing ground fault conditions and creates very high transient over voltages on the system.To control the transient over voltages, the design must permit at least 60% of the 3 phase short circuit current to flowunderground fault conditions.Example. A 6000 amp grounding reactor for a system having 10,000 amps 3 phase short circuit capacity available.Due to the high magnitude of ground fault current required to control transient over voltages, inductance grounding israrely used within industry.

Petersen Coils:

A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that the capacitivecurrent in the earth fault is compensated by an inductive current passed by the Petersen Coil. A small residualcurrent will remain, but this is so small that any arc between the faulted phase and earth will not be maintained and thefault will extinguish. Minor earth faults such as a broken pin insulator, could be held on the system without the supply

being interrupted. Transient faults would not result in supply interruptions.Although the standard ‘Peterson coil’ does not compensate the entire earth fault current in a network due to thepresence of resistive losses in the lines and coil, it is now possible to apply ‘residual current compensation’ by injectingan additional 180° out of phase current into the neutral via the Peterson coil. The fault current is thereby reduced topractically zero. Such systems are known as ‘Resonant earthing with residual compensation’, and can be considered asa special case of reactive earthing.

Resonant earthing can reduce EPR to a safe level. This is because the Petersen coil can often effectively act as a highimpedance NER, which will substantially reduce any earth fault currents, and hence also any corresponding EPRhazards (e.g. touch voltages, step voltages and transferred voltages, including any EPR hazards impressed onto nearbytelecommunication networks).Advantages:

1. Small reactive earth fault current independent of the phase to earth capacitance of the system.2. Enables high impedance fault detection.

Disadvantages:

1. Risk of extensive active earth fault losses.2. High costs associated.

(5) Earthing Transformers:

For cases where there is no neutral point available for Neutral Earthing (e.g. for a delta winding), an earthing

transformer may be used to provide a return path for single phase fault currents



In such cases the impedance of the earthing transformer may be sufficient to act as effective earthing impedance.

Additional impedance can be added in series if required. A special ‘zig-zag’ transformer is sometimes used for earthingdelta windings to provide a low zero-sequence impedance and high positive and negative sequence impedance to faultcurrents.

Conclusion:

Resistance Grounding Systems have many advantages over solidly grounded systems including arc-flash hazardreduction, limiting mechanical and thermal damage associated with faults, and controlling transient over voltages.High resistance grounding systems may also be employed to maintain service continuity and assist with locating thesource of a fault.When designing a system with resistors, the design/consulting engineer must consider the specific requirements forconductor insulation ratings, surge arrestor ratings, breaker single-pole duty ratings, and method of serving phase-to-

neutral loads.

Comparison of Neutral Earthing System:

Condition Un groundedSolidGrounded

Low ResistanceGrounded

High ResistanceGrounded

ReactanceGrounding

Immunity toTransient Overvoltages

Worse Good Good Best Best

73% Increase inVoltage StressUnder Line-to-Ground Fault

Condition

Poor Best Good Poor

Equipment Protected Worse Poor Better Best Best

Safety to Personnel Worse Better Good Best Best

Service Reliability Worse Good Better Best Best

Maintenance Cost Worse Good Better Best Best

Ease of Locating

First Ground FaultWorse Good Better Best Best

Permits Designer toCoordinateProtective

Devices

Not Possible Good Better Best Best

Reduction in

Frequency of FaultsWorse Better Good Best Best

Lighting ArrestorUngroundedneutral

type

Grounded-

neutraltype

Ungroundedneutral

type

Ungroundedneutral

type

Ungroundedneutral

type



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Current for phase-toground fault inpercent ofthree-phase fault current

Less than 1%Varies,may be100% orgreater

5 to 20% Less than 1% 5 to 25%

Reference:

By Michael D. Seal, P.E., GE Senior Specification Engineer.IEEE Standard 141-1993, “Recommended Practice for Electrical Power Distribution for Industrial Plants”Don Selkirk, P.Eng, Saskatoon, Saskatchewan Canada


EHV/HV Cable Sheath Earthing

December 21, 2011 8 Comments

7 Votes

EHV/HV Cable Sheath Earthing:

Introduction:

In urban areas, high voltage underground cables are commonly used for the transmission and distribution of electricity.Such high voltage cables have metallic sheaths or screens surrounding the conductors, and/or armour and metallic wiressurrounding the cables. During earth faults applied to directly earthed systems, these metallic paths are expected to

carry a substantial proportion of the total fault current, which would otherwise flow through the general mass of earth,while returning to system neutrals. These alternative return paths must be considered when determining the extent of thegrid potential rise at an electrical plant due to earth faults.For safety and reliable operation, the shields and metallic sheaths of power cables must be grounded. Withoutgrounding, shields would operate at a potential considerably above ground. Thus, they would be hazardous to touchand would cause rapid degradation of the jacket or other material intervening between shield and ground. This is

caused by the capacitive charging current of the cable insulation that is on the order of 1 mA/ft of conductor length.This current normally flows, at power frequency, between the conductor and the earth electrode of the cable, normallythe shield. In addition, the shield or metallic sheath provides a fault return path in the event of insulation failure,permitting rapid operation of the protection devices.In order to reduce Circulating current and electric potential difference between the sheathings of single core three-phasecables, the sheathing is grounded and bonded at one or both ends of the cables. If the cable is long, double bonding has

to be carried out which leads to circulating currents and increased total power loss. Raising the sheath’s resistance, bydecreasing its cross section and increasing its resistivity, can reduce this almost to the level of the core losses.However, in case of an earth fault, a considerable portion of the fault current flows through the increased sheathresistance, creating much higher power in the sheaths than in the faulty core. A simple solution, a conductor rod buriedinto the soil above or under the cable can divert this power from the sheaths.

Cable Screen:


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(1) Purpose of cable screen:

Cable screen controls the electric field stress in the cable insulation.Cable Screen Provides return path for Cable neutral and fault current.

If the screen is earthed at two ends than it provides Shielding for electromagnetic radiation.Enclosing dangerous high voltage with earth potential for safety.

(2) Purpose of bonding cable screens at both ends:

The electric power losses in a cable circuit are dependent on the currents flowing in the metallic sheaths of the cables soby reducing the current flows in metallic sheath by different methods of bonding we can increases the load currentcarrying capacity (ampacity) of the cable.It provides low impedance fault current return path and provides neutral point for the circuit.It provides shielding of electromagnetic field.

(3) Induced voltage & circulating circulating current in cable screen:

Electromagnetic coupling between the core and screen Electromagnetic screen.If the cable screen is single point bonded, no electrical continuity and mmf generates a voltage.If the cable screen is bonded at both ends, the mmf will cause circulating current to flow if there is electrical continuity.

The circulating current produces an opposing magnetic field.Suitable bonding method should be employed to meet the standing voltage limit and keep Circulating current to anacceptable level.

Laying Method of Cable:

The three Single core cables in a 3-phase circuit can be placed in different formations. Typical formations include trefoil(triangular) and flat formations.

(1) Trefoil Formation:

To minimize the electromechanical forces between the cables under short-circuit conditions, and to avoid eddy-currentheating in nearby steelwork due to magnetic fields set up by load currents, the three single-core cables comprising thethree phases of a 3-phase circuit are always run clamped in ‘Trefoil’ formation.

Advantage:



1. This type of Formation minimizes the sheath circulating currents induced by the magnetic flux linking the cableconductors and metallic sheath or copper wire screens.

2. This configuration is generally used for cables of lower voltages (33 to 132kV) and of smaller conductor sizes.

Disadvantages:

1. The trefoil formation is not appropriate for heat dissipation because there is an appreciable mutual heating effect of thethree cables.

2. The cumulated heat in cables and cable trench has the effect of reducing the cable rating and accelerating the cableageing.

(2) Flat Formation:

This is a most common method for Laying LT Cable.

This formation is appropriate for heat dissipation and to increase cable rating.The Formation choice is totally deepened on several factors like screen bonding method, conductor area and availablespace for installation.

Type of Core and Induced Voltage:

(1) Three Core Cable:

For LT application, typically for below 11 kV.Well balanced magnetic field from Three Phase.Induced voltages from three phases sum to zero along the entire length of the cable.Cable screen should be earthed at both endsVirtually zero induced voltage or circulating current under steady state operation.

(2) Single Core Cable:

For HV application, typically for 11 kV and above.Single–core cables neglects the use of ferromagnetic material for screen, sheath and armoring.Induced voltage is mainly contributed by the core currents in its own phase and other two phases.If cables are laid in acompact and symmetrical formation, induced in the screen can be minimized.A suitable screen bonding method should be used for single–core cables to prevent Excessive circulating current, highinduced standing voltage.igh voltage.

Accessories for HT Cable Sheath Bonding:

(1) Function of Link Box?

Link Box is electrically and mechanically one of the integral accessories of HV underground above ground cablebonding system, associated with HV XLPE power cable systems.Link boxes are used with cable joints and terminations to provide easy access to shield breaks for test purposes and to

limit voltage build-up on the sheath



Lightning, fault currents and switching operations can cause over voltages on the cable sheath. The link box optimizes

loss management in the cable shield on cables grounded both sides.In HT Cable the bonding system is so designed that the cable sheaths are bonded and earthed or with SVL in such wayas to eliminate or reduce the circulating sheath currents.Link Boxes are used with cable joints and terminations to provide easy access to shield breaks for test purposes and tolimit voltage build-up on the sheath. The link box is part of bonding system, which is essential of improving currentcarrying capacity and human protection.

(2) Sheath Voltage Limiters (SVL) (Surge Arrestors):

SVL is protective device to limit induce voltages appearing on the bonded cable system due to short circuit.It is necessary to fit SVL’s between the metallic screen and ground inside the link box. The screen separation of power

cable joint (insulated joint) will be protected against possible damages as a result of induced voltages caused by shortcircuit/break down.

Type of Sheath Bonding for HT Cable:

There is normally Three Type of Bonding for LT/HT Cable Screen.

(1) Single Point Bonded.

1. One Side Single Point Bonded System.2. Split Single Point Bonded System.

(2) Both End Bonded System

(3) Cross Bonded System

(1) Single point bonded system:

(A) One Side Single Bonded System:

A system is single point bonded if the arrangements are such that the cable sheaths provide no path for the flow ofcirculating currents or external fault currents.This is the simplest form of special bonding. The sheaths of the three cable sections are connected and grounded at onepoint only along their length. At all other points there will be a voltage between sheath and ground and between

screens of adjacent phases of the cable circuit that will be at its maximum at the farthest point from the ground bond.This induced voltage is proportional to the cable length and current. Single-point bonding can only be used for limitedroute lengths, but in general the accepted screen voltage potential limits the length



The sheaths must therefore be adequately insulated from ground. Since there is no closed sheath circuit, except throughthe sheath voltage limiter, current does not normally flow longitudinally along the sheaths and no sheath circulationcurrent loss occurs.

Open circuit in cable screen, no circulating current.Zero volt at the earthed end, standing voltage at the unearthed end.Optional PVC insulated earth continuity conductor required to provide path for fault current, if returning from earth isundesirable, such as in a coal mine.SVL installed at the unearthed end to protect the cable insulation during fault conditions.Induced voltage proportional to the length of the cable and the current carried in the cable .Zero volt with respect to the earth grid voltage at the earthed end, standing voltage at the unearthed end.Circulating current in the earth–continuity conductor is not significant, as magnetic fields from phases are partiallybalanced.The magnitude of the standing voltage is depended on the magnitude of the current flows in the core, much higher ifthere is an earth fault.High voltage appears on the unearthed end can cause arcing and damage outer PVC sheath.

The voltage on the screen during a fault also depends on the earthing condition.

Standing voltage at the unearthed end during earth fault condition.

During a ground fault on the power system the zero sequence current carried by the cable conductors could return bywhatever external paths are available. A ground fault in the immediate vicinity of the cable can cause a large differencein ground potential rise between the two ends of the cable system, posing hazards to personnel and equipment. For this reason, single-point bonded cable installations need a parallel ground conductor, grounded at bothends of the cable route and installed very close to the cable conductors, to carry the fault current during ground faultsand to limit the voltage rise of the sheath during ground faults to an acceptable level.The parallel ground continuity conductor is usually insulated to avoid corrosion and transposed, if the cables are nottransposed, to avoid circulating currents and losses during normal operating conditions.

Voltage at the unearthed end during an earth fault consists of two voltage components. Induced voltage due tofault current in the core.

Advantage:

No circulating current.No heating in the cable screen.

Economical.

Disadvantage:

Standing voltage at the un–earthed end.



Requires SVL if standing voltage during fault is excessive.Requires additional earth continuity conductor for fault current if earth returned current is undesirable. Highermagnetic fields around the cable compared to solidly bonded system.Standing voltage on the cable screen is proportional to the length of the cable and the magnitude of current in the core.Typically suitable for cable sections less than 500 m, or one drum length.

(B) Split Single Point-bonded System:

It is also known as double length single point bonding System.Cable screen continuity is interrupted at the midpoint and SVLs need to be fitted at each side of the isolation joint.Other requirements are identical to single–point–bonding system like SVL, Earth continuity Conductor, Transposition ofearth continuity conductor.Effectively two sections of single–point–bonding.No circulating current and Zero volt at the earthed ends, standing voltage at the sectionalizing joint.

Advantages:

No circulating current in the screen.No heating effect in the cable screen.Suitable for longer cable section compared to single–point–bonding system and solidly bonded single-core system.Economical.

Disadvantages:

Standing voltage exists at the screen and sectionalizing insulation joint.Requires SVL to protect the un–earthed end.Requires separate earth continuity conductor for zero sequence current.Not suitable for cable sections over 1000 m.Suitable for 300~1000 m long cable sections, double the length of single–point–bonding system.

(2) Both End Solidly Bonded (Single-core cable) systems.

Most Simple and Common method.Cable screen is bonded to earth grids at both ends (via link box).To eliminate the induced voltages in Cable Screen is to bond (Earth) the sheath at both ends of the cable circuit. This eliminates the need for the parallel continuity conductor used in single bonding systems. It also eliminates the needto provide SVL, such as that used at the free end of single-point bonding cable circuitsSignificant circulating current in the screen Proportional to the core current and cable length and de rates cable.Could lay cable in compact trefoil formation if permissible.Suitable for route length of more than 500 Meter.

Very small standing voltage in the order of several volts.



Advantages:

Minimum material required.

Most economical if heating is not a main issue.Provides path for fault current, minimizing earth return current and EGVR at cable destination.Does not require screen voltage limiter (SVL).Less electromagnetic radiation.

Disadvantages:

Provides path for circulating current.Heating effects in cable screen, greater losses .Cable therefore might need to be de–rated or larger cable required.Transfers voltages between sites when there is an EGVR at one site.Can lay cables in trefoil formation to reduce screen losses .Normally applies to short cable section of tens of meters long. Circulating current is proportional to the length of thecable and the magnitude of the load current.

(3) Cross-bonded cable system.

A system is cross-bonded if the arrangements are such that the circuit provides electrically continuous sheath runs fromearthed termination to earthed termination but with the sheaths so sectionalized and cross-connected in order to reducethe sheath circulating currents.In This Type voltage will be induced between screen and earth, but no significant current will flow.The maximum induced voltage will appear at the link boxes for cross-bonding. This method permits a cable current-carrying capacity as high as with single-point bonding but longer route lengths than the latter. It requires screenseparation and additional link boxes.For cross bonding, the cable length is divided into three approximately equal sections. Each of the three alternatingmagnetic fields induces a voltage with a phase shift of 120° in the cable shields.The cross bonding takes place in the link boxes. Ideally, the vectorial addition of the induced voltages results in U(Rise) = 0. In practice, the cable length and the laying conditions will vary, resulting in a small residual voltage and a

negligible current. Since there is no current flow, there are practically no losses in the screen.The total of the three voltages is zero, thus the ends of the three sections can be grounded.Summing up induced voltage in sectionalized screen from each phase resulting in neutralization of induced voltages inthree consecutive minor sections.Normally one drum length (500 m approx) per minor section.Sectionalizing position and cable jointing position should be coincident.Solidly earthed at major section joints.



Transpose cable core to balance the magnitude of induced voltages to be summed up.Link box should be used at every sectionalizing joint and balanced impedance in all phases.

Induced voltage magnitude profile along the screen of a major section in the cross–bonding cable system.Virtually zero circulating current and Voltage to the remote earth at the solidly earthed ends.In order to obtain optimal result, two ‘‘crosses’’ exist. One is Transposition of cable core crossing cable core at eachsection and second is Cross bond the cable screens effectively no transposition of screen.Cross bonding of cable screen: It is cancelled induced voltage in the screen at every major Section joint.Transposition of cables: It is ensure voltages to be summed up have similar magnitude .Greater standing voltage atthe screen of the outer cable.Standing voltages exist at screen and majority of section joints cable and joints must be installed as an insulated screensystem.

Requirement of transpose for cables core.

If core not transposed, not well neutralized resulting in some circulating currents.Cable should be transposed and the screen needs to be cross bonded at each sectionalizing joint position for optimalneutralization

Advantage:

Not required any earth continuity conductor.Virtually zero circulating current in the screen.



Standing voltage in the screen is controlled.Technically superior than other methods.Suitable for long distance cable network.

Disadvantage:

Technically complicated.More expensive.

Bonding Method Comparison:

Earthing MethodStanding

Voltage atCable End

SheathVoltageLimiter

Required

Application

Single End BondingYes Yes

Up to 500 Meter

Double End BondingNo No

Up to 1 Km and Substations short connections, hardly applied for HVcables, rather for MV and LV cables

Cross BondingOnly at cross

bonding pointsYes

Long distance connectionswhere jointsare required

Sheath Losses according to type of Bonding:

Sheath losses are current-dependent losses and are generated by the induced currents when load current flows in cableconductors.The sheath currents in single-core cables are induced by “transformer” effect; i.e.by the magnetic field of alternating

current flowing in cable conductor which induces voltages in cable sheath or other parallel conductors.The sheath induced electromotive forces (EMF) generate two types of losses: circulating current losses (Y1) and eddycurrent losses (Y2), so the total losses in cable metallic sheath are: Y= Y1+Y2The eddy currents circulating radially and longitudinally of cable sheaths are generated on similar principles of skin andproximity effects i.e. they are induced by the conductor currents, sheath circulating currents and by currents circulatingin close proximity current carrying conductors.They are generated in cable sheath irrespective of bonding system of single core cables or of three-core cablesThe eddy currents are generally of smaller magnitude when comparing with circuit (circulating) currents of solidlybonded cable sheaths and may be neglects except in the case of large segmental conductors and are calculated inaccordance with formulae given in the IEC60287.Circulating currents are generated in cable sheath if the sheaths form a closed loop when bonded together at the remoteends or intermediate points along the cable route.

These losses are named sheath circulating current losses and they are determined by the magnitude of current in cableconductor, frequency, mean diameter, the resistance of cable sheath and the distance between single-core cables.

Conclusion:

There is much disagreement as to whether the cable shield should be grounded at both ends or at only one end. If



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grounded at only one end, any possible fault current must traverse the length from the fault to the grounded end,imposing high current on the usually very light shield conductor. Such a current could readily damage or destroy theshield and require replacement of the entire cable rather than only the faulted section.With both ends grounded, the fault current would divide and flow to both ends, reducing the duty on the shield, withconsequently less chance of damage.Multiple grounding, rather than just grounding at both ends, is simply the grounding of the cable shield or sheath at allaccess points, such as manholes or pull boxes. This also limits possible shield damage to only the faulted section.

References:

1. Mitton Consulting.2. EMElectricals


Abstract of NEC for Size of Cable for Single or Group of Motors


6 Votes

Abstract of National Electrical Code for Size of Cable for Motors:

NEC Code 430.22 (Size of Cable for Single Motor):

Size of Cable for Branch circuit which has Single Motor connection is 125% of Motor Full Load Current Capacity.Example:what is the minimum rating in amperes for Cables supplying 1 No of 5 hp, 415-volt, 3-phase motor at 0.8Power Factor.

Full-load currents for 5 hp = 7Amp.Min Capacity of Cable= (7X125%) =8.75 Amp.

NEC Code 430.6(A) (Size of Cable for Group of Motors or Elect.Load).

Cables or Feeder which is supplying more than one motors other load(s), shall have an ampacity not less than 125 % ofthe full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motorsin the group, as determined by 430.6(A).For Calculating minimum Ampere Capacity of Main feeder and Cable is 125% of Highest Full Load Current +Sum of Full Load Current of remaining Motors.Example:what is the minimum rating in amperes for Cables supplying 1 No of 5 hp, 415-volt, 3-phase motor at 0.8Power Factor , 1 No of 10 hp, 415-volt, 3-phase motor at 0.8 Power Factor, 1 No of 15 hp, 415-volt, 3-phasemotor at 0.8 Power Factor and 1 No of 5hp, 230-volt, single-phase motor at 0.8 Power Factor?

Full-load currents for 5 hp = 7Amp

Full-load currents for 10 hp = 13AmpFull-load currents for 15 hp = 19AmpFull-load currents for 10 hp (1 Ph) = 21AmpHere Capacity wise Large Motor is 15 Hp but Highest Full Load current is 21Amp of 5hp Single Phase Motorso 125% of Highest Full Load current is 21X125%=26.25Amp


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Min Capacity of Cable= (26.25+7+13+19) =65.25 Amp.

NEC Code 430.24 (Size of Cable for Group of Motors or Electrical Load).

As specified in 430.24, conductors supplying two or more motors must have an ampacity not less than 125 % of thefull-load current rating of the highest rated motor + the sum of the full-load current ratings of all the other motors in thegroup or on the same phase. It may not be necessary to include all the motors into the calculation. It is permissible to balance the motors as evenlyas possible between phases before performing motor-load calculations.

Example:what is the minimum rating in amperes for conductors supplying 1No of 10 hp, 415-volt, 3-phase motor at0.8 P.F and 3 No of 3 hp, 230-volt, single-phase motors at 0.8 P.F.

The full-load current for a 10 hp, 415-volt, 3-phase motor is 13 amperes.The Full-load current for single-phase 3 hp motors is 12 amperes.Here for Load Balancing one Single Phase Motor is connected on R Phase Second in B Phase and third is in YPhase.Because the motors are balanced between phases, the full-load current on each phase is 25 amperes (13 + 12 =25).Here multiply 13 amperes by 125 %=(13 × 125% = 16.25 Amp). Add to this value the full-load currents of theother motor on the same phase (16.25 + 12 = 28.25 Amp). The minimum rating in amperes for conductors supplying these motors is 28 amperes.

NEC 430/32 Size of Overload Protection for Motor:

Overload protection (Heater or Thermal cut out protection) would be a device that thermally protects a given motorfrom damage due to heat when loaded too heavy with work.All continuous duty motors rated more than 1HP must have some type of an approved overload device.An overload shall be installed on each conductor that controls the running of the motor rated more than onehorsepower. NEC 430/37 plus the grounded leg of a three phase grounded system must contain an overload also. ThisGrounded leg of a three phase system is the only time you may install an overload or over – current device on agrounded conductor that is supplying a motor.To Find the motor running overload protection size that is required, you must multiply the F.L.C. (full load current) withthe minimum or the maximum percentage ratings as follows;

Maximum Overload

Maximum overload = F.L.C. (full load current of a motor) X allowable % of the maximum setting of an overload,

130% for motors, found in NEC Article 430/34.Increase of 5% allowed if the marked temperature rise is not over 40 degrees or the marked service factor is not lessthan 1.15.

Minimum Overload

Minimum Overload = F.L.C. (full load current of a motor) X allowable % of the minimum setting of an overload,115% for motors found in NEC Article 430/32/B/1.Increase of 10% allowed to 125% if the marked temperature rise is not over 40 degrees or the marked service factor isnot less than 1.15.


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HIPOT Testing


4 Votes

What is HIPOT Testing (Dielectric Strength Test):

Hipot Test is short name of high potential (high voltage) Teat and It also known as Dielectric Withstand Test. A hipottest checks for “good isolation.” Hipot test makes surety of no current will flow from one point to another point.Hipot test is the opposite of a continuity test.Continuity Test checks surety of current flows easily from one point to another point while Hipot Test checks surety ofcurrent would not flow from one point to another point (and turn up the voltage really high just to make sure no currentwill flow).

Importance of HIPOT Testing:

The hipot test is a nondestructive test that determines the adequacy of electrical insulation for the normally occurringover voltage transient. This is a high-voltage test that is applied to all devices for a specific time in order to ensure thatthe insulation is not marginal.Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive orcorrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in cables. Inadequatecreepage and clearance distances introduced during the manufacturing process.HIPOT test is applied after tests such as fault condition, humidity, and vibration to determine whether any degradation

has taken place.The production-line hipot test, however, is a test of the manufacturing process to determine whether the construction ofa production unit is about the same as the construction of the unit that was subjected to type testing. Some of theprocess failures that can be detected by a production-line hipot test include, for example, a transformer wound in such away that creepage and clearance have been reduced. Such a failure could result from a new operator in the windingdepartment. Other examples include identifying a pinhole defect in insulation or finding an enlarged solder footprint.As per IEC 60950, The Basic test Voltage for Hipot test is the 2X (Operating Voltage) + 1000 VThe reason for using 1000 V as part of the basic formula is that the insulation in any product can be subjected to normalday-to-day transient over voltages. Experiments and research have shown that these over voltages can be as high as1000 V.

Test method for HIPOT Test:

Hipot testers usually connect one side of the supply to safety ground (Earth ground). The other side of the supply isconnected to the conductor being tested. With the supply connected like this there are two places a given conductor



can be connected: high voltage or ground.When you have more than two contacts to be hipot tested you connect one contact to high voltage and connect allother contacts to ground. Testing a contact in this fashion makes sure it is isolated from all other contacts.

If the insulation between the two is adequate, then the application of a large voltage difference between the twoconductors separated by the insulator would result in the flow of a very small current. Although this small current isacceptable, no breakdown of either the air insulation or the solid insulation should take place.Therefore, the current of interest is the current that is the result of a partial discharge or breakdown, rather than thecurrent due to capacitive coupling.

Time Duration for HIPOT Test:

The test duration must be in accordance with the safety standard being used.The test time for most standards, including products covered under IEC 60950, is 1 minute.A typical rule of thumb is 110 to 120% of 2U + 1000 V for 1–2 seconds.

Current Setting for HIPOT Test:

Most modern hipot testers allow the user to set the current limit. However, if the actual leakage current of the product isknown, then the hipot test current can be predicted.The best way to identify the trip level is to test some product samples and establish an average hipot current. Once thishas been achieved, then the leakage current trip level should be set to a slightly higher value than the average figure.Another method of establishing the current trip level would be to use the following mathematical formula: E(Hipot) /E(Leakage) = I(Hipot) / 2XI(Leakage)The hipot tester current trip level should be set high enough to avoid nuisance failure related to leakage current and, atthe same time, low enough not to overlook a true breakdown in insulation.

Test Voltage for HIPOT Test:

The majority of safety standards allow the use of either ac or dc voltage for a hipot test.When using ac test voltage, the insulation in question is being stressed most when the voltage is at its peak, i.e., either atthe positive or negative peak of the sine wave.Therefore, if we use dc test voltage, we ensure that the dc test voltage is under root 2 (or 1.414) times the ac testvoltage, so the value of the dc voltage is equal to the ac voltage peaks. For example, for a 1500-V-ac voltage, the equivalent dc voltage to produce the same amount of stress on theinsulation would be 1500 x 1.414 or 2121 V dc.

Advantage / Disadvantage of use DC Voltage for Hipot Test:

One of the advantages of using a dc test voltage is that the leakage current trip can be set to a much lower value thanthat of an ac test voltage. This would allow a manufacturer to filter those products that have marginal insulation, which

would have been passed by an ac tester.when using a dc hipot tester, the capacitors in the circuit could be highly charged and, therefore, a safe-dischargedevice or setup is needed. However, it is a good practice to always ensure that a product is discharged, regardless ofthe test voltage or its nature, before it is handled.It applies the voltage gradually. By monitoring the current flow as voltages increase, an operator can detect a potentialinsulation breakdown before it occurs. A minor disadvantage of the dc hipot tester is that because dc test voltages aremore difficult to generate, the cost of a dc tester may be slightly higher than that of an ac tester.The main advantage of the dc test is DC Voltage does not produce harmful discharge as readily occur in AC.It can be applied at higher levels without risk or injuring good insulation. This higher potential can literally “sweep-out”far more local defects.The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage



current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thusproducing a “snow balling” effect which leads to the small visible dielectric puncture usually observed. Since the dc isfree of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in thedielectric which have lower resistance.

Advantage / Disadvantage of use AC Voltage for Hipot Test:

One of the advantages of an ac hipot test is that it can check both voltage polarities, whereas a dc test charges the

insulation in only one polarity. This may become a concern for products that actually use ac voltage for their normaloperation. The test setup and procedures are identical for both ac and dc hipot tests.A minor disadvantage of the ac hipot tester is that if the circuit under test has large values of Y capacitors, then,depending on the current trip setting of the hipot tester, the ac tester could indicate a failure. Most safety standardsallow the user to disconnect the Y capacitors prior to testing or, alternatively, to use a dc hipot tester. The dc hipottester would not indicate the failure of a unit even with high Y capacitors because the Y capacitors see the voltage butdon’t pass any current.

Step for HIPOT Testing:

Only electrically qualified workers may perform this testing.Open circuit breakers or switches to isolate the circuit or Cable that will be hi-pot tested.

Confirm that all equipment or Cable that is not to be tested is isolated from the circuit under test.The limited approach boundary for this hi-pot procedure at 1000 volts is 5 ft. (1.53m) so place barriers around theterminations of cables and equipment under test to prevent unqualified persons from crossing this boundary.Connect the ground lead of the HIPOT Tester to a suitable building ground or grounding electrode conductor. Attachthe high voltage lead to one of the isolated circuit phase conductors.Switch on the HIPOT Tester. Set the meter to 1000 Volts or pre decide DC Voltage. Push the “Test” button on themeter and after one minute observe the resistance reading. Record the reading for reference.At the end of the one minute test, switch the HIPOT Tester from the high potential test mode to the voltage measuringmode to confirm that the circuit phase conductor and voltage of HIPOT Tester are now reading zero volts.Repeat this test procedure for all circuit phase conductors testing each phase to ground and each phase to each phase.When testing is completed disconnect the HIPOT Tester from the circuits under test and confirm that the circuits areclear to be re-connected and re-energized.

To PASS the unit or Cable under Test must be exposed to a minimum Stress of pre decide Voltage for 1 minutewithout any Indication of Breakdown. For Equipments with total area less than 0.1 m2, the insulation resistance shallnot be less than 400 MΩ. For Equipment with total area larger than 0.1 m2 the measured insulation resistance times thearea of the module shall not be less than 40 MΩ⋅m2.

Safety precautions during HIPOT Test:

During a HIPOT Test, There may be at some risk so to minimize risk of injury from electrical shock make sure HIPOTequipment follows these guidelines:

1. The total charge you can receive in a shock should not exceed 45 uC.2. The total hipot energy should not exceed 350 mJ.3. The total current should not exceed 5 mA peak (3.5 mA rms)4. The fault current should not stay on longer than 10 mS.

5. If the tester doesn’t meet these requirements then make sure it has a safety interlock system that guarantees you cannotcontact the cable while it is being hipot tested.

For Cable:



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1. Verify the correct operation of the safety circuits in the equipment every time you calibrate it.2. Don’t touch the cable during hipot testing.3. Allow the hipot testing to complete before removing the cable.4. Wear insulating gloves.5. Don’t allow children to use the equipment.6. If you have any electronic implants then don’t use the equipment.


What is Earthing

November 27, 2011 12 Comments

4 Votes

The main reason for doing earthing in electrical network is for the safety. When all metallic parts in electrical equipments aregrounded then if the insulation inside the equipments fails there are no dangerous voltages present in the equipment case. If thelive wire touches the grounded case then the circuit is effectively shorted and fuse will immediately blow. When the fuse isblown then the dangerous voltages are away.

Purpose of Earthing:

(1) Safety for Human life/ Building/Equipments:

To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the

fault current to flow so that it will not endanger the userTo protect buildings, machinery & appliances under fault conditions.To ensure that all exposed conductive parts do not reach a dangerous potential.To provide safe path to dissipate lightning and short circuit currents.To provide stable platform for operation of sensitive electronic equipments i.e. To maintain the voltage at any part ofan electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment.

(2) Over voltage protection:

Lightning, line surges or unintentional contact with higher voltage lines can cause dangerously high voltages to theelectrical distribution system. Earthing provides an alternative path around the electrical system to minimize damages inthe System.

(3) Voltage stabilization:

There are many sources of electricity. Every transformer can be considered a separate source. If there were not acommon reference point for all these voltage sources it would be extremely difficult to calculate their relationships to

each other. The earth is the most omnipresent conductive surface, and so it was adopted in the very beginnings ofelectrical distribution systems as a nearly universal standard for all electric systems.

Conventional methods of earthing:


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(1) Plate type Earthing:

Generally for plate type earthing normal Practice is to useCast iron plate of size 600 mm x600 mm x12 mm. ORGalvanized iron plate of size 600 mm x600 mm x6 mm. ORCopper plate of size 600 mm * 600 mm * 3.15 mm

Plate burred at the depth of 8 feet in the vertical position and GI strip of size 50 mmx6 mm bolted with the plate isbrought up to the ground level.These types of earth pit are generally filled with alternate layer of charcoal & salt up to 4 feet from the bottom of the pit.

(2) Pipe type Earthing:

For Pipe type earthing normal practice is to useGI pipe [C-class] of 75 mm diameter, 10 feet long welded with 75 mm diameter GI flange having 6 numbers of holesfor the connection of earth wires and inserted in ground by auger method.These types of earth pit are generally filled with alternate layer of charcoal & salt or earth reactivation compound.

Method for Construction of Earthing Pit (Indian Electricity Board):

Excavation on earth for a normal earth Pit size is 1.5M X 1.5M X 3.0 M.Use 500 mm X 500 mm X 10 mm GI Plate or Bigger Size for more Contact of Earth and reduce Earth Resistance. Make a mixture of Wood Coal Powder Salt & Sand all in equal part Wood Coal Powder use as good conductor of electricity, anti corrosive, rust proves for GI Plate for long life.The purpose of coal and salt is to keep wet the soil permanently.The salt percolates and coal absorbs water keeping the soil wet.Care should always be taken by watering the earth pits in summer so that the pit soil will be wet.Coal is made of carbon which is good conductor minimizing the earth resistant.

Salt use as electrolyte to form conductivity between GI Plate Coal and Earth with humidity.Sand has used to form porosity to cycle water & humidity around the mixture.Put GI Plate (EARTH PLATE) of size 500 mm X 500 mm X 10 mm in the mid of mixture.Use Double GI Strip size 30 mm X 10 mm to connect GI Plate to System Earthling. It will be better to use GI Pipe of size 2.5″ diameter with a Flange on the top of GI Pipe to cover GI Strip fromEARTH PLATE to Top Flange.Cover Top of GI pipe with a T joint to avoid jamming of pipe with dust & mud and also use water time to time throughthis pipe to bottom of earth plate.Maintain less than one Ohm Resistance from EARTH PIT conductor to a distance of 15 Meters around the EARTHPIT with another conductor dip on the Earth at least 500 mm deep.Check Voltage between Earth Pit conductors to Neutral of Mains Supply 220V AC 50 Hz it should be less than 2.0Volts.

Factors affecting on Earth resistivity:

(1) Soil Resistivity:

It is the resistance of soil to the passage of electric current. The earth resistance value (ohmic value) of an earth pitdepends on soil resistivity. It is the resistance of the soil to the passage of electric current.It varies from soil to soil. It depends on the physical composition of the soil, moisture, dissolved salts, grain size anddistribution, seasonal variation, current magnitude etc.

In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its distribution, seasonalvariation, current magnitude.



(2) Soil Condition:

Different soil conditions give different soil resistivity. Most of the soils are very poor conductors of electricity when theyare completely dry. Soil resistivity is measured in ohm-meters or ohm-cm.Soil plays a significant role in determining the performance of Electrode.Soil with low resistivity is highly corrosive. If soil is dry then soil resistivity value will be very high.If soil resistivity is high, earth resistance of electrode will also be high.

(3) Moisture:

Moisture has a great influence on resistivity value of soil. The resistivity of a soil can be determined by the quantity of

water held by the soil and resistivity of the water itself. Conduction of electricity in soil is through water.The resistance drops quickly to a more or less steady minimum value of about 15% moisture. And further increase ofmoisture level in soil will have little effect on soil resistivity. In many locations water table goes down in dry weatherconditions. Therefore, it is essential to pour water in and around the earth pit to maintain moisture in dry weatherconditions. Moisture significantly influences soil resistivity

(4) Dissolved salts:

Pure water is poor conductor of electricity.Resistivity of soil depends on resistivity of water which in turn depends on the amount and nature of salts dissolved in it.

Small quantity of salts in water reduces soil resistivity by 80%. common salt is most effective in improving conductivityof soil. But it corrodes metal and hence discouraged.

(5) Climate Condition:

Increase or decrease of moisture content determines the increase or decrease of soil resistivity.Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low.

(6) Physical Composition:

Different soil composition gives different average resistivity. Based on the type of soil, the resistivity of clay soil may bein the range of 4 – 150 ohm-meter, whereas for rocky or gravel soils, the same may be well above 1000 ohm-meter.

(7) Location of Earth Pit :

The location also contributes to resistivity to a great extent. In a sloping landscape, or in a land with made up of soil, orareas which are hilly, rocky or sandy, water runs off and in dry weather conditions water table goes down very fast. Insuch situation Back fill Compound will not be able to attract moisture, as the soil around the pit would be dry. The earthpits located in such areas must be watered at frequent intervals, particularly during dry weather conditions.Though back fill compound retains moisture under normal conditions, it gives off moisture during dry weather to the drysoil around the electrode, and in the process loses moisture over a period of time. Therefore, choose a site that is

naturally not well drained.

(8) Effect of grain size and its distribution:

Grain size, its distribution and closeness of packing are also contributory factors, since they control the manner in whichthe moisture is held in the soil.Effect of seasonal variation on soil resistivity: Increase or decrease of moisture content in soil determines decrease orincrease of soil resistivity. Thus in dry weather resistivity will be very high and during rainy season the resistivity will below.

(9) Effect of current magnitude:



Soil resistivity in the vicinity of ground electrode may be affected by current flowing from the electrode into thesurrounding soil.The thermal characteristics and the moisture content of the soil will determine if a current of a given magnitude and

duration will cause significant drying and thus increase the effect of soil resistivity

(10) Area Available:

Single electrode rod or strip or plate will not achieve the desired resistance alone. If a number of electrodes could be installed and interconnected the desired resistance could be achieved. The distancebetween the electrodes must be equal to the driven depth to avoid overlapping of area of influence. Each electrode,therefore, must be outside the resistance area of the other.

(11) Obstructions:

The soil may look good on the surface but there may be obstructions below a few feet like virgin rock. In that eventresistivity will be affected. Obstructions like concrete structure near about the pits will affect resistivity. If the earth pitsare close by, the resistance value will be high.

(12) Current Magnitude:

A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soilresistivity.

Measurement of Earth Resistance by use of Earth Tester:

For measuring soil resistivity Earth Tester is used. It is also called the “MEGGER”.It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connectterminal to Earth Electrode and Spikes.It is measured by using Four Terminal Earth Tester Instrument. The terminals are connected by wires as in illustration.P=Potential Spike and C=Current Spike. The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, and50M.

All spikes are equidistant and in straight line to maintain electrical continuity. Take measurement in different directions.Soil resistivity =2πLR.R= Value of Earth resistance in ohm.Distance between the spikes in cm.π = 3.14P = Earth resistivity ohm-cm.Earth resistance value is directly proportional to Soil resistivity value

Measurement of Earth Resistance (Three point method):

In this method earth tester terminal C1 & P1 are shorted to each other and connected to the earth electrode (pipe)



under test.Terminals P2 & C2 are connected to the two separate spikes driven in earth. These two spikes are kept in same line atthe distance of 25 meters and 50 meters due to which there will not be mutual interference in the field of individualspikes.If we rotate generator handle with specific speed we get directly earth resistance on scale.Spike length in the earth should not be more than 1/20th distance between two spikes.

Resistance must be verified by increasing or decreasing the distance between the tester electrode and the spikes by 5meter. Normally, the length of wires should be 10 and 15 Meter or in proportion of 62% of ‘D’.Suppose, the distance of Current Spike from Earth Electrode D = 60 ft, Then, distance of Potential Spike would be 62% of D = 0.62D i.e. 0.62 x 60 ft = 37 ft.

Four Point Method:

In this method 4 spikes are driven in earth in same line at the equal distance. Outer two spikes are connected to C1 &C2 terminals of earth tester. Similarly inner two spikes are connected to P1 & P2 terminals. Now if we rotategenerator handle with specific speed, we get earth resistance value of that place.In this method error due to polarization effect is eliminated and earth tester can be operated directly on A.C.

GI Earthing Vs Copper Earthing:

As per IS 3043, the resistance of Plate electrode to earth (R) = (r/A) X under root(P/A).Where r = Resistivity of Soil Ohm-meter.A=Area of Earthing Plate m3.The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d).Where L= Length of Pipe/Rod in cmd=Diameter of Pipe/Rod in cm.The resistivity of the soil and the physical dimensions of the electrode play important role of resistance of Rod with

earth.The material resistivity is not considered important role in earth resistivity.Any material of given dimensions would offer the same resistance to earth. Except the sizing and number of the earthingconductor or the protective conductor.

Pipe Earthing Vs Plate Earthing:

Suppose Copper Plate having of size 1.2m x 1.2m x 3.15mm thick. soil resistivity of 100 ohm-m,The resistance of Plate electrode to earth (R)=( r/A)X under root(π/A) = (100/2.88)X(3.14/2.88)=36.27 ohmNow, consider a GI Pipe Electrode of 50 mm Diameter and 3 m Long. soil resistivity of 100 Ohm-m,The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d) = (100X100/2X3.14X300) X loge(4X300/5) =29.09 Ohm.From the above calculation the GI Pipe electrode offers a much lesser resistance than even a copper plate electrode.As per IS 3043 Pipe, rod or strip has a much lower resistance than a plate of equal surface area.

Length of Pipe Electrode and Earthing Pit:

The resistance to earth of a pipe or plate electrode reduces rapidly within the first few feet from ground (mostly 2 to 3meter) but after that soil resistivity is mostly uniform.

After about 4 meter depth, there is no appreciable change in resistance to earth of the electrode. Except a number ofrods in parallel are to be preferred to a single long rod.



Amount of Salt and Charcoal (more than 8Kg) :

To reduce soil resistivity, it is necessary to dissolve in the moisture particle in the Soil.

Some substance like Salt/Charcoal is highly conductive in water solution but the additive substance would reduce theresistivity of the soil, only when it is dissolved in the moisture in the soil after that additional quantity does not serve thePurpose.5% moisture in Salt reduces earth resistivity rapidly and further increase in salt content will give a very little decrease insoil resistivity.The salt content is expressed in percent by weight of the moisture content in the soil. Considering 1M3 of Soil, themoisture content at 10 percent will be about 144 kg. (10 percent of 1440 kg). The salt content shall be 5% of this (i.e.)5% of 144kg, that is, about 7.2kg.

Amount of Water Purring:

Moisture content is one of the controlling factors of earth resistivity.Above 20 % of moisture content, the resistivity is very little affected. But below 20% the resistivity increases rapidlywith the decrease in moisture content.If the moisture content is already above 20% there is no point in adding quantity of water into the earth pit, exceptperhaps wasting an important and scarce national resource like water.

Length Vs Diameter of Earth Electrode:

Apart from considerations of mechanical strength, there is little advantage to be gained from increasing the earthelectrode diameter with the object in mind of increasing surface area in contact with the soil.The usual practice is to select a diameter of earth electrode, which will have enough strength to enable it to be driveninto the particular soil conditions without bending or splitting. Large diameter electrode may be more difficult to drivethan smaller diameter electrode.The depth to which an earth electrode is driven has much more influence on its electrical resistance characteristics than

has its diameter.

Maximum allowable Earth resistance:

Major power station= 0.5 Ohm.Major Sub-stations= 1.0 Ohm

Minor Sub-station = 2 OhmNeutral Bushing. =2 OhmService connection = 4 OhmMedium Voltage Network =2 OhmL.T.Lightening Arrestor= 4 OhmL.T.Pole= 5 OhmH.T.Pole =10 OhmTower =20-30 Ohm

Treatments to for minimizing Earth resistance:

Remove Oxidation on joints and joints should be tightened.Poured sufficient water in earth electrode.Used bigger size of Earth Electrode.Electrodes should be connected in parallel.



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Earth pit of more depth & width- breadth should be made.


Abstract of National Electrical Code for Transformer’s Protection

November 3, 2011 1 Comment

8 Votes

Abstract of National Electrical Code for Transformer’s Protection:

NEC, Code 450.4: (Calculate over current Protection on the Primary)

According to NEC 450.4, “each transformer 600 volts, nominal, or less shall be protected by an individual over currentdevice installed in series with each ungrounded input conductor.Such over current device shall be rated or set at not more than 125% of the rated full-load input current of the autotransformer. Further, according to NEC Table 450.3(B), if the primary current of the transformer is less than 9 amps, an overcurrent device rated or set at not more than 167% of the primary current shall be permitted. Where the primary current

is less than 2 amps, an over current device rated or set at not more than 300% shall be permitted.Example: Decide Size of circuit breaker (over current protection device) is required on the primary side to protect a75kva 440v-230v 3ø transformer.75kva x 1,000 = 75,000va75,000va / (440V x √3) = 98.41 amps.The current (amps) is more than 9 amps so use 125% rating.123 amps x 1.25 = 112.76 ampsUse 125amp 3-pole circuit breaker (the next highest fuse/fixed-trip circuit breaker size per NEC 240.6).The over current device on the primary side must be sized based on the transformer KVA rating and not sized basedon the secondary load to the transformer.

NEC, Code 450.3B: (Calculate over current Protection on the Secondary)

According to NEC Table 450.3(B), where the secondary current of a transformer is 9 amps or more and 125% of thiscurrent does not correspond to a standard rating of a fuse or circuit breaker, the next higher standard rating shall berequired. Where the secondary current is less than 9 amps, an over current device rated or set at not more than 167%of the secondary current shall be permitted.Example: Decide Size of circuit breaker (over current protection device) is required on the secondary side to protect a75kva 440v-230v 3ø transformer.We have Calculate the secondary over current protection based on the size of the transformer, not the total connectedload.75kva x 1,000 = 75,000va75,000va / (230V x √3) = 188.27 amps. (Note: 230V 3ø is calculated)The current (amps) is more than 9 amps so use 125% rating.


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188.27 amps x 1.25 = 235.34 amps.

Therefore: Use 300amp 3-pole circuit breaker (per NEC 240.6).

NEC, Section 450-3(a): (Transformers over 600 volts, Nominal)

For primary and secondary protection with a transformer impedance of 6% or less, the primary fuse must not be largerthan 300% of primary Full Load Amps (F.L.A.) and the secondary fuse must not be larger than 250% of secondaryF.L.A.

NEC, Section 450-3(b): (Transformers over 600 volts, Nominal)

For primary protection only, the primary fuse must not be larger than 125% of primary F.L.A. For primary and secondary protection the primary feeder fuse must not be larger than 250% of primary F.L.A. if thesecondary fuse is sized at 125% of secondary F.L.A.

NEC, Section 450-3(b): (Potential (Voltage) Transformer)

These shall be protected with primary fuses when installed indoors or enclosed

NEC, Section 230-95 Ground-Fault Protection of Equipment).

This section show that 277/480 volt “wye” only connected services, 1000 amperes and larger, must have ground faultprotection in addition to conventional over current protection.The ground fault relay (or sensor) must be set to pick up ground faults which are 1200 amperes or more and actuatethe main switch or circuit breaker to disconnect all ungrounded conductors of the faulted circuit.

NEC, Section 110-9 – Interrupting Capacity.

Any device used to protect a low voltage system should be capable of opening all fault currents up to the maximumcurrent available at the terminal of the device.Many over current devices, today, are used in circuits that are above their interrupting rating.By using properly sized Current Limiting Fuses ahead of these devices, the current can usually be limited to a valuelower than the interrupting capacity of the over current devices.

NEC, Section 110-10 – Circuit Impedance and Other Characteristics.

The over current protective devices, along with the total impedance, the component short-circuit withstand ratings, andother characteristics of the circuit to be protected shall be so selected and coordinated so that the circuit protectivedevices used to clear a fault will do so without the occurrence of extensive damage to the electrical components of thecircuit. In order to do this we must select the over current protective devices so that they will open fast enough to preventdamage to the electrical components on their load side.


Guideline to Design Electrical Network for Building / Small Area.


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October 31, 2011 9 Comments

5 Votes

Guideline to Design Electrical Network for Building / Small Area.

(1) Calculate Electrical Load:

Find out built up area in Sqft.of per flat per House/Dwelling unit.Multiply area in Sqft. by Load/Sqft according to following Table

Type of Load Load/Sqft

Industrial 100 Watt/Sqft

Commercial 30 Watt/Sqft

Domestic 15 Watt/Sqft

Apply the diversity factor and Compute the load of all dwelling units in the area.

Type of Load Diversity Factor

Industrial 0.5

Commercial 0.8

Domestic 0.4

Add the load of common services such as Auditorium, Street Lights, Lifts and Water Pumps etc. For simplicity purpose

0.5kW/dwelling units may be considered as common load.Compute the “Total Load” of the area by adding load observed at above.Apply the power factor of 0.8 to determine the load in kVA.Compute the Load in kVA= “Total Load”/0.8Take transformer loading of 65% considering the network arrangement Ring Main Circuit.

(2) Decide voltage grade for Electrical Load:

If load is equal to or more than 2.50MVA, the area shall be fed through 33kV feeder. For such loads, the land spacefor 33/11kV Sub-station shall have to be allocated by builder / Society/ Authority.For load between 1 MVA to 2.5MVA, dedicated 11kV feeder shall be preferred.

For load below 1 MVA, existing 11kV feed can be tapped through VCB or RMU.

(3) Decide Size of Transformer:

Select T.C Size of 25 KVA,63 KVA,100 KVA,200 KVA or 400 KVA according to your Load.The maximum capacity of distribution transformer acceptable is 400 kVA as a standard capacity.Only two-no of transformer at one location shall be acceptable. If there is more number of transformers HT shall berequired to extend using underground cables to locate additional transformer.

(4) RMU / LT Panel:

Either VCB or Ring Main Circuit shall be used to control transformers. There cables should have metering arrangementat 11kV. The protection system at incoming supply shall be using numerical relays.On LT side of transformer, LT main feeder pillar shall be provided. The Incoming shall be protected by MCCB/SFU.The distribution pillar-box shall be connected into Ring Main Unit.The incomer of distribution pillar shall have MCCB / SFU. The outgoing shall have HRC fuses.



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(5) The LT cables from T.C to LT panel / Main feeder pillar:

Decide Size of LT Cable from T.C to LT Panel as per following Table.

Transformer Size Cable

630kVA transformers 2 no x 1C x 630 Sq mm, Al, XLPE Cable

400kVA transformers 1 no x 1C x 630 Sq mm, Al, XLPE

250kVA transformers 3 ½ C x 400 Sq mm, Al, XLPE



(6) Considering various Factors & Length of Cable:

The factors for cable loading shall be taken as 70%.The factor for multiplicity of cables from same cable trench shall be 80%.The suggested maximum length of LT cable feeder shall be 250 Mtrs.

The LT cables shall be connected in ring main circuit.The load on sub-feeder pillar shall be restricted to 150kW.

(7) LT cables from main feeder pillars to distribution pillar boxes:

Load on distribution pillar

LT Cable Size

Up to 50kW 3 ½ C x 150 sqmm, AL, XLPE

Up to 100kW 3 ½ C x 300 sqmm, AL, XLPE

Up to 150 kW 3 ½ C x 400sqmm, AL, XLPE

(8) Calculate Voltage Drop and T&D Losses:

The entire system has to be designed for a voltage drop of 2.0% from 11kV Side of transformer to meteringequipment at end consumer premises.The entire system has to be designed for T&D losses of service maximum 2.0% from 11kV to end consumermeter including of service cable.

Ref:

1. NPC Limited.2. Electrical code.



Likell



Demand Factor-Diversity Factor-Utilization Factor-Load Factor


8 Votes

(1) Demand factor:

Demand Factor = Maximum demand of a system / Total connected load on the system

Demand factor is always less than one. Example: if a residence having 6000W equipment connected has a maximum demand of 3000W,Than demand factor =3000W / 6000W = 55%.

The lower the demand factor, the less system capacity required to serve the connected load.Feeder-circuit conductors should have an ampere sufficient to carry the load; the ampere of the feeder-circuit need notalways be equal to the total of all loads on all branch-circuits connected to it.Remember that the demand factor permits a feeder-circuit ampere to be less than 100% of the sum of all branch-circuitloads connected to the feeder.

Example: One Machine Shop has

1. Fluorescent fixtures=1 No, 5kw each, Receptacle outlets =1 No, 1500w each.2. Lathe=1No, 10 Hp, Air Compressor=1 No, 20 Hp, Fire Pump=1 No, 15 Hp.

After questioning the customer about the various loads, the information is further deciphered as follows:

1. The shop lights are on only during the hours of 8 a.m. to 5 p.m.2. The receptacle outlets are in the office only, and will have computers and other small loads plugged into them.3. The lathe is fully loaded for 5 minutes periods. The rest of the time is setup time. This procedure repeats every 15

minutes.4. The air compressor supplies air to air tools and cycles off and on about half the time.

5. The fire pump only runs for 30 minutes when tested which is once a month after hours.

Calculation:

Lighting Demand Factor = Demand Interval Factor x Diversity Factor.= (15 minute run time/ 15 minutes) x 1.0 = 1.0Lighting Demand Load = 5 kW x 1.0 = 5 kWReceptacle Outlet Demand Factor = Demand Interval Factor x Diversity Factor= (15 minute run time / 15 minutes) x 0.1 = 0.1Receptacle Outlet Demand Load = 15 x 1500 watts x 0.1 = 2.25 kWLathe Demand Factor = Demand Interval Factor x Diversity Factor.= (5 minute run time / 15 minutes) x 1.0 =0 .33Lathe Demand Load = 10 hp x .746 x .33 = 2.46 kWAir Compressor Demand Factor = Demand Interval Factor x Diversity Factor.= (7.5 minute run time / 15 minutes) x 1.0 = 0.5

Air Compressor Demand Load = 20 hp x .746 x .5 = 7.46 kWFire Pump Demand Factor = Demand Interval Factor x Diversity Factor.= (15 minute run time/ 15 minutes) x 0.0 = 0.0Fire Pump Demand Load = 15 hp x .746 x 0.0 = 0.0 kWSummary of Demand Loads :



Equipment kW D.F. Demand KW

Lighting 5 1 5

Receptacle Outlets 22.5 .1 2.25

Lathe 7.5 .33 2.46

Air Compressor 15 0.5 7.46

Fire Pump 11.25 0.0 0.0

TOTAL 61.25 Kw 17.17 Kw

(2) Diversity factor / simultaneity factor (Ks):

Diversity Factor = Sum of Individual Max. Demand. / Max. Demand on Power Station.

Diversity Factor = Installed load. / Running load.

Diversity factor is usually more than one. (Since the sum of individual max. demands >Max. Demand)

The load is time dependent as well as being dependent upon equipment characteristics. The diversity factor recognizesthat the whole load does not equal the sum of its parts due to this time Interdependence (i.e. diverseness).When the maximum demand of a supply is being assessed it is not sufficient to simply add together the ratings of allelectrical equipment that could be connected to that supply. If this is done, a figure somewhat higher than the truemaximum demand will be produced. This is because it is unlikely that all the electrical equipment on a supply will beused simultaneously.The concept of being able to De-rate a potential maximum load to an actual maximum demand is known as theapplication of a diversity factor.70% diversity means that the device in question operates at its nominal or maximum load level 70% of the time that it isconnected and turned on.If total installed full load ampere is twice your running load ampere then the diversity factor is two.If total installed full load ampere is four times your load a ampere then the diversity factor is four.

If everything (all electrical equipment) was running at full load at the same time the diversity factor is equal to OneGreater the diversity factor, lesser is the cost of generation of power.Diversity factor in a distribution network is the ratio of the sum of the peak demands of the individual customers to thepeak demand of the network.This will be determined by the type of service, i.e., residential, commercial, industrial and combinations of such.Example-I: A distribution feeder serves 5 houses, each of which has a peak demand of 5 KW. The feeder peak turnsout to be 20 kw. The diversity is then 20/25 or 0.8. This results from the timing differences between the individualheating/cooling, appliance usages in the individual customers.As supply availability decreases, the diversity factor will tend to increase toward 1.00. This can be demonstrated whenrestoring service after outages (called “cold starts”) as the system initial surge can be much greater than the historicalpeak loads.Example-II: A sub-station has three outgoing feeders:

1. feeder 1 has maximum demand 10 MW at 10:00 am,2. feeder 2 has maximum demand 12 MW at 7:00 pm and3. feeder 3 has maximum demand 15 MW at 9:00 pm,4. While the maximum demand of all three feeders is 33 MW at 8:00 pm.

Here, the sum of the maximum demand of the individual sub-systems (feeders) is 10 + 12 + 15 = 37 MW, while thesystem maximum demand is 33 MW. The diversity factor is 37/33 = 1.12. The diversity factor is usually greater than 1;its value also can be 1 which indicates the maximum demand of the individual sub-system occurssimultaneously.



Diversity is the relationship between the rated full loads of the equipment downstream of a connection point, and therated load of the connection point. To illustrate:

1. The building at these co-ordinates is fitted with a 100A main supply fuse.2. The distribution board has 2no. 6A breakers, 1no. 20A breaker and 5no. 32A breakers, a total, potentially, of 192A.

Not all these rated loads are turned on at once. If they were, then the 100A supply fuse would rupture, as it cannotpass 192A. So the diversity factor of the distribution board can be said to be 192A/100A, or 1.92, or 52%.Many designers prefer to use unity as the diversity factor in calculations for planning conservatism because of plant loadgrowth uncertainties. Local experience can justify using a diversity factor larger than unity, and smaller service entranceconductors and transformer requirements chosen accordingly.The diversity factor for all other installations will be different, and would be based upon a local evaluation of the loadsto be applied at different moments in time. Assuming it to be 1.0 may, on some occasions, result in a supply feeder andequipment rating that is rather larger than the local installation warrants, and an over-investment in cable and equipmentto handle the rated load current. It is better to evaluate the pattern of usage of the loads and calculate an acceptablediversity factor for each particular case.In the case of the example given above, achieving a diversity of 1.0 or 100% would require well over twice the cross-

sectional area of copper cable to be installed in a deep trench underneath a field, the rebuild of a feeder cabinet tolarger dimensions, more substantial overhead supply cables for a distance exceeding 2km northwards and a differenttariff, where one pays rather more for a kWh than at present. The investment required to achieve 1.0 simply isn’tjustifiable in this particular case.Diversity factor is mostly used for distribution feeder size and transformer as well as to determine the maximum peakload and diversity factor is always based on knowing the process. You have to understand what will be on or off at agiven time for different buildings and this will size the feeder. Note for typical buildings diversity factor is always one.You have to estimate or have a data records to create 24 hours load graph and you can determine the maximumdemand load for node then you can easily determine the feeder and transformer size.The diversity factor of a feeder would be the sum of the maximum demands of the individual consumers divided by themaximum demand of the feeder. In the same manner, it is possible to compute the diversity factor on a substation, atransmission line or a whole utility system.The residential load has the highest diversity factor. Industrial loads have low diversity factors usually of 1.4, street light

practically unity and other loads vary between these limits.

Diversity Factor in distribution Network:

Elements ofSystem

Diversity Factors

Residential CommercialGeneralPower

LargeIndustrial

Between individualusers

2.00 1.46 1.45

Betweentransformers

1.30 1.30 1.35 1.05

Between feeders 1.15 1.15 1.15 1.05

Between substations 1.10 1.10 1.10 1.10

From users totransformers

2.00 1.46 1.44

From users to feeder 2.60 1.90 1.95 1.15

From users tosubstation

3.00 2.18 2.24 1.32



From users togenerating station

3.29 2.40 2.46 1.45

Diversity Factor for distribution switchboards:

Number of circuits Diversity Factor(ks)

Assemblies entirely tested 2 and 3 0.9

4 and 5 0.8

6 to 9 0.7

10 and more 0.6

Assemblies partially tested in every casechoose

1

Diversity Factor for according to circuit function (IEC 60439):

Circuits Function Diversity Factor(ks)

Lighting 0.9

Heating and air conditioning 0.8

Socket-outlets 0.7

Lifts and catering hoist

For the most powerful motor 1

For the second most powerful motor 0.75

For all motors 0.8

Diversity Factor for an apartment block:

Apartment Diversity Factor (ks)

2 To 4 1

5To 19 0.78

10To 14 0.63

15To 19 0.53

20To 24 0.49

25To 29 0.46

30 To 34 0.44

35 To 39 0.42

40To 40 0.41

50 To Above 0.40

Example: 5 storey apartment building with 25 consumers, each having 6 kVA of installed load.The total installed load for the building is: 36 + 24 + 30 + 36 + 24 = 150 kVA

The apparent-power supply required for the building is: 150 x 0.46 = 69 kVAIt is a matter of common experience that the simultaneous operation of all installed loads of a given installation neveroccurs in practice, i.e. there is always some degree of diversity and this fact is taken into account for estimatingpurposes by the use of a simultaneity factor / Diversity Factor (ks).The Diversity factor ks is applied to each group of loads (e.g. being supplied from a distribution or sub-distributionboard). The determination of these factors is the responsibility of the designer, since it requires a detailed knowledge ofthe installation and the conditions in which the individual circuits are to be exploited. For this reason, it is not possible to



give precise values for general application.

Designing Size of Electrical Switchgear by use of Demand Factor and Diversity Factor:

Diversity factors are used by utilities for distribution transformer sizing and load predictions.Demand factors are more conservative and are used by NEC for service and feeder sizing.Demand factors and diversity factors are used in design.For example, the sum of the connected loads supplied by a feeder is multiplied by the demand factor to determine theload for which the feeder must be sized. This load is termed the maximum demand of the feeder. The sum of themaximum demand loads for a number of sub feeders divided by the diversity factor for the sub feeders will give the

maximum demand load to be supplied by the feeder from which the sub feeders are derived.Example-1: Suppose We have four individual feeder-circuits with connected loads of 250 kVA, 200 kVA, 150 kVAand 400 kVA and demand factors of 90%, 80%, 75% and 85% respectively.Use a diversity factor of 1.5.Calculating demand for feeder-circuits

250 kVA x 90% = 225 kVA200 kVA x 80% = 160 kVA150 kVA x 75% = 112.5 kVA400 kVA x 85% = 340 kVA837.5 kVAThe sum of the individual demands is equal to 837.5 kVA.If the main feeder-circuit were sized at unity diversity: kVA = 837.5 kVA ÷ 1.00 = 837.5 kVA.The main feeder-circuit would have to be supplied by an 850 kVA transformer.

However, using the diversity factor of 1.5, the kVA = 837.5 kVA ÷ 1.5 = 558 kVA for the main feeder.For diversity factor of 1.5, a 600 kVA transformer could be used.Example-2: A conveyor belt made up of six sections, each driven by a 2 kW motor. As material is transportedalong this belt, it is first carried by section 1, and then each section in succession until the final section is reached.In this simple example only one section of conveyor is carrying material at any point in time. Therefore fivemotors are only handling no-load mechanical losses (say .1 kW) keeping the belts moving whilst one motor ishandling the load (say 1 kW). The demand presented by each motor when it is carrying its load is 1 kW, the sumof the demand loads is 6 kW but the maximum load presented by the system at any time is only 1.5 kW.Diversity factor =Sum of Individual Max. Demand / Max. Demand = 6 Kw / 1.5 Kw =4.Demand Factor = Maximum demand / Total connected load = 1.5 Kw / 12 Kw = 0.125.

(3) Load factor:

Load Factor = Average load. /Maximum load during a given period.

It can be calculated for a single day, for a month or for a year.Its value is always less than one. Because maximum demand is always more than avg. demand.It is used for determining the overall cost per unit generated. Higher the load factor, lesser will be the costper unit.Load Factor = Load that a piece of equipment actually draws / Load it could draw (full load).Example: Motor of 20 hp drives a constant 15 hp load whenever it is on. The motor load factor is then 15/20 = 75%.Load factor is term that does not appear on your utility bill, but does affect electricity costs. Load factor indicates howefficiently the customer is using peak demand.Load Factor = ( energy (kWh per month) ) / ( peak demand (kW) x hours/month )A high load factor means power usage is relatively constant. Low load factor shows that occasionally a high demand isset. To service that peak, capacity is sitting idle for long periods, thereby imposing higher costs on the system. Electrical

rates are designed so that customers with high load factor are charged less overall per kWh.For Example



Customer A – High Load Factor82% load factor = (3000 kWh per month x 100%) / 5 kW x 730 hours/month.Customer B – Low Load Factor41% load factor = (3000 kWh per month x 100%) / 10kW x 730 hours/month.To encourage the efficient use of installed capacity, electricity rates are structured so the price per kWh above a certainload factor is lower. The actual structure of the price blocks varies by rate.

(4) Utilization factor (Ku):

In normal operating conditions the power consumption of a load is sometimes less than that indicated as its nominalpower rating, a fairly common occurrence that justifies the application of an utilization factor (ku) in the estimation ofrealistic values.Utilization Factor = The time that a equipment is in use./ The total time that it could be in use.Example: The motor may only be used for eight hours a day, 50 weeks a year. The hours of operation would then be2000 hours, and the motor Utilization factor for a base of 8760 hours per year would be 2000/8760 = 22.83%. Witha base of 2000 hours per year, the motor Utilization factor would be 100%. The bottom line is that the use factor is

applied to get the correct number of hours that the motor is in use.

This factor must be applied to each individual load, with particular attention to electric motors, which are very rarelyoperated at full load. In an industrial installation this factor may be estimated on an average at 0.75 for motors.For incandescent-lighting loads, the factor always equals 1.For socket-outlet circuits, the factors depend entirely on the type of appliances being supplied from the socketsconcerned.

Maximum demand:

Maximum demand (often referred to as MD) is the largest current normally carried by circuits, switches and protectivedevices. It does not include the levels of current flowing under overload or short circuit conditions. Assessment of maximum demand is sometimes straightforward. For example, the maximum demand of a 240 V single-

phase 8 kW shower heater can be calculated by dividing the power (8 kW) by the voltage (240 V) to give a current of33.3 A. This calculation assumes a power factor of unity, which is a reasonable assumption for such a purely resistiveload.There are times, however, when assessment of maximum demand is less obvious. For example, if a ring circuit feedsfifteen 13 A sockets, the maximum demand clearly should not be 15 x 13 = 195 A, if only because the circuitprotection will not be rated at more than 32 A. Some 13 A sockets may feed table lamps with 60 W lamps fitted,whilst others may feed 3 kW washing machines; others again may not be loaded at all.Lighting circuits pose a special problem when determining MD. Each lamp-holder must be assumed to carry the currentrequired by the connected load, subject to a minimum loading of 100 W per lamp holder (a demand of 0.42 A perlamp holder at 240 V). Discharge lamps are particularly difficult to assess, and current cannot be calculated simply bydividing lamp power by supply voltage. The reasons for this are:

1. Control gear losses result in additional current,2. the power factor is usually less than unity so current is greater, and3. Chokes and other control gear usually distort the waveform of the current so that it contains harmonics which are

additional to the fundamental supply current.

So long as the power factor of a discharge lighting circuit is not less than 0.85, the current demand for the circuit can becalculated from:current (A) = (lamp power (W) x 1.8) / supply voltage (V)For example, the steady state current demand of a 240 V circuit supplying ten 65 W fluorescent lamps would be: I =10X65X1.8A / 240 = 4.88A



Switches for circuits feeding discharge lamps must be rated at twice the current they are required to carry, unless theyhave been specially constructed to withstand the severe arcing resulting from the switching of such inductive andcapacitive loads.

(5) Coincidence factor:

The coincidence factor =Max. demand of a system / sum of the individual maximum demandsThe coincidence factor is the reciprocal of the diversity factor

Demand Factor & Load Factor according to Type of Industries:

Type of Industry DemandFactor

LoadFactor

Utilization Factor(DF x LF)

Arc Furnace 0.55 0.80 0.44

Induction Furnace 0.90 0.80 0.72

Steel Rolling mills 0.80 0.25 0.20

Mechanical/ Electrical

a) Single Shift 0.45 0.25 0.11

b) Double Shift 0.45 0.50 0.22

Cycle Industry 0.40 0.40 0.16

Wire products 0.35 0.40 0.14

Auto Parts 0.40 0.50 0.20

Forgings 0.50 0.35 0.17

Cold Storage

a) Working Season 0.60 0.65 0.39

b) Non-Working Season 0.25 0.15 0.04

Rice Sheller’s



Ice Candy Units



Ice Factories



Cotton Ginning



Spinning Mills 0.60 0.80 0.48

Textile Industry 0.50 0.80 0.40

Dyeing and Printing 0.40 0.50 0.20

Ghee Mills 0.50 0.50 0.25

Oil Mills 0.70 0.50 0.35

Solvent Extraction Mills 0.45 0.50 0.22

Plastic 0.60 0.25 0.11

Soap 0.50 0.25 0.12

Rubber (Foot Wear) 0.45 0.35 0.16



Distilleries 0.35 0.50 0.17

Chemical Industry 0.40 0.50 0.20

Gas Plant Industry 0.70 0.50 0.35

Pain and Colour Factory 0.50 0.40 0.20

Sugar 0.30 0.45 0.13

Paper 0.50 0.80 0.40

Flour Mills(Single Shift) 0.80 0.25 0.20

Atta Chakies 0.50 0.25 0.12

Milk Plants 0.40 0.80 0.32

Printing Presses 0.35 0.30 0.10

Repair Workshops 0.40 0.25 0.10

Bottling Plants 0.40 0.35 0.14

Radio Stations 0.55 .0.45 0.25

Telephone exchange 0.50 0.90 0.45

Public Water Works 0.75 0.40 0.30

Medical Colleges 0.60 0.25 0.15

Hospitals 0.25 0.90 0.22

Nursing Homes 0.50 0.50 0.25

Colleges and Schools 0.50 0.20 0.10

Hotels and Restaurants 0.75 0.40 0.30

Marriage Palaces 1.00 0.25 0.25

Demand Factor & Load Factor according to Type of Buildings:

Individual Facilities DemandFactor

Load Factor

Communications – buildings 60-65 70-75

Telephone exchange building 55-70 20-25

Air passenger terminal building 65-80 28-32

Aircraft fire and rescue station 25-35 13-17

Aircraft line operations building 65-80 24-28

Academic instruction building 40-60 22-26

Applied instruction building 35-65 24-28

Chemistry and Toxicology Laboratory 70-80 22-28

Materials Laboratory 30-35 27-32

Physics Laboratory 70-80 22-28

Electrical and electronics systemslaboratory

20-30 3-7

Cold storage warehouse 70-75 20-25

General warehouse 75-80 23-28

Controlled humidity warehouse 60-65 33-38

Hazardous/flammable storehouse 75-80 20-25

Disposal, salvage, scrap building 35-40 25-20

Hospital 38-42 45-50



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Laboratory 32-37 20-25

Dental Clinic 35-40 18-23

Medical Clinic 45-50 20-23

Administrative Office 50-65 20-35

Single-family residential housing 60-70 10-15

Detached garages 40-50 2-4

Apartments 35-40 38-42

Fire station 25-35 13-17

Police station 48-53 20-25

Bakery 30-35 45-60

Laundry/dry cleaning plant 30-35 20-25

K-6 schools 75-80 10-15

7-12 schools 65-70 12-17

Churches 65-70 5-25

Post Office 75-80 20-25

Retail store 65-70 25-32

Bank 75-80 20-25

Supermarket 55-60 25-30

Restaurant 45-75 15-25

Auto repair shop 40-60 15-20

Hobby shop, art/crafts 30-40 25-30

Bowling alley 70-75 10-15

Gymnasium 70-75 20-45

Skating rink 70-75 10-15

Indoor swimming pool 55-60 25-50

Theater 45-55 8-13

Library 75-80 30-35

Golf clubhouse 75-80 15-20

Museum 75-80 30-35


Calculate Transverse Load on PCC/RCC/Tublar/RSJ Pole.


3 Votes

Calculate Transverse Load on PCC/RCC/RSJ/Tublar/RSJ Pole.


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Calculate Crippling load on Pole

Calculate Wind Load on Pole.Calculate wind Load on all Conductors.Calculate Transverse Load on PoleDecide spacing between Two Conductor as per IS:5613.Calculate required Voltage level of Overhead Line according to Distance.

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Working Principle of ELCB and RCB


4 Votes

Working Principle of ELCB and RCB:

An Earth Leakage Circuit Breaker (ELCB) is a device used to directly detect currents leaking to earth from an installation andcut the power and mainly used in TT earthing systems.

There are two types of ELCBs,

1. Voltage Earth Leakage Circuit Breaker (voltage-ELCB)2. Current Earth Leakage Current Earth Leakage Circuit Breaker (Current-ELCB).

Voltage-ELCBs were first introduced about sixty years ago and Current-ELCB was first introduced about forty years ago.For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCBsbecause it was a simpler name to remember. But the use of a common name for two different devices gave rise toconsiderable confusion in the electrical industry. If the wrong type was used on an installation, the level of protection given


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could be substantially less than that intended. To ignore this confusion, IEC decided to apply the term Residual Current Device(RCD) to differential current operated ELCBs. Residual current refers to any current over and above the load current

Voltage Base ELCB.

Voltage-ELCB is a voltage operated circuit breaker. The device will function when the Current passes through theELCB. Voltage-ELCB contains relay Coil which it being connected to the metallic load body at one end and it isconnected to ground wire at the other end.If the voltage of the Equipment body is rise (by touching Phase to metal Part or Failure of Insulation of Equipment)which could cause the difference between earth and load body voltage, the danger of electric shock will occur. Thisvoltage difference will produce an electric current from the load metallic body passes the relay loop and to earth. Whenvoltage on the equipment metallic body rose to the danger level which exceed to 50Volt, the flowing current throughrelay loop could move the relay contact by disconnecting the supply current to avoid from any danger electric shock.The ELCB detects fault currents from live to the earth (ground) wire within the installation it protects. If sufficientvoltage appears across the ELCB’s sense coil, it will switch off the power, and remain off until manually reset. Avoltage-sensing ELCB does not sense fault currents from live to any other earthed body.

These ELCBs monitored the voltage on the earth wire, and disconnected the supply if the earth wire voltage was over50 volts.These devices are no longer used due to its drawbacks like if the fault is between live and a circuit earth, they willdisconnect the supply. However, if the fault is between live and some other earth (such as a person or a metal waterpipe), they will NOT disconnect, as the voltage on the circuit earth will not change. Even if the fault is between live anda circuit earth, parallel earth paths created via gas or water pipes can result in the ELCB being bypassed. Most of thefault current will flow via the gas or water pipes, since a single earth stake will inevitably have a much higher impedancethan hundreds of meters of metal service pipes buried in the ground.



The way to identify an ELCB is by looking for green or green and yellow earth wires entering the device.They rely on voltage returning to the trip via the earth wire during a fault and afford only limited protection to theinstallation and no personal protection at all. You should use plug in 30mA RCD’s for any appliances and extensionleads that may be used outside as a minimum.

Advantages

ELCBs have one advantage over RCDs: they are less sensitive to fault conditions, and therefore have fewer nuisancetrips.While voltage and current on the earth line is usually fault current from a live wire, this is not always the case, thus thereare situations in which an ELCB can nuisance trip.When an installation has two connections to earth, a nearby high current lightning strike will cause a voltage gradient in

the soil, presenting the ELCB sense coil with enough voltage to cause it to trip.If the installation’s earth rod is placed close to the earth rod of a neighboring building, a high earth leakage current in theother building can raise the local ground potential and cause a voltage difference across the two earths, again trippingthe ELCB.If there is an accumulated or burden of currents caused by items with lowered insulation resistance due to olderequipment, or with heating elements, or rain conditions can cause the insulation resistance to lower due to moisturetracking. If there is a some mA which is equal to ELCB rating than ELCB may give nuisance Tripping.If either of the earth wires become disconnected from the ELCB, it will no longer trip or the installation will often nolonger be properly earthed.Some ELCBs do not respond to rectified fault current. This issue is common for ELCBs and RCDs, but ELCBs are onaverage much older than RCB so an old ELCB is more likely to have some uncommon fault current waveform that itwill not respond to.

Voltage-operated ELCB are the requirement for a second connection, and the possibility that any additional connectionto earth on the protected system can disable the detector.Nuisance tripping especially during thunderstorms.

Disadvantages:

They do not detect faults that don’t pass current through the CPC to the earth rod.They do not allow a single building system to be easily split into multiple sections with independent fault protection,because earthing systems are usually use common earth Rod.They may be tripped by external voltages from something connected to the earthing system such as metal pipes, a TN-S earth or a TN-C-S combined neutral and earth.

As electrically leaky appliances such as some water heaters, washing machines and cookers may cause the ELCB totrip.ELCBs introduce additional resistance and an additional point of failure into the earthing system.

Can we assume whether Our Electrical System is protected against Earth Protection or notby only Pressing ELCB Test Switch?

Checking the health of the ELCB is simple and you can do it easily by pressing TEST Push Button Switch of ELCB.The test push-button will test whether the ELCB unit is working properly or not. Can we assume that If ELCB is Tripafter Pressing TEST Switch of ELCB than your system is protected against earth protection? Then you are wrong.The test facility provided on the home ELCB will only confirm the health of the ELCB unit, but that test does notconfirm that the ELCB will trip when an electric shock hazard does occur. It is a really sad fact that all the while thismisunderstanding has left many homes totally unprotected from the risk of electric shocks.This brings us or alarming us to think over second basic requirement for earth protection. The second requirement forthe proper operation of a home shock protection system is electrical grounding.

We can assume that the ELCB is the brain for the shock protection, and the grounding as the backbone. Therefore,



without a functional grounding (Proper Earthing of Electrical System) there is totally no protection against electricalshocks in your house even if You have installed ELCB and its TEST switch show proper result. Looking after theELCB alone is not enough. The electrical Earthing system must also be in good working order for the shock protection

system to work. In addition to routine inspections that should be done by the qualified electrician, this grounding shouldpreferably be inspected regularly at shorter intervals by the homeowner and need to pour Water in Earthing Pit atRegular interval of Time to minimize Earth Resistance.

Current-operated ELCB (RCB):

Current-operated ELCBs are generally known as Residual-current devices (RCD). These also protect against earthleakage. Both circuit conductors (supply and return) are run through a sensing coil; any imbalance of the currents meansthe magnetic field does not perfectly cancel. The device detects the imbalance and trips the contact.When the term ELCB is used it usually means a voltage-operated device. Similar devices that are current operated arecalled residual-current devices. However, some companies use the term ELCB to distinguish high sensitivity currentoperated 3 phase devices that trip in the milliamp range from traditional 3 phase ground fault devices that operate atmuch higher currents.

Typical RCB circuit:

The supply coil, the neutral coil and the search coil all wound on a common transformer core.On a healthy circuit the same current passes through the phase coil, the load and return back through the neutral coil.Both the phase and the neutral coils are wound in such a way that they will produce an opposing magnetic flux. With thesame current passing through both coils, their magnetic effect will cancel out under a healthy circuit condition.In a situation when there is fault or a leakage to earth in the load circuit, or anywhere between the load circuit and theoutput connection of the RCB circuit, the current returning through the neutral coil has been reduced. Then the magneticflux inside the transformer core is not balanced anymore. The total sum of the opposing magnetic flux is no longer zero.

This net remaining flux is what we call a residual flux.The periodically changing residual flux inside the transformer core crosses path with the winding of the search coil. Thisaction produces an electromotive force (e.m.f.) across the search coil. An electromotive force is actually an alternatingvoltage. The induced voltage across the search coil produces a current inside the wiring of the trip circuit. It is this



current that operates the trip coil of the circuit breaker. Since the trip current is driven by the residual magnetic flux (theresulting flux, the net effect between both fluxes) between the phase and the neutral coils, it is called the residualcurrent devise.With a circuit breaker incorporated as part of the circuit, the assembled system is called residual current circuit breaker(RCCB) or residual current devise (RCD). The incoming current has to pass through the circuit breaker first before

going to the phase coil. The return neutral path passes through the second circuit breaker pole. During tripping when afault is detected, both the phase and neutral connection is isolated.

RCD sensitivity is expressed as the rated residual operating current, noted IΔn. Preferred values have beendefined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value.High sensitivity (HS): 6- 10- 30 mA (for direct-contact / life injury protection)Standard IEC 60755 (General requirements for residual current operated protective devices) defines three typesof RCD depending on the characteristics of the fault current.Type AC: RCD for which tripping is ensured for residual sinusoidal alternating currents

Sensitivity of RCB:

Medium sensitivity (MS): 100- 300- 500- 1000 mA (for fire protection)

Low sensitivity (LS): 3- 10- 30 A (typically for protection of machine)

Type of RCB:

Type A: RCD for which tripping is ensured

for residual sinusoidal alternating currentsfor residual pulsating direct currentsFor residual pulsating direct currents superimposed by a smooth direct current of 0.006 A, with or without phase-anglecontrol, independent of the polarity.

Type B: RCD for which tripping is ensured

as for type Afor residual sinusoidal currents up to 1000 Hzfor residual sinusoidal currents superposed by a pure direct currentfor pulsating direct currents superposed by a pure direct currentfor residual currents which may result from rectifying circuits

three pulse star connection or six pulse bridge connectiontwo pulse bridge connection line-to-line with or without phase-angle monitoring, independently of the polarityThere are two groups of devices:

Break time of RCB:

1. G (general use) for instantaneous RCDs (i.e. without a time delay)

Minimum break time: immediateMaximum break time: 200 ms for 1x IΔn, 150 ms for 2x IΔn, and 40 ms for 5x IΔn

1. S (selective) or T (time delayed) for RCDs with a short time delay (typically used in circuits containing surgesuppressors)

Minimum break time: 130 ms for 1x IΔn, 60 ms for 2x IΔn, and 50 ms for 5x IΔnMaximum break time: 500 ms for 1x IΔn, 200 ms for 2x IΔn, and 150 ms for 5x IΔn



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Single Earthed Neutral and Multi Earthed Neutral.

September 6, 2011 4 Comments

5 Votes

Single Earthed Neutral and Multi Earthed Neutral:

In Distribution System Three Phase load is unbalance and non linear so The Neutral plays an important role inDistribution system.Generally, distribution networks are operated in an unbalanced configuration and also service to consumers. Thiscauses current flowing through neutral conductor and voltage dropping on neutral wire. The unbalance load and

excessive current in neutral wire is one of the issues in three phase four-wire distribution systems that causes voltagedrop through neutral wire and makes tribulations for costumers. The existence of Neutral earth Voltage makesunbalance in three phase voltages for three phase customers and reduction of phase to neutral voltage for single phasecustomers.MULTI-GROUNDED three-phase four-wire service is widely adopted in modern power distribution systems due tohaving lower installation costs and higher sensitivity of fault protection than three-phase three-wire service. The neutralsplay an important role in power quality and safety problems.The multi grounded neutral system is the predominant electrical distribution system used in the United States.It allow an uncontrolled amount of electric current to flow over the earth unrestrained, posing the potential of harm tothe public and to animals causing electric shocks and is presumed responsible for undetected electrocutions.The protective grounding used in low voltage,600-volt and below, applications will be described and used to explainthe hazards involved with the present day multi grounded neutral distribution System, used in the United States. This will

allow the reader to see the parallels between the safe low voltage distribution system and the dangerous medium voltagemulti grounded neutral distribution system.The reasons for the development of the three phase, four-wire, multi-grounded systems involve a combination of safetyand economic considerations. The three-phase, four-wire multi-grounded design has been successfully used for manyyears and is well documented in the standards including the National Electrical Code (NEC).It is Crucial decisions to adopt Multi Grounded Neutral System “save money” by the adoption of the multi groundedneutral electrical distribution system in the cost of the public’s safety.

Multi Grounded Neutral System (MEN):


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Fig shows the multi-grounded neutral systems commonly used by the electric utilities in North America. The neutralgrounding reactor is used by some utilities to reduce the available ground fault current while at the same time stillmaintaining an effectively grounded system.The multiple earthed neutral (MEN) system of earthing is one in which the low voltage neutral conductor is used as thelow resistance return path for fault currents and where its potential rise is kept low by having it connected to earth at anumber of locations along its length. The neutral conductor is connected to earth at the distribution transformer, at each

consumer’s installation and at specified poles or underground pillars. The resistance between the neutral conductor ofthe distribution system and the earth must not exceed 10 ohms at any location.NEC Article 250 Part X Grounding of Systems and Circuits 1 kV and Over (High Voltage)(A) Multiple Grounding: The neutral of a solidly grounded neutral system shall be permitted to be grounded at morethan one point.(B) Multi-grounded Neutral Conductor: Ground each transformer, Ground at 400 m intervals or less, Ground shieldedcables where exposed to personnel contact.

Single Grounded Neutral:

Fig Show Single Grounded Neutral Which is different from Multi Grounded System .Figure shows the neutral alsoconnected to earth, but the neutral conductor is extended along with the phase conductors. The configuration shown infigure allows electrical loads, transformers to be placed between any of the three phase conductors, phase-to-phaseand/or phase-to-neutral.This connection, phase to neutral will force electric current to flow over the neutral back to the transformer. So far, this

electrical connection is acceptable, as long as the neutral is insulated or treated as being potentially energized, butmodifications will be made in the future that will negate safety for the public and animals.The ground connection would typically be located in the distribution substation. This may appear insignificant, but thedifferences are significant

Advantages of Multiple Grounded Neutral Systems:

(1) Optimize the Size of Surge Arrestor:



Surge arresters are applied to a power system based on the line-to-ground voltage under normal condition and

abnormal conditions. Under ground-fault conditions, the line-to-ground voltage can increase up to 1.73 per unit on thetwo, un faulted.Application of surge arresters on a power system is dependent on the effectiveness of the system grounding. The overvoltage condition that can occur during a ground fault can be minimized by keeping the zero sequence impedance low.Therefore, optimization in sizing the surge arresters on the system is dependent on the system grounding.An effectively grounded power system allows the use of a lower rated surge arrester. The lower rated surge arresterprovides better surge protection at a lower cost. An effectively grounded system can only be accomplished using aproperly sized, multi-grounded system neutral.With Single Grounded Neutral System require the use of full line-to-line voltage rated arresters. This increases the costof the surge arresters while at the same time reduces the protection provided by the surge arrester. In addition, if thefourth wire neutral is not multi grounded, it would be good practice to place surge arresters at appropriate locations onthat conductor.

(2) The zero sequence impedance is lower for a multi grounded system than the single point grounded neutral system.

(3) Freezing and arctic conditions have an adverse impact on the zero sequence impedance. A multi-grounded system neutralwill still lower the zero sequence impedance over a single point ground. In fact, without the multi-grounded system, it is moreprobable that insufficient fault current will flow to properly operate the ground fault protection.

(4) Cost of Equipment for the multi-grounded system is lower.

(5) Safety Concerns on Cable Shields.

Medium voltage and high voltage cables typically have cable shields (NEC requirement above 5 kV) that need to begrounded. There are several reasons for this shield:

To confine electric fields within the cable

To obtain uniform radial distribution of the electric fieldTo protect against induced voltagesTo reduce the hazard of shock

If the shield is not grounded, the shock hazard can be increased. With the shield grounded at one point, induced voltageon the shield can be significant and create a shock hazard. Therefore, it is common practice to apply multiple groundson the shield to keep the voltage limited to 25 volts.This practice of multi grounding cable shields includes the grounding of concentric neutrals on power cables thereby

extending the need for multi grounding of neutrals on the power system.

Disadvantages of Multiple Neutral Grounding:

(1) Less Electrical Safety in Public and Private Property.

With a multi grounded neutral distribution system it is necessary to have an electrical connection to earth at least 4times per mile to keep the voltage on the multi grounded neutral from exceeding approximately 25 volts making it safefor the linemen should they come into contact with the neutral and the earth.As per NESC Rule 096 C in the section with the multi grounded neutral conductor connected to earth at least 4 timesper mile and at each transformer and lightning arrester there are now multiple paths over and through the earth that thehazardous electric current can flow over continuously, uncontrolled.The path that this current flow takes through the earth cannot be determined. We cannot put an isotope on each

electron and trace its path as it flows uncontrolled through the earth. It is irresponsible to permit stray uncontrolledelectric current to flow into and over private property.The National Electrical Code (NEC) requires the neutral in the service disconnect and over current panel board to beconnected to the earth also. Now the secondary neutral is connected to earth a second time. A parallel connection of



the neutral to earth now exists permitting hazardous electric current to flow continuously uncontrolled over the earth.

(2) Earth Fault Protection Relay setting is complicated.

Advantages of Single Grounded Neutral System:

(1) More Reliable and Safe System.

(2) Protection Relay Setting is more easy in Single Grounded Neutral:

Protective relays need to sense abnormal conditions, especially those involving a ground fault. The single pointgrounded system, with or without a neutral conductor, current flowing into the ground should be considered abnormal(excluding normal charging current). For sensing of ground faults are:

A current transformer in the location where the neutral is grounded can be used to sense the ground fault (zerosequence) current.A zero sequence CT enclosing the three phase and neutral conductors.

Four CT residue circuit (Three CT residual with neutral CT cancellation).

Protecting against ground faults on a multi-grounded neutral system is more difficult than the single point groundedsystem since both neutral and ground fault currents must be considered.Neutral current and likewise ground fault current can flow in both the neutral and the ground. So, We have mustcalculate both current as the amount of neutral current which may flow in the circuit, and the ground fault setting mustbe above this neutral current. This is self explanatory from Fig.

(3) Sensing of Ground Fault current :

While the sensing of the ground fault current in the single point grounded system is less complex than the multi groundedsystem, the amount of ground fault current on the single-point grounded system may be greatly limited due to the factthat all ground fault current must return through the earth. This is especially true where the earth resistivity is high, the

soil is frozen or the soil is extremely dry.

Reference:



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John P. Nelson Fellow, IEEE ANSI/IEEE Std 142-1991

Westinghouse Electric Corporation, Electrical Transmission and Distribution Reference Book NFPA 70.

Jeffery Leib, Train-Car Crashes on the Rise,Denver Post Newspaper, November 7, 2002

R.T. Beck and Luke Yu, Design Considerations for Grounding Systems.

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Type of Electrical Power Distribution systems

September 5, 2011 1 Comment

3 Votes

Type of Electrical Power Distribution Systems:

Electrical power is distribution either three wires or Four wires (3 wire for phases and 1 wire for Neutral). Voltagebetween Phase to Phase Called Line Voltage and Voltage between Phase and Neutral is Called Phase Voltage.This Forth wire may or may not be distributed in Distribution System and Same way this neutral may or may not be

earthedDepending of this neutral condition (Earthed-not Earthed-access-not access) there are various type of earthing System.The neutral may be directly connected to earth or connected through a resistor or a reactor. This system is calleddirectly earthed or Earthed System.When a connection has not been made between the neutral point and earth, we say that the neutral is unearthed.In a network, the earthing system plays a very important role. When an insulation fault occurs or a phase is accidentallyearthed, the values taken by the fault currents, the touch voltages and over voltages are closely linked to the type ofneutral earthing connection.A directly earthed neutral strongly limits over voltages but it causes very high fault currents, here as an unearthed neutrallimits fault currents to very low values but encourages the occurrence of high over voltages.In any installation, service continuity in the event of an insulation fault is also directly related to the earthing system. Anunearthed neutral permits service continuity during an insulation fault. Contrary to this, a directly earthed neutral, or low

impedance-earthed neutral, causes tripping as soon as the first insulation fault occurs.The choice of earthing system in both low voltage and medium voltage networks depends on the type of installation aswell as the type of network. It is also influenced by the type of loads and service continuity required.The Main objectives of an earthing system are Provide an alternative path for the fault current to flow so that it will notendanger the user, Ensure that all exposed conductive parts do not reach a dangerous potential, Maintain the voltage atany part of an electrical system at a known value and prevent over current or excessive voltage on the appliances orequipment.Different earthing systems are capable of carrying different amounts of over current. Since the amount of over currentproduced in different types of installation differs from each other, required type of earthing will also differ according tothe type of installation. so in order to ensure that the installation goes with the existing earthing system or else to do anymodification accordingly, we need to have a proper idea of the present earthing system. It would enhance the safety aswell as the reliability


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As per IEC 60364-3 There are three types of systems:

(1) Unearthed System:

IT System.

(2) Earthed System:

TTTN (TN-S, TN-C, TN-C-S).The first letter defines the neutral point in relation to earth:

1. T = directly earthed neutral (from the French word Terre)2. I =unearthed or high impedance-earthed neutral (e.g. 2,000 Ω)

The second letter defines the exposed conductive parts of the electrical installation in relation to earth:

1. T =directly earthed exposed conductive parts2. N =exposed conductive parts directly connected to the neutral conductor

Unearthed System:

(1) IT system unearthed (High Impedance earthed neutral)

First Letter I= the neutral is unearthed at Transformer or Generator side.Second Letter T= Frame parts of the loads are interconnected and earthed at Load Side

It is compulsory to install an over voltage limiter between the MV/LVtransformer neutral point and earth.

If the neutral is not accessible, the overvoltage limiter is installed between a phase and earth.It runs off external over voltages, transmitted by the transformer, to the earth and protects the low voltage network froma voltage increase due to flashover between the transformer’s medium voltage and low voltage windings.

Advantages:

1. System providing the best service continuity during use.2. When an insulation fault occurs, the short-circuit current is very low.3. Higher operational safety only a capacitive current flows, which is caused by the system leakage capacitance if an earth

fault occurs.4. Better accident prevention the fault current is limited by the body impedance, earthing resistance and the high



impedance of the earth fault loop.

Disadvantages:

1. Requires presence of maintenance personnel to monitor and locate the first fault during use.2. Requires a good level of network insulation (High leakage current must be supplied by insulating transformers).3. Overvoltage limiters must be installed.4. Requires all the installation’s exposed conductive parts to be Same Voltage level. If this is not possible RCDs must be

installed.5. Locating faults is difficult in widespread networks.6. When an insulation fault with reference to the earth occurs, the voltage of the two healthy phases in relation to the earth

take on the value of the phase-to-phase voltage So when Select Size of equipments it is need to higher insulation levelof the Equipments.

7. The risk of high internal over voltages making it advisable to reinforce the equipment insulation.

8. The compulsory insulation monitoring, with visual and audible indication of the first fault if tripping is not triggered untilthe second fault occurs.

9. Protection against direct and indirect contact is not guaranteed.10. 10. Short-circuit and earth fault currents may cause fires and destroy parts of the plant.

Earthed System:

(1) TT system directly earthed neutral

First letter T=the neutral is directly earthed.Second letter T= the exposed conductive parts of the loads are interconnected and earthed.The transformer neutral is earthed;The frames of the electrical loads are also connected to an earth connection

System characteristics:

1. High earth fault loop impedance2. Low earth fault current3. Utility company need not to provide earth for consumer

Advantages:

1. save earth wires2. The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come

through the neutral wire from various electrical equipment connected to it.3. TT has always been preferable for special applications like telecommunication sites that benefit from the interference-

free earthing

4. Does not have the risk of a broken neutral.



5. The simplest system to design, implement, monitor and use.

6. Easily find location of faults.7. Upon occurrence of an insulation fault, the short-circuit current is small.8. Reduces the risk of over voltages occurring.9. Authorizes the use of equipment with a normal phase to earth insulating level.

Disadvantages:

1. High demand of E/F relays.2. Individual earth system needs higher investment.3. Higher touch voltage.4. Induce Potential gradient.5. Switching upon occurrence of the first insulation fault.6. Use of an RCD on each outgoing feeder to obtain total selectivity.7. Special measures must be taken for the loads or parts of the installation causing high leakage currents during normal

operation in order to avoid spurious tripping (feed the loads by insulating transformers or use high threshold RCDs,compatible with the exposed conductive part earth resistance).

8. Very high fault currents leading to maximum damage and disturbance in telecommunication networks.9. The risk for personnel is high while the fault lasts; the touch voltages which develop being high.

10. Requires the use of differential protection devices so that the fault clearance time is not long. These systems are costly.

(2)TN System: Neutral-connected exposed conductive part

First Letter T = the neutral is directly earthed at Transformer.Second Letter N=the Frames of Electrical loads are connected to the neutral Conductor.There are two types of TN systems, depending on whether the neutral conductor and Earth conductor are combined ornot:

(a)TN-C:

In TNC System (the third letter C=combined Neutral and Earth Conductor), the neutral and Earth conductors arecombined in a single conductor and earthed at source end. This Combined Neutral-Earth wire is than distributed to Load side.In This System Earthing connections must be evenly placed along the length of the Neutral-(Earth) conductor to avoidpotential rises in the exposed conductive parts at Load Side if a fault occurs.

This system must not be used for copper cross-sections of less than 10 mm² and aluminum cross-sections of less than16 mm², as well as downstream of a TNS system (As per IEC 60364-5).

System Characteristics:

1. Low earth fault loop impedance.



2. High earth fault current.3. More than one earth fault loops.

Advantages:

1. No earth wire required; allow of multi-point earth,2. Better earthing continuity.

3. Neutral never have float voltage.4. Impedance of earth fault loop could be predicted.5. The TNC system may be less costly upon installation (elimination of one switchgear pole and one conductor).

Disadvantages:

1. If not multi-point earthed, and the neutral earth broken, the exposed metallic part may have float voltage.2. High earth fault level,3. intervene the operation of earth fault protective device.4. current operated type device is not appropriated, voltage detected type could be employed.5. Third and multiples of third harmonics circulate in the protective conductor (TNC system).6. The fire risk is higher and, moreover, it cannot be used in places presenting a fire risk (TNC system).

(b)TN-S:

In TN-S system (the third letter S=Separate Neutral and Earth Conductor) neutral of the source of energy is connectedwith earth at one point only, generally near to the Source. The neutral and Earth conductors are separately distributedto load.

In This System Earthing connections must be evenly placed along the length of the Neutral-(Earth) conductor to avoidpotential rises in the exposed conductive parts at Load Side if a fault occurs.This system must not be used upstream of a TNC system.

System characteristic:

1. Low earth fault loop impedance2. High earth fault current

Advantages:

1. Use of over current protective devices to ensure protection against indirect contact.2. Earth fault protection device operates faster.3. Allow multi point earth, better earthing continuity; minimize the use of earth fault relay because of low earth fault loop

impedance.

Disadvantages:

1. Switching on occurrence of the first insulation fault.



2. The TNC system involves the use of fixed and rigid trunkings3. Requires earthing connections to be evenly placed in the installation so that the protective conductor remains at the

same potential as the earth.4. A tripping check on occurrence of the insulation fault should be carried out, if possible, when the network is being

designed using calculations, and must be performed during commissioning using measurements; this check is the onlyguarantee that the system operates both on commissioning and during operation, as well as after any kind of work onthe network (modification, extension).

5. Passage of the protective conductor in the same trunkings as the live conductors of the corresponding circuits.6. high earth fault level under earth fault condition,7. low power factor (high inductance of long cable)8. Requires extra equal potential bonding.9. On occurrence of an insulation fault, the short-circuit current is high and may cause damage to equipment or

electromagnetic disturbance.

(c)TN-C-S System:

The Neutral and Earth wires are combined within the supply cable.Typically this will be a concentric cable, with the live as the central core, and a ring of wires around this for thecombined neutral and earth.At the property, the Neutral and Earth are separated, with the earth terminal usually being on the side of the cutout.Inside the cutout, the live and neutral are linked.Throughout the supply network, the combined earth/neutral conductor is connected to the ground in multiple places,

either buried underground or at the poles for overhead supplies.This multiple earthing is why a TNCS supply is often called PME (Protective Multiple Earthing).

Advantages:

Cost for core cable is cheaper than a 3 core. As the outer sheath is usually plastic, there are no problems with corrosion.

Disadvantage:

When the combined earth/neutral conductor is broken. This results in a voltage appearing on the exposed metalwork inthe customer’s property, which can be a shock risk.This happens as the earth and neutral are connected in the cutout, and there is no direct connection to the ground otherthan in the supply network.In the event of a fault, the current flowing in the customer’s earthing conductors can be much greater that for a TNS

system.It is also possible to get unusual circulating earth currents between properties, particularly where some properties havemetal water pipes and others have plastic

Reference:



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Protection of Electrical Network-Christophe Prévé.

=====================================================================================


Type of Cable Tray

August 13, 2011 5 Comments

3 Votes

THE NEED FOR A CABLE TRAY SYSTEM: Download in PDF

As technology advances, so too does the need for effective support systems. Today, plants and buildings are moving

more and more towards automation. Requiring complex system of wiring and cable laying.

Old methods of cable management become obsolete under these demanding conditions.

1. Regular inspections must be carried out, & faults located2. Many entry/exit points are required3. New cables may need to be installed, and old ones removed4. Ventilation, essential to power and similar cables, must be provided

Today cable trays have become a necessary part of industrial and commercial construction by offering quick,economical and flexible solutions to these problems. Cable trays are capable of supporting all types of wiring:

1. High Voltage Power Lines2. Power Distribution Cables3. Sensitive Control Wiring4. Telecommunication Wiring5. Optical Cables

Cable Tray Materials:

Most cable tray systems are fabricated from a corrosion-resistant metal (low-carbon steel, stainless steel or an

aluminum alloy) or from a metal with a corrosion-resistant finish (zinc or epoxy).

The choice of material for any particular installation depends on the installation environment (corrosion and electricalconsiderations) and cost.

Aluminum

Cable trays fabricated of extruded aluminum are often used for their high strength-to-weight ratio, superior resistance tocertain corrosive environments, and ease of installation. They also offer the advantages of being light weight(approximately 50% that of a steel tray) and maintenance free, and since aluminum cable trays are non-magnetic,electrical losses are reduced to a minimum.

Cable tray products are formed from the 6063 series alloys which by design are copper free alloys for marineapplications. These alloys contain silicon and magnesium in appropriate proportions to form magnesium silicate,


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allowing them to be heat treated. These magnesium silicon alloys possess good formability and structural properties, aswell as excellent corrosion resistance.The unusual resistance to corrosion, including weathering, exhibited by aluminum is due to the self-healing aluminumoxide film that protects the surface. Aluminum’s resistance to chemicals in the application environment should be testedbefore installation.

Steel

Steel cable trays are fabricated from structural quality steels using a continuous roll-formed process. Forming andextrusions increase the mechanical strength.The main benefits of steel cable tray are its high strength and low cost. Disadvantages include high weight, low electricalconductivity and relatively poor corrosion resistance.The rate of corrosion will vary depending on many factors such as the environment, coating or protection applied andthe composition of the steel. T&B offers finishes and coatings to improve the corrosion resistance of steel. These include pre-galvanized, hot dip galvanized (after fabrication), epoxy and special paints.

Stainless Steel

Stainless steel offers high yield strength and high creep strength, at high ambient temperatures.Stainless steel cable tray is roll-formed from AISI Type 316 stainless steel.Stainless Steel is resistant to dyestuffs, organic chemicals, and inorganic chemicals at elevated temperatures. Higherlevels of chromium and nickel and a reduced level of carbon serve to increase corrosion resistance and facilitatewelding. Type 316 includes molybdenum to increase high temperature strength and improve corrosion resistance,especially to chloride and sulfuric acid. Carbon content is reduced to facilitate welding.

Finishing of Cable Tray

Galvanized Coatings

The most widely used coating for cable tray is galvanizing. It is cost-effective, protects against a wide variety of environmental chemicals, and is self-healing if an area becomes unprotected through cuts or scratches.Steel is coated with zinc through electrolysis by dipping steel into a bath of zinc salts. A combination of carbonates,hydroxides and zinc oxides forms a protective film to protect the zinc itself. Resistance to corrosion is directly related tothe thickness of the coating and the harshness of the environ ment.

Pre-Galvanized

Pre-galvanized, also known as mill-galvanized or hot dip mill-galvanized, is produced in a rolling mill by passing steelcoils through molten zinc. These coils are then slit to size and fabricated.Areas not normally coated during fabrication, such as cuts and welds, are protected by neighboring zinc, which worksas a sacrificial anode. During welding, a small area directly affected by heat is also left bare, but the same self-healingprocess occurs.G90 requires a coating of .90 ounces of zinc per square foot of steel, or .32 ounces per square foot on each side of themetal sheet. In accordance with A653/A653M-06a, pre-galvanized steel is not generally recommended for outdoor

use or in industrial environments.

Hot-Dip Galvanized

After the steel cable tray has been manufactured and assem bled, the entire tray is immersed in a bath of molten zinc,resulting in a coating of all surfaces, as well as all edges, holes and welds.Coating thickness is determined by the length of time each part is immersed in the bath and the speed of removal. Hotdip galvanizing after fabrication creates a much thicker coating than the pre-galvanized process, a minimum of 3.0ounces per square foot of steel or 1.50 ounces per square foot on each side of the sheet (according to



ASTMA123,grade 65).The process is recommended for cable tray used in most outdoor environments and many harsh industrial environment

applications.

Type of Cable Trays:

Cable trays are made of either steel, aluminum or fiber reinforced plastic (FRP) and are available in six basic types,

(1) Ladder Cable Tray

Generally used in applications with intermediate to long support spans—12 to 30 feet. Ladder cable tray is used for about 75 percent of the cable tray wiring system installations. It is the predominate cabletray type due to its many desirable features: A ladder cable tray without covers permits the maximum free flow of air across the cables. This allows the heatproduced in the cable’s conductors to effectively dissipate. Under such conditions, the conductor insulation in thecables of a properly designed cable tray wiring system will not exceed its maximum operating temperature. The cableswill not prematurely age due to excessive operating temperatures.The rungs of the ladder cable trays provide convenient anchors for tying down the cables in the non-horizontal cabletray runs or where the positions of the cables must be maintained in the horizontal cable tray runs. This capability is amust for single conductor cable installations. Under fault conditions (short circuit), the magnetic forces produced by the

fault current will force the single conductor cables from the cable tray if they are not securely anchored to the cable tray.Cables may exit or enter the ladder cable trays through the top or the bottom of the cable tray. Where the cables enteror exit conduit, the conduit to cable tray clamps may be installed upright or inverted to terminate conduits on the top orbottom of the cable tray side rail.Moisture can’t accumulate in ladder cable trays.

If cable trays are being installed where working space is a problem, hand access through the cable tray bottom may

help to facilitate the installation of small diameter cables: control instrumentation, signal, etc.The most common rung spacing for ladder cable tray is 9 inches. This spacing may be used to support all sizes ofcables. This spacing is desirable for the small diameter Type PLTC and TC cables as the support distance is such thatthere is no visible drooping of the small cables between rungs. 12 or 18 inch rung spacing provides adequate cablesupport but the slight amount of small diameter cable drooping between rungs may be aesthetically objectionable forsome installations. The maximum allowable distance between supports for 1/0 through 4/0 AWG single conductorcables is 9 inches [1993 NEC Section 318-3(b) (1)].Ventilated Trough Cable TrayThe only reason to select a ventilated trough cable tray over a ladder type cable tray is aesthetics. No drooping of smallcables is visible. The ventilated trough cable tray does provide more support to the cables than does the ladder cabletray but this additional support is not significant. It doesn’t have any impact on the cables service record or life.

Characteristics

1. Solid side rail protection and system strength with smooth radius fittings and a wide selection of materials and finishes

2. Maximum strength for long span applications3. Standard widths of 6, 12, 18, 24, 30 & 36 inches4. Standard depths of 3, 4, 5 & 6 inches5. Standard lengths of 10, 12, 20 & 24 feet6. Rung spacing of 6, 9, 12 & 18 inches.



(2) Solid Bottom Cable Tray

Generally used for minimal heat generating electrical or telecommunication applications with short to intermediatesupport spans—5 to 12 feet.The main reason for selecting solid bottom cable tray (with covers) is the concern of EMI/ RFI shielding protection forvery sensitive circuits. A solid bottom steel cable tray with steel covers provides a good degree of shielding if there areno breaks or holes in the completed installation.The solid bottom cable tray system has a disadvantage in that moisture can build up in the cable trays. This can becontrolled by drilling 1/4 inch drain holes in the bottom of the cable tray at three foot intervals (at the middle and verynear the sides) if the cable tray is not being used for EMI/RFI shielding.Some engineers and designers specify solid bottom cable trays (often with covers) in the belief that all electrical circuitshave to be totally enclosed by metal. The cable trays are just supporting cables that are designed for such installations.

Cable failures in cable tray runs rarely happen. Cable failures due to cable support problems in cable trays arenonexistent.

Characteristics

1. Non-ventilated continuous support for delicate cables with added cable protection available in metallic and fiberglass2. Solid bottom metallic with solid metal covers for non-plenum rated cable in environmental areas3. Standard widths of 6, 12, 18, 24, 30 & 36 inches4. Standard depths of 3, 4, 5 & 6 inches5. Standard lengths of 10, 12, 20 & 24 feet.

(3) Trough Cable Tray

Generally used for moderate heat generating applications with short to intermediate support spans—5 to 12 feet

Characteristics

1. Moderate ventilation with added cable support frequency—with the bottom configuration providing cable support

every four inches.2. Available in metal and nonmetallic materials3. Standard widths of 6, 12, 18, 24, 30 & 36 inches4. Standard depths of 3, 4, 5 & 6 inches5. Standard lengths of 10, 12, 20 & 24 feet6. Fixed rung spacing of 4 inches on center.

(4) Channel Cable Tray

Used for installations with limited numbers of tray cable when conduit is undesirable. Support frequency with short tomedium support spans—5 to 10 feet.

Characteristics

1. Economical support for cable drops and branch cable runs from the backbone cable tray system



2. Standard widths of 3, 4, & 6 inches in metal systems and up to 8 inches in nonmetallic systems3. Standard depths of 1 1/4 to 1 3/4 inches in metal systems and 1, 1 1/8, 1 5/8 & 2 3/16 inches in nonmetallic systems4. Standard length of 10, 12, 20 & 24 feet.

(5) Wire Mesh Cable Tray

Generally used for telecommunication and fiber optic applications, installed on short support spans—4 to 8 feet.

Characteristics

1. A job site, field adaptable support system primarily for low voltage, telecommunication and fiber optic cables. Thesesystems are typically steel wire mesh, zinc plated

2. Standard widths of 2, 4, 6, 8, 12, 16, 18, 20 & 24 inches

3. Standard depths of 1, 2 & 4 inches4. Standard length of about 10 feet.

(6) Single Rail Cable Tray

Generally used for low voltage and power cable installations where maximum cable freedom, side fill and speed toinstall are factors.

Characteristics

1. These aluminum systems are the fastest systems to install and provide the maximum freedom for cable to enter and exitthe system

2. Single hung or wall mounted systems in single or multiple tiers3. Standard widths are 6, 9, 12, 18 & 24 inches4. Standard depths are 3, 4 & 6 inches5. Standard lengths are 10 & 12 feet.

Thermal Expansion and Contraction of Cable Tray:

A cable tray system may be affected by thermal expansion and contraction, which must be taken into account duringinstallation.

To determine the number of expansion splice plates you need, decide the length of the straight cable tray runs and thetotal difference between the minimum winter and maximum summer temperatures.To function properly, expansion splice plates require accurate gap settings between trays.The support nearest the midpoint between expansion splice plates should be anchored, allowing the tray longitudinalmovement in both directions.When a cable tray system is used as an equipment grounding conductor, it is important to use bonding jumpers at allexpansion connections to keep the electrical circuit continuous.

MAX DISTANCE BETWEEN EXPANSION JOINTS(For 1” Movement)

TemperatureDifferential (oF)

Steel (Feet) Aluminum(Feet)

25 512 260

50 256 130



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75 171 87

100 128 65

125 102 52

150 85 43

175 73 37


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Jignesh.Parmar

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