Project 132kv

Introduction :’

The present day electrical power system is a.c. i.e. electric power is

generated, transmitted and distributed in the form of Alternating current. The electric

power is produce at the power station, which are located at favorable places, generally

quite away from the consumers. It is delivered to the consumer through a large network

of transmission and distribution. At many place in the line of power system, it may be

desirable and necessary to change some characteristic ( e.g. Voltage, ac to de, frequency

p.f. etc.) of electric supply. This is accomplished by suitable apparatus called sub-

station for example, generation voltage (11kv or 6.6kv) at the power station is stepped

up to high voltage (Say 220kv to 132kv) for transmission of electric power. Similarly

near the consumer’s localities, the voltage may have to be stepped down to utilization

level.

This job is again accomplished by suitable apparatus called sub-station.

Definition of sub-station :-

“The assembly of apparatus used to change some characteristics (e.g. Voltage al to de freq. p.f. etc) of electric supply is called sub-station”.

Classification of sub-station

There are several ways of classifying sub-station. However the two most important way of classifying them are.

I) According to service requirement :-

According to service requirement sub-station may be classified into.

1) Transformer sub-station :-

Those sub-station which change the voltage level of electrical supply are called TIF s/s.

2) Switching sub-station :-

These sub-station simply perform the switching operation of power line.

3)Power factor correction S/S :-

These sub-station which improve the p.f. of the system are called p.f. correction s/s. these are generally located at receiving end s/s.

4) Frequency changer S/S :-

Those sub-stations, which change the supply frequency, are known as frequency changer s/s. Such s/s may be required for industrial utilization

5) Converting sub-station :-

Those sub-station which change a.c. power into d.c. power are called converting s/s ignition is used to convert AC to dc power for traction, electroplating, electrical welding etc.

6) Industrial sub-station :-

Those sub-stations, which supply power to individual industrial concerns, are known as industrial sub-station.

II) According to constructional features :-

According to constructional features, the sub-station are classified as :

1) Outdoor Sub-Station :-

For voltage beyond 66KV, equipment is invariably installed outdoor. It is because for such Voltage the clearances between conductor and the space required for switches, C.B. and other equipment becomes so great that it is not economical to installed the equipment indoor.

2) Indoor Sub-station :-

For voltage up to 11KV, the equipment of the s/s is installed indoor because of economic consideration. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltage up to 66KV.

3) Under ground sub-station :-

In thickly populated areas, the space available for equipment and building is limited and the cost of the land is high. Under such situations, the sub-station is created

underground transformer, switch gear & other equipments are installed.The design of underground s/s requires more careful consideration .

1) The size of the s/s should be as minimum as possible. 2)There should be responsible access for both equipment & personal.

3)There should be provision for emergency lighting and protection against fire. There should be good ventilation

4) Pole-mounted sub-station :-

This is an outdoor sub-station with equipment installed overhead on

H.pole or 4-pole structure. It is the cheapest from of s/s for voltage not exceeding

11KV(or 33KV in some cases). Electric power is almost distributed in localities through

such sub-station. For complete is given bellow. Fig. Shows the typical 4-pole sub-

station, it is a distribution sub-station placed overhead on a pole. Fig No.2 shows the

schematic connections, the transformer & other equipment are mounted on H-type pole.

The 11KV line is connected to the T/F (11KV/440V) through gang isolator and fuses.

The lighting arresters are installed on the H.T. Side to protect the sub-station from

lighting strokes. The T/F step down voltage to 400 V, 3 phase, 4 wire supply. The

voltage between any two lines is 400 V & between line & neutral is 230V. The oil ckt

breaker installed on the L.T. side automatically isolates the mounted sub-station. T/F

are generally in the event of fault generally 200KVA T/F is used.

Functions of a Substation:

1 - Supply of required electrical power.2 - Maximum possible coverage of the supply network.3 - Maximum security of supply.4 - Shortest possible fault-duration.5 - Optimum efficiency of plants and the network.

6 - Supply of electrical power within targeted frequency limits, (49.5 Hz and50.5 Hz).7 - Supply of electrical power within specified voltage limits.8 - Supply of electrical energy to the consumers at the lowest cost.

Equipment Used in a Sub-Station :

The equipment required for a transformer Sub-Station depends upon the type of Sub-Station, Service

requirement and the degree of protection desired. TIF Sub-Station has the following major equipments.

1) Bus - bar :-

When a no. of lines operating at the same voltage have to be directly connected electrically, bus-bar are used, it is made up of copper or aluminum bars (generally of rectangular X-Section) and operate at constant voltage.

Fig. Shows the arrangement of Duplicate bus-bar, generally it consist of two bus-bars a “main” bus-bar and spare bus-bar. The incoming and outgoing lines can be connected to either b/b. With the help of a bus-bar coupler, which consist of a ckt breaker and isolators.

However, in case of repair of main bus-bar or fault accusing on it, the continuity of supply to the circuit can be maintain by transforming it to the spare bus-bar for voltage exceeding 33KV, Duplicate bus-bar is frequently used.

2) Insulators :-

The insulator serves two purpose. They support the conductor ( or bus bar ) and confine the current to the conductor. The most commonly used material for the manufactures of insulators is porcelain. There are several type of insulator (i.e. pine type, suspension type etc.) and there used in Sub-Station will depend upon the service requirement.

3) Isolating Switches :-

In Sub-Station, it is often desired to disconnect a part of the system for general

maintenance and repairs. This is accomplished by an isolating switch or isolator. An isolator is

essentially a kniff Switch and is design to often open a ckt under no load, in other words, isolator

Switches are operate only when the line is which they are connected carry no load. For example,

consider that the isolator are connected on both side of a cut breaker, if the isolators are to be opened,

the C.B. must be opened first.

3) Instrument Transformer :-

The line in Sub-Station operate at high voltage and carry current of thousands of amperes. The measuring instrument and protective devices are designed for low voltage (generally 110V) and current (about 5A). Therefore, they will not work satisfactory if mounted directly on the power lines. This difficulty is overcome by installing Instrument transformer, on the power lines. There are two types of instrument transformer.

i) Current Transformer :-

A current transformer is essentially a step-down transformer which steps-down the current in a known ratio, the primary of this transformer consist of one or more turn of thick wire connected in series with the line, the secondary consist of thick wire connected in series with line having large number of turn of fine wire and provides for measuring instrument, and relay a current which is a constant faction of the current in the line. The current transformer (CT) is often treated as a ‘‘black box.’’ It is atransformer that is governed by the laws of electromagnetic induction:

ε = k β Ac Nf Where ε = Induced voltage

β = Flux densityAc = Core cross-sectional areaN = Turnsf = Frequencyk = Constant of proportionality

CT Connections:

As previously mentioned, some dev ices are sensitive to the direction ofcurrent flow. It is often critical in three-phase schemes to maintain proper phase shifting . Residually connected CTs in three-phase ground-fault scheme sum to zero when the phases are balanced. Reversed polarity of a CT could cause a ground-fault relay to trip under a normal balanced condition. Another scheme to detect zero-sequence faults uses one CT to simultaneously monitor all leads and neutral.

In differential protection schemes current-source phase and magnitude are compared. Reverse polarity of a CT could effectively double the phase current flowing into the relay, thus causing a nuisance tripping of a relay. When two CTs are driving a three-phase ammeter through a switch, a reversed CT could show 1.73 times the monitored current flowing in the unmonitored circuit.

(a) Over current and ground-fault protection scheme.

(b) Differentialprotection scheme. (c) Zero sequencescheme with all three-phase leads going through the window of the CT. Thisconnection, as well as the residual connection in Figure 7.17a, will cancel out the positive- andnegative-sequence currents leaving only the zero-sequence current to the 50G device.

Sometimes the ground or neutral lead will be included. The diagram on the right shows sheathed cable. It is important that one ground point go back through the window to avoid the possibility of a shorted electrical turn via the ground path.

ii) Voltage Transformer :-

It is essentially a step - down transformer and step down the voltage in

known ratio. The primary of these transformer consist of a large number of turn of fine

wire connected across the line. The secondary way consist of a few turns and provides

for measuring instruments and relay a voltage which is known fraction of the line

voltage.

5) Metering and Indicating Instrument :-

There are several metering and indicating Instrument (e.g. Ammeters,

Volt-meters, energy meter etc.) installed in a Sub-Station to maintain which over the ckt

quantities. The instrument transformer are invariably used with them for satisfactory

operation.

6) Miscellaneous equipment :-

In addition to above, there may be following equipment in a Sub-Station. i) Fuses. ii) Carrier-current equipment. iii) Sub-Station auxiliary supplies.

7) Protective relay :-

“A protective relay is a device that detects the fault and initiates the

operation of the C.B. to isolate the defective element from the rest of the system”. The

relay detects the abnormal condition in the electrical ckt by constantly measuring the

electrical quantities, which are different under normal and fault condition. The electrical

quantities which may change under fault condition are voltage, current, frequency and

phase angle. Having detect the fault, the relay operate to close the trip ckt of C.B.

POTENTIAL VOLTAGE TRANSFORMER INTRODUCTION:

The transformer that is used to increased voltage range of volt meter is called

potential

voltage transformer. The PT is used to measure high voltage from a low range volt

meter

PT is similar to power transformer. Due to work PT is step down transformer.

Normally PT is a shell type transformer. Its construction of two windings similar to

power transformer accept its power rating is very small. It has many metric tons

weight.

CONSTRUCTION AND WORKING It has two windings, primary and secondary winding. The primary winding is connected parallel with in put voltage. The secondary winding

is free

with supply in put. The volt meter connected parallel to secondary winding. The

winding also earthed.

The load is connected to secondary winding side. During the construction, proper turn ratio, low leakage reluctance ,low voltage drop should properly decided.

The both winding are in concern, for reduce the leakage reluctance.

In place of insulation, the cotton tap and vanished cambric.

The solid fiber separator are used between coils.

The high voltage condition, the PT is consist of oil immersed, but in low

voltage.

the PT completely sealed with compound. There is high insulated provided between primary and secondary winding because these winding should be a design for high voltage. The circular shield coil used for winding to detect the electrical stress. The low voltage coil is wounded over paper core, and high voltage coil also assemble under that core. the complete assembly is sealed in oil and coil assembly keep held in a cover. There are bushing used on steel cover. Then steel cover is completely with immersed oil.

In normal condition the primary voltage remains constant, so volt meter can not show reading. The secondary winding connected volt meter, relay contacts etc. In abnormal condition the primary winding voltage changes, so voltages across volt

meter change,

and it indicates these changes. The change of voltage effect on relay plunger

contacts

and relay contacts close with each other and relay gives trip single to circuit

breaker.

Buchholz Relays:-

The following protective devices are used so that, upon a fault development inside aTransformer, an alarm is set off or the Transformer is disconnected from the circuit.

In the event of a fault, oil or insulations decomposes by heat, producing gas or developing an impulse oil flow. To detect these phenomena, a Buchholz relay is installed. The Buchholz relay is installed at the middle of the connection pipe

between the Transformer tank and the conservator. There are a 1st stage contact and a 2nd stage contact as shown in Fig. The 1st stage contact is used to detect minor faults.

When gas produced in the tank due to a minor fault surfaces to accumulate in the relay chamber within a certain amount (0.3Q-0.35Q) or above, the float lowers and closes the contact, thereby actuating the alarm device.

The 2nd stage contact is used to detect major faults. In the event of a major fault, abrupt gas production causes pressure in the tank to flow oil into the conservator. In this case, the float is lowered to close the contact, thereby causing the Circuit Breaker to trip or actuating the alarm device. Buchholz Relay figure show below.

Fig Buchholz Relay

STRUCTURE OF BUCHHOLZ REALY

Attraction relays: Attraction relays can be supplied by AC or DC, and operate by the movement of a piece of metal when it is attracted by the magnetic field produced by a coil. There are two main types of relay in this class.

The attracted armature relay, which is shown in figure 1, consists of a bar or plate of metal which pivots when it is attracted towards the coil.The armature carries the moving part of the contact, which is closed or opened according to the design when the armature is attracted to the coil. The other type is the piston or solenoid relay, illustrated in Figure 2, in which α bar or piston is attracted axially within the field of the solenoid. In this case, the piston also carries the operating contacts.

It can be shown that the force of attraction is equal to K1I2 - K2, where Κ1 depends upon the number of turns on the operating solenoid, the air gap, the effective area and the reluctance of the magnetic circuit, among other factors. K2 is the restraining force, usually produced by a spring. When the relay is balanced, the resultant force is zero and thereforeΚ112 = K2, So that I=K2/K1= constant

In order to control the value at which the relay starts to operate, the restraining tension of the spring or the resistance of the solenoid circuit can be varied, thus modifying the restricting force. Attraction relays effectively have no time delay and, for that reason, are widely used wheninstantaneous operations are required.

8) Circuit breaker :-

A circuit breaker is an equipment, which can open or close a ckt under

normal as well as fault condition. It is so designed that it can be operated manually ( or

by remote control) under normal conditions and automatically under fault condition. For

the latter operation a relay wt. is used with a C.B. generally bulk oil C.B. are used for

voltage upto 66 KV while for high voltage low oil & SF6 C.B. are used. For still higher

voltage, air blast vacuum or SF6 cut breaker are used.

CIRCUIT BREAKER

The process of fault clearing has the following sequence:

1-Fault Occurs.

As the fault occurs, the fault impedance being low,the currents increase and the relay gets actuated.The moving part of the relay move because of the increase in the operating torque. The relay takes some time to close its contacts.

2 - Relay contacts close the trip circuit of the Circuit Breaker closes andtrip coil is energized.

3 - The operating mechanism starts operating for the openingoperation.The Circuit Breaker contacts separate.

4 - Arc is drawn between the breaker contacts. The arc isextinguished in the Circuit Breaker by suitable techniques. The current reaches final zero as the arc is extinguished and does not restrict again. The Trip-Circuit Fig (1) below illustrates the basic connections of the Circuit Breaker control for the opening operation.

The type of the Circuit Breaker :

The type of the Circuit Breaker is usually identified according to the medium of arc extinction. The classification of the Circuit Breakers based on the medium of arc extinction is as follows:

(1) Air break' Circuit Breaker. (Miniature Circuit aker).(2) Oil Circuit Breaker (tank type of bulk oil)(3) Minimum oil Circuit Breaker.(4) Air blast Circuit Breaker.(5) Vacuum Circuit Breaker.(6) Sulphur hexafluoride Circuit Breaker. (Singlepressure or Double Pressure).

9) Power Transformer:-

ANSI=IEEE defines a transformer as a static electrical device, involving nocontinuously moving parts, used in electric power systems to transfer power between circuits throughthe use of electromagnetic induction. The term power transformer is used to refer to those transformers used between the generator and the distribution circuits, and these are usually rated at 500 kVA and above.

Power systems typically consist of a large number of generation locations, distribution points, and interconnections within the system or with nearby systems, such as a neighboring utility. The complexity of the system leads to a variety of transmission and distribution voltages.

Power transformers must be used at each of these points where there is a transition between voltage levels.Power transformers are selected based on the application, with the emphasis toward custom design being more apparent the larger the unit. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up (GSU) transformers, and for step-down operation, mainly used to feed distribution circuits. Power transformers are available as single-phase or three-phase apparatus.

Transformer is a vital link in a power system which has made possible the power generated at low voltages (6600 to 22000 volts) to be stepped up to extra high voltages for transmission over long distances and then transformed to low voltages for utilization at proper load centers.

This flux induces an electro-motive force in the secondary winding too. When load is connected across this winding, current flows in the secondary circuit.This produces a demagnetizing effect, to counter balance this the primary winding draws more current from the supply so that

IP.NP = IS.NS

POWER TRANSFORMER

CONSTRUCTION :

1- Transformer Core Construction in which the iron circuit is surrounded by windings and forms a low reluctance path for the magnetic flux set up by the voltage impressed on the primary.

Windings:

The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported, and cooled to withstand operational and test conditions.The terms winding and coil are used interchangeably in this discussion.Copper and aluminum are the primary materials used as conductors in power-transformer windings.While aluminum is lighter and generally less expensive than copper, a larger cross section of aluminumconductor must be used to carry a current with similar performance as copper.

Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable. In cases where extreme forces are encountered, materials such as silver-bearing copper can be used for even greater strength. The conductors used in power transformers are t y pically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors.

Multiple strands can be wound in parallel and joined together at the ends of the winding, in which case it is necessary to transpose the strands at various points throughout the winding to prevent circulating currents around the loop(s) created by joining the strands at the ends.Individual strands may be subjected to differences in the flux field due to their respective positions within the winding , which create differences in voltages between the strandsand drive circulating currents through the conductor loops.

Proper transposition of the strands cancels out these voltage differences and eliminates or greatly reduces the circulating currents. A variation of this technique, involving many rectangular conductor strands combined into a cable, is called continuously transposed cable (CTC), as shown in Figure

Continuously transposed cable (CTC)

Continuously transposed cable (CTC). Concentric arrangement, outer coil being lowered onto core leg over top of inner coil.

A variety of different types of windings have been used in power transformersthrough the years. Coils can be wound in an upright, vertical orientation, as is necessary with larger, heavier coils; or they can be wound horizontally and placed upright upon completion. As mentioned previously, the type of winding depends on the transformer rating as well as the core construction. Several of the more common winding types are discussed here..

Pancake Windings :

Several types of windings are commonly referred to as ‘‘pancake’’ windings due to the arrangement of conductors into discs. However, the term most often refers to a coil type thatis used almost exclusively in shell-form transformers. The conductors are wound around a rectangular form, with the widest face

Pancake winding during winding process

of the conductor oriented either horizontally or vertically. Figure 2.12illustrates how these coils are typically wound. This type of winding lends itself to the interleaved arrangement previously discussed.

Layer (Barrel) Windings:

Layer (barrel) windings are among the simplest of win dings in that the insulated conductors are wound directly next to each other around the cylinder and spacers. Several layers can be wound on top of one another, with the layers separated by solid insulation, ducts, or a combination. Several strands can be wound in parallel if the current magnitude so dictates.

Variations of this winding are often used for applications such as tap windings used in load-tap-changing (LTC) transformers and for tertiary windings used for, among other things, third-harmonic suppression.Figu are shows a layer winding during assembly that will be used as a regulating winding in an LTC transformer.

Stacked pancake windings

Layer windings (single layer with two strands wound in parallel)

Helical Windings:

Helical windings are also referred to as screw or spiral windings, with eachterm accurately characterizingthe coil’s construction. A helical winding consists of a few to more than 100

insulated strands wound inparallel continuously along the length of the cylinder, with spacers insertedbetween adjacent turns ordiscs and suitable transpositions included to minimize circulating currentsbetween parallel strands. Themanner of construction is such that the coil resembles a corkscrew. Figure2.15 shows a helical windingduring the winding process. Helical windings are used for the higher-currentapplications frequentlyencountered in the lower-voltage classes.

Helical winding during assembly.

Tank:

The tank has two main parts: a –The tank is manufactured by forming and welding steel plate to be used as a container for holding the core and coil assembly together with insulating oil.

The base and the shroud, over which a cover is sometimes bolted. These parts are manufactured in steel plates assembled together via weld beads.The tank is provided internally with devices usually made of wood for fixing the magnetic circuit and the windings.

In addition, the tank is designed to withstand a total vacuum during the treatment process. Sealing between the base and shroud is provided by weld beads. The other openings are sealed with oil-resistant synthetic rubber joints, whose compression is limited by steel stops.

Finally the tank is designed to withstand the application of the internal overpressure specified, without permanent deformation.

Fig Power Transformer 30 MVA 132 / 11 KV

Conservator:

The tank is equipped with an expansion reservoir (conservator) which allowsfor the expansion of the oil during operation. The conservator is designed tohold a total vacuum and may be equipped with a rubber membranepreventing direct contact between the oil and the air.

The dehydrating breather:

The dehydrating breather is provided at the entrance of the conservator ofoil immersed equipment such as Transformers and reactors.The conservator governs the breathing action of the oil system on forming to the temperature change of the equipment, and the dehydrating breather removes the moisture and dust in the air inhaled and prevents the deterioration of the Transformer oil due to moisture absorption.Construction and Operation See Fig.. The dehydrating breather uses silica - gel as the desiccatingAgent and is provided with an oil pot at the bottom to filtrate the inhaled air.The specifications of the dehydrating breather are shown in Table (1) and the operation of the component parts in Table (2)

Fig. Dehydrating breathe

Bushing:

Having manufactured various types of bushings ranging from 6kV-class to 800kV-class, Toshiba has accumulated many years of splendid actual results in their operation.Plain-type Bushing Applicable to 24 kV-classes or below, this type of bushing is available in astandard series up to 25,000A rated current. Consisting of a single porcelain tube through which passes a central conductor, this bushing is of simplified construction and small mounting dimensions; especially, this type proves to be advantageous when used as an opening of equipment to be placed in a bus duct Fig

Oil-impregnated, Paper-insulated Condenser Bushing

Fig. 800 KV bushing

Temperature Measuring Device:

Liquid Temperature Indicator (like BM SERIES Type) is used to measure oil temperature as a standard practice. With its temperature detector installed on the tank cover and with its indicating part installed at any position easy to observe on the front of the Transformer, the dial temperature detector is used to measure maximum oil temperature.

The indicating part, provided with an alarm contact and a maximum temperature pointer, is of airtight construction with moisture absorbent contained therein; thus, there is no possibility of the glass interior collecting moisture whereby it would be difficult to observe the indicator Fig. (30&31).

Further, during remote measurement and recording of the oil temperatures,on request a search coil can be installed which is fine copper wire wound on a bobbin used to measure temperature through changes in its resistance.

Winding Temperature Indicator Relay (BM SERIES). The winding temperature indicator relay is a conventional oil temperature indicator supplemented with an electrical heating element.

The relay measures the temperature of the hottest part of the Transformer winding. If specified, the relay can be fitted with a precision potentiometer with the same characteristics as the search coil for remote indication.

Fig. (29) Construction of Winding Temperature Indicator Relay

Fig. (30) Winding Temperature Indicator

The temperature sensing system is filled with a liquid, which changes in volume with varying temperature. The sensing bulb placed in a thermometer well in the Transformer tank cover senses the maximum oil temperature.

The heating elements with a matching resistance is fed with current from the Transformer associated with the loaded winding of the Transformer and compensate the indicator so that a temperature increase of the heating element is thereby proportional to a temperature increase of the winding- over-the maximum- oil temperature.

Therefore, the measuring bellows react to both the temperature increase of the winding-over-the-maximum-oil temperature and maximum oil temperature. In this way the instrument indicates the temperature in the hottest part of the Transformer winding.The matching resistance of the heating element is preset at the factory.

Pressure Relief Device:

When the gauge pressure in the tank reaches abnormally To 0.35-0.7 kg/cm.sq. The pressure relief device starts automatically to discharge the oil

.When the pressure in the tank has dropped beyond the limit through discharging, the device is automatically reset to prevent more oil thanrequired from being discharged.

Fig. (32) Pressure Relief Device

PARALLEL OPERATION OF THREE-PHASE TRANSFORMERS:

Ideal parallel operation between Transformers occurs when (1) there are no circulating currents on open circuit, and (2) the load division between the Transformers is proportional to their kVA ratings.

These requirements necessitate that any - two or more three phase Transformers, which are desired to be operated in parallel, should possess:

1) The same no load ratio of transformation;2) The same percentage impedance;3) The same resistance to reactance ratio;4) The same polarity;5) The same phase rotation;6) The same inherent phase-angle displacement between primary

and secondary terminals. The above conditions are characteristic of all three phase Transformers whether two winding or three winding. With three winding Transformers, however, the

following additional requirement must also be satisfied before the Transformers can bedesigned suitable for parallel operation.

7) The same power ratio between the corresponding windings. The first four conditions need no explanation being the same as in single phase Transformers.

The fifth condition of phase rotation is also a simple requirement. It assumes that the standard direction of phase rotation is anti-clockwise. In case of any difference in the phase rotation it can be set right by simply interchanging two leads either on primary or secondary. It is the intention here to discuss the last two i.e., sixth and seventh conditions in detail.

– CT's For Bus-Bar protection and metering. – Circuit Breaker. – Maintenance Earth Switches.

Bus Section:

1 – Bus-Bar Isolator. (Disconnector Switch)2 – Maintenance Earth Switches.3 – CT's For Bus-Bar protection and metering.4 – Circuit Breaker.6 – Maintenance Earth Switches.7 – Bus-Bar Isolator.

Bus coupler:

Circuit breaker using SF6 gas:

Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc extinguishing properties. The dielectric strength of the gas increases with pressure and is more than of dielectric strength of oil at 3 kg/cm2. SF6 is now being widely used in electrical equipment like high voltage metal enclosed cables; high voltage metal clad switchgear, capacitors, circuit breakers, current transformers, bushings, etc. The gas is liquefied at certain low temperature, liquefaction temperature increases with pressure.

Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur.

The steel box is made gas tight. The gas thus obtained contains other fluorides such as S2F10, SF4 and must be purified further SF6 gas generally supplier by chemical firms. The cost of gas is low if manufactured in large scale.

During the arcing period SF6 gas is blown axially along the arc. The gas removes the heat from the arc by axial convection and radial dissipation. As a result, the arc diameter reduces during the decreasing mode of the current wave. The diameter becomes small during the current zero and the arc is extinguished.

Due to its electronegativity, and low arc time constant, the SF6 gas regains its dielectric strength rapidly after the current zero, the rate of rise of dielectric strength is very high and the time constant is very small.

Lightening arrestor:

These lightening arrestors are used to prevent the lightening from damaging the instruments in the substation.

Lightening arrestors are the instrument that are used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation. Even the instruments are very costly, so to prevent any damage lightening arrestors are used.

The lightening arrestors do not let the lightening to fall on the station. If some lightening occurs the arrestors pull the lightening and ground it to the earth. In any substation the main important is of protection which is firstly done by these lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the lightening to the ground. The lightening arrestor works with an angle of 30° to 45° making a cone.

DC BATTERIES :

The led acid battery used at grid station. Electrolyte for battery is hydro sulfuric acid (H2SO4).

The led acid battery is made of hoppeck by Germany. The quantity of batteries used at grid station is 55. The ampere hour capacity of battery is 480 ampere hour. The date of all batteries is 25/09/2004. The float voltages are 119.5V.

DC BETTERY

CONSTRUCTION :

The led acid battery contains two plates and electrolyte.

The plates are made of led per oxide (Pbo2) and use for positive plate and negative plate is made of led (Pb).

H2SO4 use as electrolyte.

The battery use for large scale. There is a compound used for grid. The compound contains lead and antimony (Pb & Sb). There is a layer of PbO2 is also use for surfing on grid. After charging Pbo2 change in acto material.

The plates are divided into negative and positive charges.

The negative plates are large in quantity. Normally one more then positive plate.

The negative plates should spread around the positive plates to complete the chemical reaction.

The separators are used between these plates. These separators are any insulating material.

The separator has two sides. One side consists of grooved used positive to

positive plate for circulating of electrolyte around positive plate.

There is hole on cell upper side to filling and testing of oil. There is a filler

cap on the hole. Filler GP is vent hole used for gas exit.

Battery case and cell covers are made of rabber and acid proof selling compound are used for outer side sealed.

POSITIVE PLATE:

To prepare positive plate grid frame is made of antimony and led. There is surface of lead per oxide is use o outer side and positive plates becomes.

Its colour is dark brown.

It has two works first is to support actio material and is to pass current to the post.

NEGATIVE PLATE:

Its colour is grey.

The difference between positive and negative plates are the actio material is roughf type.

SEPARATORS:

The separators are held between positive and negative plates for to protect the short circuit. Because separator are insulater material.

The material use for separator are e.g glass wood, micro poric etc.

ELECTROLYTE:

The electrolyte is prepare with H2SO4 and water compound.

Both plates are immersed in the electrolyte.

To prepare electrolyte we take 5% of water and 1% of H2SO4.during preparationof electrolyte should be mix one drop step by step into water and a rod should be used for mixing.

During process gloves, apron and gaggles should be used.

During preparation the temperature of electrolyte increase, so specific gravity should check with hydrometer.

Normally the specific gravity is 1.205 to 1.215 if specific gravity is less than correct value than we increase the quantity of H2SO4 should be increased but higher value of specific gravity the water should be used in electrolyte.

Emergancy lighting used for battery with electrolyte has 1.2 specific gravity

ELEMENT:

The assembly of positive and negative plates in any cell is called element cell

CELL COVERS AN D VENT PLUGS:

These are made of hard rubber. Two terminal are shown from cell cover. The vent whole is used for entered the electrolyte the in cell. Vent plugs are made of rubber.

BATTERY TERMINAL:

Battery has two terminals the positive terminal is long and has increase in area than negative plate.

CELL CONNECTORS:

The cell connectors are used to connect the cell in series

CONTAINER:

The container has electrolyte and battery plates.

WORKING PRINCIPLE:

There are two simple leads strips immersed into H2SO4 electrolyte tank. If these are connected with respect to the lamp in series with 115 volt dc source. The current passes through these strips and produces gas bubble. After some time oneplate change its colour and other plate does not change its colour.During thatprocess the cell charged and volt meter show reading. When the cell discharge itsvoltage reduced to 1.75 volt. After this the process of

voltage drop increased and voltage becomes zero. The positive plate colour changes from dark brown to light brown and at last it colour return to its initial state.

The positive plate has dark choclate that plate how is componently in spongy. Led

is negative plate maximum bubble exit from negative plate the bubbles are as a

reason of hydrogen gas but some bubbles also come from positive plate due to

oxygen gas the change metalic led into led per oxide. The negative plates can not

changes with chemical reaction. That plate change normally solid led to spongy

led. There is emf produced between these plates and cell will be charged.

CHARGING OF LED ACID BATTERY :

The discharge battery charge with DC supply. The direction of current should be positive than during discharging situation. The battery terminals that are positive and negative should be connected with negative and positive terminals of source.

The first state of battery charging that the lower specific gravity of electrolyte should be used.

During charging H2 ions moves to cathode and SO4 ions moves to anod. So anod and cathode becomes to Pb2 and Pb.

The anod becomes to brown and cathode becomes to gray colour.

The specific gravity of H2SO4 increased from 1.18 to 1.27.

The voltage of cell increased from 1.8 to 2.3V.

The cell obsorb energy.

When cell charge it produced gas.

The voltage to charge the battery should be 25% higher the battery voltage.

EARTHING :

All equipment used at 132 kv grid station all earthed.

Some equipments are placed in open but some protection equipment are placed in building. Every equipment has separately earthing system.

E A R T H I N G

S I N G L E L I N E D I A G R A M E

O F

1 3 2 K V

G R I D S T A T I O N

Single line diagram (SLD)

A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of the concernedElectrical System which includes all the required ELECTRICAL EQUIPMENT connection

sequence wise from the point of entrance of Power up to the end of the scope of the mentionedWork.

As in the case of 132KV Substation, the SLD shall show Lightening Arrestor, State ElectricityBoard's C.T/P.T Unit, Isolators, Protection and Metering P.T & C.T. Circuit Breakers, againIsolators and circuit Breakers, Main Power Transformer, all protective devices/relays and otherspecial equipment like NGR, CVT, GUARD RINGS, SDR etc as per design criteria.

2.1. Fig: Single line diagram of substation.

As these feeders enter the station they are to pass through various instruments. The instruments have their usual functioning. They are as follows in the single line diagram. 1. Lightening arrestors, 2. C V T3. Wave trap 4. Current transformer 5. Isolators with earth switch

6. Circuit breaker

7. Line isolator

8. BUS

9. Potential transformer in the bus with a bus isolator

10. Isolator

11. Current transformer

12. Circuit breaker

13. Lightening arrestors

14. Transformer

15. Lightening arrestors with earth switch

16. Circuit breaker


18. Isolator

19. Bus

20. Potential transformer with a bus isolator

21. A capacitor bank attached to the bus.

Brief descriptions of the instruments in the line diagram are-


Lightening arrestors are the instrument that are used in the incoming feeders so that to preventthe high voltage entering the main station. This high voltage is very dangerous to the instrumentsused in the substation. Even the instruments are very costly, so to prevent any damage lighteningarrestors are used. The lightening arrestors do not let the lightening to fall on the station. If somelightening occurs the arrestors pull the lightening and ground it to the earth. In any substation themain important is of protection which is firstly done by these lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the lightening to the ground. The lightening arrestor works with an angle of 30° to 45° making a cone.

2. C V T

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-downextra high voltage signals and provide low voltage signals either for measurement or to operate aprotective relay. In its most basic form the device consists of three parts: two capacitors acrosswhich the voltage signal is split, an inductive element used to tune the device to the supplyfrequency and a transformer used to isolate and further step-down the voltage for theinstrumentation or protective relay. The device has at least four terminals, a high-voltageterminal for connection to the high voltage signal, a ground terminal and at least one set ofsecondary terminals for connection to the instrumentation or protective relay. CVTs are typicallysingle-phase devices used for measuring voltages in excess of one hundred kilovolts where theuse of voltage transformers would be uneconomical. In practice the first capacitor, C1, is oftenreplaced by a stack of capacitors connected in series. This results in a large voltage drop acrossthe stack of capacitors that replaced the first capacitor and a comparatively small voltage dropacross the second capacitor, C2, and hence the secondary terminals.

3. Wave trap

Wave trap is an instrument using for tripping of the wave. The function of this trap is that it trapsthe unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is connected tothe main incoming feeder so that it can trap the waves which may be dangerous to theinstruments here in the substation.


Current transformers are basically used to take the readings of the currents entering thesubstation. This transformer steps down the current from 800 amps to 1 amp. This is donebecause we have no instrument for measuring of such a large current. The main use of thistransformer is (a) distance protection; (b) backup protection; (c) measurement.


Lightening arrestors after the current transformer are used so as to protect it from lightening i.e. from high voltage entering into it. This lightening arrestor has an earth switch, which can directly earth the lightening. The arrestor works at 30° to 45° angel of the lightening making a cone. The earth switch can be operated manually, by pulling the switch towards ground. This also helps in breaking the line entering the station. By doing so maintenance and repair of any instrument can b performed.

6. Circuit breaker

The circuit breakers are used to break the circuit if any fault occurs in any of the instrument.These circuit breaker breaks for a fault which can damage other instrument in the station. Forany unwanted fault over the station we need to break the line current. This is only doneautomatically by the circuit breaker. There are mainly two types of circuit breakers used for anysubstations. They are (a) SF6 circuit breakers; (b) spring circuit breakers.

The use of SF6 circuit breaker is mainly in the substations which are having high input kv input,say above 220kv and more. The gas is put inside the circuit breaker by force ie under highpressure. When if the gas gets decreases there is a motor connected to the circuit breaker. Themotor starts operating if the gas went lower than 20.8 bar. There is a meter connected to thebreaker so that it can be manually seen if the gas goes low. The circuit breaker uses the SF6 gasto reduce the torque produce in it due to any fault in the line. The circuit breaker has a direct linkwith the instruments in the station, when any fault occur alarm bell rings.

The spring type of circuit breakers is used for small kv stations. The spring here reduces the torque produced so that the breaker can function again. The spring type is used for step down side of 132kv to 33kv also in 33kv to 11kv and so on. They are only used in low distribution side.

7. Line isolator

The line isolators are used to isolate the high voltage from flow through the line into the bus.This isolator prevents the instruments to get damaged. It also allows the only needed voltage andrest is earthed by itself.

8. BUS

The bus is a line in which the incoming feeders come into and get into the instruments for furtherstep up or step down. The first bus is used for putting the incoming feeders in la single line.There may be double line in the bus so that if any fault occurs in the one the other can still havethe current and the supply will not stop. The two lines in the bus are separated by a little distanceby a conductor having a connector between them. This is so that one can work at a time and theother works only if the first is having any fault.

9. Potential transformers with bus isolators

There are two potential transformers used in the bus connected both side of the bus. Thepotential transformer uses a bus isolator to protect itself. The main use of this transformer is tomeasure the voltage through the bus. This is done so as to get the detail information of thevoltage passing through the bus to the instrument. There are two main parts in it (a)measurement; (b) protection.

10. Isolators

The use of this isolator is to protect the transformer and the other instrument in the line. Theisolator isolates the extra voltage to the ground and thus any extra voltage cannot enter the line.Thus an isolator is used after the bus also for protection.

11. Current transformer Current transformers are used after the bus for measurement of the current going out through the feeder and also for protection of the instruments.

12. Circuit breaker The circuit breakers are used to break the circuit if any fault occurs in the circuit of the any

feeders.


The use of lightening arrestors after the bus is to protect the instrument in the station so that

lightening would not affect the instruments in the station.

14. Transformer

There are three transformers in the incoming feeders so that the three lines are step down at

the

same time. In case of a 220kv or more kv line station auto transformers are used. While in case

of lower kv line such as less than 132kv line double winding transformers are used.


The lightening arrestors are used with earth switch so that lightening would not pass through

the

instruments in the station.

16. Circuit breaker

The circuit breakers are used to break the circuit for any fault.


Current transformers are used to measure the current passing through the transformer. Its

main

use is of protection and measurement.

18. Isolator

These are used to ground the extra voltage to the ground.

19. Bus

This bus is to carry the output stepped down voltage to the required place.

20. Potential transformer with a bus isolator

Two PT are always connected across the bus so that the voltage across the bus could be

measured.

21. Capacitor bank attached to the bus.

The capacitor banks are used across the bus so that the voltage does not gets down

till the require place.

G R O U P N O 1 1

P R E S E N T E D B Y : *

H A S E E B A L I

1 4 P 2 - 2 1 0 0 1 5

N A S E E R G U L Z A R

1 4 P 2 - 2 1 0 0 2 8

W A Q A R A S L A M

1 4 P 2 - 2 1 0 0 3 9

Z A H E E R A S L A M

1 4 P 2 - 2 1 0 0 4 8

P R O J E C T

O F

1 3 2 K V

G R I D S T A T I O N

S U B M I T E D T O * H A M I D

R A Z A

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Project 132kv