SWITCH GEAR & PROTECTIVE DEVICE (EEN-437)

SWITCHGEAR & PROTECTION

PREPARED BY:Md.Foyez Ahammad

Dept:EEEID:13205100

Air-Blast Circuit Breaker

Air-Blast Circuit Breaker (Cont. 1)* The breakers employ a high pressure air-blast as quenching medium.

* Air blast is obtained by opening the valve of high pressure air reserver.

* During opening the contacts high pressure air blast is forced through the contacts.

* Air blast cools the arc and enhances rapid increase of dielectric strength.

* Air blast sweeps away the arc and arcing products to the atmosphere.

* Restriking voltage can not breakdown the dielectric strength of the medium and arc is extenguished.

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Air-Blast Circuit Breaker (Cont. 2)

* Risk of fire is eliminated as no oil is used.

* As dielectric strength development is very rapid, final contact gap requirement is smaller, resulting reduced size of circuit breaker.

* As the arcing time is much less than that of oil CB, less damage is done to the contacts due to tremendous arcing heat.

* Air-blast CB is suitable in a place of frequent operation as arc energy dissipated is smaller than that of oil CB.

Advantage :

* Arc extinction is independent of fault current as required blast is obtained from external source than arc energy.

* Arcing products are completely removed to the air, so medium is never deteriorated as happens in oil circuit breaker.

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Air-Blast Circuit Breaker (Cont. 3)

* Air has relatively inferior arc extinguishing quality in comparison with Sulphur hexaflouride (SF6).

* Considerable maintenance is required for high pressure air tank.

* Installation area :

* Widely used in high power installation.

Disadvantage :

* Mainly used where voltage level is beyond 11 KV.

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Types of Air-blast Circuit Breaker

i. Axial-blast type : Air blast is directed along the arc path.ii. Cross-blast type : Air blast is directed at right angle to the arc path.iii. Radial-blast type : Air blast is directed radially.

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Description of Air-blast Circuit Breakersi. Axial-blast air circuit breaker :

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* Air blast is directed along the arc path.

* Describe construction and common features of Air-blast Circuit Breakers

* Contact separation required for arc extinction is smaller (1.75 cm or so).

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Axial-blast air circuit breaker : (Cont.)8

* This small gap constitutes inadequate clearance for normal voltage operation.

* An isolating switch is incorporated in series as a part of this type of CB.* Just after arc extinction isolator is opened providing necessary clearance for insulation.

Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, [email protected]

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Description of Air-blast Circuit Breakers

ii. Cross-blast air circuit breaker :

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* Air blast is directed at right angle to the arc path.* The cross-blast lengthens and forces the arc into a suitable chute for arc extinction.

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* The chute consists of arc splitter and baffle.

ii. Cross-blast air circuit breaker : (Cont.) 10

* The splitters serve to increase the length of the arc and the baffles gives improved cooling.* As a result, arc is extinguished and flow of current is interrupted.

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* Arc extinction is independent of fault current as required blast is obtained from external source than arc energy.* After arc extinction contact gap is big enough to provide required clearance .

* So, series isolating switch is not required to ensure insulation like axial-blast air circuit breaker.

Sulphur Hexaflouride (SF6) Circuit Breaker 12

* Sulphur Hexaflouride (SF6) gas is used as arc quenching medium.* Sulphur Hexaflouride is an electronegative gas and has strong tendency of absorbing free electrons.

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* Free electrons of arc is absorbed by SF6 and become immobile negative ions.

Sulphur Hexaflouride (SF6) Circuit Breaker: (Cont.) 13

* This loss of conducting electrons in the arc builds up enough insulating properties in the medium and arc is extinguished .* Construction and working procedure.

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* Fixed and moving contacts are enclosed in a chamber, called interruption chamber, containing SF6 .* The interruption chamber is connected to a high pressure SF6 gas reservior through valve with opening and closing mechanism.* Both the contacts are hollow inside and equipped with horns to safeguard from flashover.* At normal condition SF6 gas pressure in interruption chamber is 2.8 Kg/cm2 and in gas reservior is 14 Kg/cm2.

Sulphur Hexaflouride (SF6) Circuit Breaker: (Cont.)14

* When arc is struck, valve is opened, blast of SF6 gas flows between the contacts through the hollow passage to the recycling chamber and arc is extinguished .

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* After each extinction exhaust SF6 gas is recycled and fed into the high pressure gas reservior.* The tips of contacts and arcing horns are coated with arc resistant copper-tungsten material to prevent damage.* Due to the high degree of arc quenching capabilities of SF6 gas, this circuit breaker is widely used in high voltage and high power service.* To prevent leakage of expensive SF6 gas, the whole circuit is built in air tight mechanism.

Sulphur Hexaflouride (SF6) Circuit Breaker: (Cont.) 15

* Advantages :

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* Due to the superior arc quenching properties of SF6, such circuit breakers have very short arcing time. * Since dielectric strength of SF6 gas is 2 to 3 times greater than that of air, such breakers can interrupt much larger fault current.* Provides noiseless service due to close gas circuit.* The closed gas enclosure keeps the interior dry so there is no moisture problem.* There is no risk of fire as SF6, gas in non-inflammable. * Requires low maintenance, low foundations and auxiliary equipments. * Suitable to use in an explosion hazard environment like coal mine as it is completely enclosed and sealed from atmosphere.

Sulphur Hexaflouride (SF6) Circuit Breaker: (Cont.)16

* Disadvantages :

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* SF6 breakers are costly due to high cost of SF6.* Additional equipments are to be incorporated for reconditioning of SF6.

* Applications :* A typical SF6 unit is capable of dealing 60 KA and 50-80 KV.* Number of units are incorporated in series to serve higher ratings.* Conventional ratings of SF6 breakers are 230 KV, 10-20 MVA and interrupting time less than 3 sec.

Note on Flashover & Horn17

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* Flashover is defined as high intensity arc between two conductors over insulation .* Horn is a mechanical arrangement of metallic conductor to perform flashover in a guided way in order to protect insulation.

Vacum Circuit Breaker (VCB) 19

* Vacum is used as arc quenching medium.

* Range of vacum is from 10-7 to 10-5 torr. 1 torr = 1 mm Hg = 1/760 atm. pressure

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* Since vacum offers highest insulating strength, it has highest arc quenching capabilities than any other medium.

Vacum Circuit Breaker (Cont.) 20

* Dielectric strength of vacum builds up thousands time faster than any other medium.* So, arcing current interrupts at first current zero.

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* Principal :* When contacts are opened in the vacum, arc is produced through the ions of metallic vapours as per Field emission theory.

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* Metal vapours condense rapidly at the contacts and at inner surface of the breaker and ensures instant arc extinction.

* Construction :* It consists of fixed contact, moving contact and arc shield mounted inside a vacum chamber. * The movable contact is mounted on stainless steel bellows and the whole chamber is sealed preventing leakage . * A glass or ceramic vessel is used as the outer insulating body.

* At very short contact separation ( 0.625 cm) arc is extinguished as dielectric strength development of vacun is very high.


* Advantages :* They are compact, reliable and have longer life.

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* There is no fire risk and hence suitable to use in fire hazard .* They can interrupt low as well as very high fault current.

* They require little maintenance and are quiet in operation.

* They can successfully withstand lightning surge.* Low arc energy is dissipated in VCB.* VCB has low inertia and hence require smaller controlling mechanism.* Due to little maintenance requirement VCBs are used in outdoor and rural areas handling around 66 KV and 100 MVA.

Switchgear components 24

i. Bushings

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ii. Circuit breaker contacts iii. Instrument transformersiv. Bus-bars conductors

Bushing :

Switchgear components(Cont.) 25

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* Bushing is the arrangement of insulating material between or around high voltage conductor and surrounding earthed metallic plate. * The function of Bushing is to prevent breakdown between high voltage conductor and surrounding earthed metallic plate.

* Breakdown occurs in the form of puncture or Flashover. * puncture damages the insulating material and renders it incapable of withstanding even the normal working voltage.

* Flashover occurs between the exposed portion of conductor and earthed metallic plate over the insulation causing little damage and remains the bushing serviceable.

* So bushing is so designed that flashover happens before puncture.

ii. Circuit breaker contacts 26

* Circuit breaker contacts handle both normal current and short circuit current.

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* During normal current flow temperature rise and voltage drop at the junction of contacts should not exceed limit.* During fault current tremendous heat due to arc should not melt and vaporize contact tips.

a) Tulip type contacts b) Finger and wedge contactsc) Butt contacts

* Following three types of contacts have been designed to serve the purpose and They are :


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a) Tulip type contacts

* Moving contact moves inside the fixed contact.* Arc is produced between the tips of the contacts and hence arcing damage remains confined at the tips.* During normal operation electricity flows throughout the whole body i.e. non damaged portions of the contacts.* This technique results in less heating and less voltage drop across the contacts during normal operation.


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b) Finger and wedge contacts

* Spring pressure keeps the contacts tightly closed results in less heating and less voltage drop across the contacts during normal operation.* As per construction this type of contacts are not suitable for use with arc controlling devices.* This type of contacts are largely used in low voltage oil circuit breaker.


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c) Butt contacts

* Spring pressure is used for making forced contacts ensuring less heating and less voltage drop across the contacts during normal operation.* Use of higher quality materials, 85% silver and 15% nickel, at contact tips ensures less arcing decay.* End-to-end mating eliminates the affects of frictional wear and allows for quick-break (15-millisecond).* Widely used for dealing with higher short-circuit current.

iii. Instrument Transformers 30

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* Modern power system carries very high voltages with thousands of amperes of current.* Measuring instruments can not be used directly in the power line of such high ratings to have accurate readings.* To overcome this problem instrument transformers are used which provide accurate fractional output at the secondary ensuring accurate readings and suitable operation.

iii. Instrument Transformers 31

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* Instrument transformers are of two types. They are :a) Current transformer (C.T.)b) Potential transformer (P. T.)

* In high power system C.T. is connected in series while P.T. is connected in parallel rendering suitable fraction of input parameter.* Use of Instrument transformers help in following ways: ■ permits isolation between measuring instruments and high voltage. ■ wires with minimum insulation and cross section in secondary.

iv. Bus-bars and conductors* In some cases of outdoor installation CBs are connected directly to the overhead lines while for some structural outdoor and indoor arrangement CBs are connected to Bus-bars.

Problem of Circuit Interruption 32

* Inductive and capacitive loads In electric system stores and releases energy at each cycle.

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* When circuit breaker is tripped due to fault current, this stored energy partly dissipates at CB and rest of the energy is dissipated at the system as oscillatory surge.

* For efficient design of circuit breaker some topics should be understood clearly. They are:

* Circuit breaker should be capable enough to withstand dissipated energy and oscillatory surge.

i. Rate of rise of re-striking voltage (R.R.R.V.)ii. Current chopping

iii. Capacitive current breaking

Problem of Circuit Interruption 33

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* Fig. (i) shows generator, transformer, CB and fault point and Fig. (ii) represents equivalent circuit. Fig. 19.18 shows restriking.

i. Rate of rise of re-striking voltage (R.R.R.V.)

* It is rate of increase of re-striking voltage and is abbreviated by (R.R.R.V.). Conventional unit : KV/µ sec.* Consider fig (ii) up to fault point L represents inductance an C represents capacitance of single phase transmission line.

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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]

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* Before interruption , The capacitor is considered shorted due to fault current.

i. Rate of rise of re-striking voltage (R.R.R.V.) (Cont.)

* The fault current is limited only by inductance L and the whole generator voltage appears across the inductor .

* Fig . 19.18 shows system voltage e, current i, arc voltage ea where i lags e by 900.

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* When the contacts are opened and the arc finally extinguishes at some current zero, the generator voltage e is suddenly applied to the inductance and capacitance in series.* This L-C combination forms an oscillatory circuit with transient frequency fn. Where

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* The transient voltage known as re-striking voltage reaches instantaneous peak voltage of 2Em, double the system peak voltage.


* The transient voltage appears across C and hence across contacts.

* The system losses decreases transient peaks but initial overshoot may cause restrike for arc continuation.* If rate of rise of re-striking voltage (R.R.R.V.) is greater than rate of rise of dielectric strength, only then arc continues otherwise not.

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R.R.R.V. depends on (a) recovery voltage (b) natural frequency of oscillation.

* Short circuit occurring near bus-bar of power station, C becomes low resulting high natural frequency fn.. R.R.R.V. becomes very high a worse situation for circuit breaker.

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M05_ ii. Current chopping

* Current chopping is the phenomenon of current interruption before current zero is reached.* This phenomenon is happened in air blast type CB.* The powerful deionising effect of air-blast causes the arc current to fall abruptly to zero before natural current zero is reached.* This phenomenon is called current chopping and produces high voltage transient across the contacts of circuit breaker.

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M05_ ii. Current chopping ( Cont. )

* Suppose, as per Fig. 19.19( ii), at point a arc current is i and chop occurs due to air-blast.

* At this moment energy stored in the inductor is Li2/2.

* This energy instantly transfers to Capacitor C charging it to a prospective voltage e. The relation is as follows :

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M05_ ii. Current chopping ( Cont. )

* The value of prospective voltage e is very high and causes restrike and first chop happens.* As current i decreases gradually, number of successive chop with lower intensity occurs and at last arc is extinguished far before current zero is reached.* Excessive surge voltage due to current chopping is limited by shunting the contacts of the breaker with a resistor of suitable value.

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M05_ iii. Capacitive current breaking

* Capacitive current breaking happens when a circuit breaker opens a capacitive load.* Though unloaded, current I flows through the circuit breaker due to capacitance C of transmission line with respect to earth. * Fig. 19.20 shows circuit diagram while Fig. 19.21 represents capacitive current breaking phenomenon.* Consider Fig. 19.21, at point 1 current I is zero and voltage across capacitor is Vgm, the maximum voltage of generator.

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M05_ iii. Capacitive current breaking (Cont.)

* Suppose, at this moment contacts of CB are opened and voltage Vr across the contacts of CB is zero.* As Vg lags I by 900, at next half cycle voltage at point ‘A’ will be - Vgm.

* Vr becomes 2 Vgm , big enough for restriking and arc happens.

* Instantly capacitor C1 discharges and transient voltage at point ‘B’ reaches -4 Vgm , Vr becomes 3 Vgm .

* The arc current quickly reaches its first zero as it varies at natural frequency and once again the two halves of circuit become separated.

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M05_ iii. Capacitive current breaking (Cont.)

* About half a cycle later the same phenomenon occurs again and voltage difference across CB, Vr becomes almost 5 Vgm .

* Theoretically this phenomenon should proceed infinitely increasing voltage Vr by 2Vgm at every half cycle rendering a very serious condition.

* But voltage increase limits at 5 Vgm , due to leakage and corona.

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M05_ Resistance switching

* Connecting a resistance of suitable value across the contacts of CB is called resistance switching.* Resistance switching minimizes voltage surge across open contacts during current chopping and capacitive current breaking.

* Part of arc current passes through the shunt resistance R, consequently voltage surge is minimized.

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M05_ Resistance switching (Cont.)

* With R natural frequency of oscillation reduces and can be calculated with the equation

* The effect of R causes the restriking voltage grow exponentially to recovery voltage .

* The circuit becomes critically damped if the value of R is so chosen that R equals .5(L/C)½ .

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M05_ Circuit breaker ratings

* A circuit breaker should be capable to provide service at both normal and faulty condition.

a) Opening fault circuit and consequently breaking fault current.

* During faulty condition a circuit breaker should be capable to provide following three services. They are :

b) Being closed at a virtual fault situation.

c) Carrying fault current for short time.

* Corresponding to the above three duties circuit breakers have following three ratings.

i) Breaking capacity.

ii) Making capacity.

iii) Short-time capacity.

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M05_ i) Breaking capacity

* Breaking capacity is defined as the value of current(r.m.s.) a circuit breaker is capable of breaking at given recovery voltage.

* Fault current may contains a little portion of DC current for some while.

* Adjacent picture shows the status of fault current.* Consider, D-D´ is the moment of contact separation.

x = maximum value of a.c. componenty = d.c. component

Symmetrical breaking current = r.m.s. value of a.c. component = x/√2

asymmetrical breaking current = r.m.s. value of total current = ((x/√2)2 + y2) ½

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M05_ i) Breaking capacity (cont.)

* It is common practice to express breaking capacity in MVA.

* If I is the rated breaking current in amperes and V is rated line voltage in volts then for a 3-phase circuit,

Breaking capacity = √3 x V x I x 10-6 MVA

* The peak value of current during the first cycle of current wave after the closure of circuit breaker is known as Making capacity.

ii) Making capacity

* Just after contact all the reactive elements of the circuit consumes huge energy generating virtual effect of fault current due to short circuit.

Making capacity = 2.55 x Symmetrical breaking capacity.

2.55 = √2 x 1.8, multiplication with √2 converts r.m.s. value of symmetrical current into peak value.

Multiplication with 1.8 is for doubling effect.

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M05_ iii) Short-time capacity

* The period for which circuit breaker can carry fault current safely is known as Short-time capacity.

* Some times in the system fault persists for short time, 1 or 2 seconds, and then removes automatically.* To maintain continuity in power flow, circuit breaker should be capable to carry fault current for some designated time.

* If fault persists after that time, circuit breaker should trip.

* The short-time capacity depends on (a) the electromagnetic force effects (b) temperature rise at contacts.

Normal current rating

* Normal current rating is defined as the r.m.s. value of current the circuit breaker can carry continuously at its rated frequency.

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M05_ Some problems

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M05_ Some problems (cont.)

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M05_ Damping in oscillation

Generally, damped harmonic oscillators satisfy the second-order : where ω0 is the undamped of the oscillator and ζ is a constant called the .The value of the damping ratio ζ determines the behavior of the system. A damped harmonic oscillator can be:Overdamped (ζ > 1): The system returns () to equilibrium without oscillating. Larger values of the damping ratio ζ return to equilibrium more slowly. Critically damped (ζ = 1): The system returns to equilibrium as quickly as possible without oscillating. This is often desired for the damping of systems such as doors. Underdamped (0 < ζ < 1): The system oscillates (at reduced frequency compared to the undamped case) with the amplitude gradually decreasing to zero. Undamped (ζ = 0): The system oscillates at its natural resonant frequency (ωo).

Engineering

SWITCH GEAR & PROTECTIVE DEVICE (EEN-437)