NTDCL 220/132 kV Grid Station (Gakkhar)

Internship Report

(Summer internship 2014)

Submitted by: Naseeb Ali 2011-EE-502 Shahvaiz Ali 2011-EE-511 Ibrar Hussain 2011-EE-517 Ihtasham Ali 2011-EE-550 Zeesham Afzal 2011-EE-574 Ans Saljook 2011-EE-576 M.Zahid 2011-EE-587 M.Kashif Junaid 2011-EE-588

Rachna College of Engineering and Technology, Gujranwala

(A Constituent College of UET Lahore)

Starting Date: 15-JUN-2014 End Date: -JUL-2014

Preface

The grooming of an engineering student is incomplete without proper

exposure to the industry.we are completing our Bachelor’s degree in

Electrical Engineering at Rachna College of Engineering and Technology,

Gujranwala. We are students of final year in specialization of Power

engineering therefore, we have to conduct an internship for learning

purposes. This report documents the work done and learning during the

summer internship at National Dispatch and Transmission Company Ltd.

220/132 kV grid station Gakkhar. The internship report contains and

overview of the internship company and the activities, tasks that we have

worked on during our internship. This report shall give and overview of tasks

completed and technical details we have learnt during the period of

internship and the discussions and overview of lectures taken.

We have tried our best to write the complete knowledge we gain during this

internship in report.

ACKNOWLEDGMENTS

The work culture in grid station really motivates.we could not have done this

work without the lots of help we received cheerfully from whole NTDCL.

Everybody is such a friendly and cheerful companion here that work stress is

never comes in way.Special thanks to Sir Akbar Ali and Sir Mansor for providing

the nice ideas to work upon. Their lectures were very informative and made

clear a lot of our concepts about field work and related to our Electrical

Engineering perspective.

Contents:

Why there is a need of Grid Stations?

An introduction to NTDCL Gakkhar Grid Station

Single line diagram

Explanation of Equipment used

Transmission line

Isolators

Terminal Tower

Wave Trap

CCVT

Earthing Switch

Current Transformer (CT)

Potential Transformer (PT)

Circuit Breaker (CB)

Auto Transformer

Operation

Limitations

Parallel Operation of transformer

Vector group of transformer

PROTECTION OF GRID

Transformer

DC Protection

Why there is a need of Grid Stations?

An electrical grid (also referred to as an electricity grid or electric grid) is an

interconnected network for delivering electricity from suppliers to consumers. It

consists of generating stations that produce electrical power, high-voltage transmission

lines that carry power from distant sources to demand centers, and distribution lines

that connect individual customers.

Power stations may be located near a fuel source, at a dam site, or to take advantage of

renewable energy sources, and are often located away from heavily populated areas.

They are usually quite large to take advantage of the economies of scale. The electric

power which is generated is stepped up to a higher voltage-at which it connects to the

transmission network.

When voltage level of a power is increased, the electric current of the power is reduced

which causes reduction in ohmic or 𝐼2𝑅losses in the system, reduction in cross sectional

area of the conductor i.e. reduction in capital cost of the system and it also improves the

voltage regulation of the system(as low I results in low voltage drop in the line). Because

of these, low level power must be stepped up for efficient electrical power transmission.

The transmission network will move the power long distances, sometimes across

international boundaries, until it reaches its wholesale customer (usually the company

that owns the local distribution network).

On arrival at a substation, the power will be stepped down from a transmission level

voltage to a distribution level voltage. As it exits the substation, it enters the distribution

wiring. Finally, upon arrival at the service location, the power is stepped down again from

the distribution voltage to the required service voltage(s).

Candle light dinners are most enjoyable when they are not forced. In today’s modern

world of electrical home appliances we realize that poor maintenance of the electricity

supply may mean a loss of a few expensive appliances alongside added inconvenience.

Thus in order to keep your passionately light decorated houses glimmering there are

grid stations to ensure high reliability power supply.

There are different kinds of Grid stations such as:

66 KV Grid Station

132KV Grid Station

220 KV Grid Station

500 KV Grid Station

In this report only 220 KV Grid Station is discussed.

GRID STATION GAKHAR

132kv Ghakhar G/Stn. which was commissioned during 01/77 was upgraded to

220KV level with commissioning on 11.06.1982. 220KV Ghakhar G/Stn. has 04Nos.

160MV, 220/132KV Power T/Fs and 02 Nos. 26 MVA, 132/11KV Power T/Fs

installed at the G/Stn. The G/Stn. is being fed from Mangla Power House through 02

Nos. 220KV D/C T/Lines. This G/S is also connected with 220KV G/S Sialkot through

Single Circuit and 500KV Nokhar and Mangla Power House through IN Out

arrangement.

An introduction to 220/132 kV NTDCL Grid Station (Gakkhar) This is 220/132 kV grid station, located at Gakkhar (near Gujranwala), under NTDCL (National

Transmission and Dispatch Company Ltd.). The lines coming from MANGLA-I, MANGLA-II and

NOKHAR (500/220 kV grid) and outgoing to SIALKOT. (Although, any of lines can be used to

send or receive power so, don’t confuse yourself with the direction of arrows in single line diagram-

as shown in fig (1) below). It uses double bus scheme on both of its 220 and 132 kV sides with

sectionalizing scheme on 132 kV side. Which will be discussed below in single line diagram.

Single line diagram:-

In power engineering, a one-line diagram or single-line diagram (SLD) is a

simplified notation for representing a three-phase power system. Electrical elements,

such as circuit breakers, transformers, capacitors, bus bars, and conductors are

shown by standardized schematic symbols. Instead of representing each of three

phases with a separate line or terminal, only one conductor is represented. It is a

form of block diagram graphically depicting the paths for power flow between entities

of the system. Elements on the diagram do not represent the physical size or location

of the electrical equipment.

Fig (1) single line diagram of grid station

There are some conventions and symbols for single line diagram, such as: the

colour of different potential lines are different. Which are given below:

500kV Brown

220kV Green

132kV Red

11kV Blue

Same like, there are symbols for equipment to show on single line diagram for

complete understanding. Some of them are shown below:

The equipment in single line diagram are denoted by some of symbols which

are given in LEGEND with it. Here, the circuit breakers are represented by “Q” and

for understanding its potential and position, there are also keywords, like:

132kV EnQn

220kV DnQn

500kV BnQn

Where, n=1, 2, 3….

Which represent the location and exact name of the circuit breaker and make ease

in communication.

Some other key words are:

L.A Lightning Arresters

W.T Wave Trap

C.T Current Transformer

P.T Potential Transformer

Others are represented by their symbols. Shown in fig.

The single line diagram of this grid station is shown below. According to which

there are total four lines, coming form MANGLA-I, MANGLA-II and NOKHAR and going

to SIALKOT. On each of line (entering or leaving) there are some common equipment

installed such as: Instrument Transformers (C.T & P.T), Wave Trap, CCVT,

Isolators, Grounding Switches, Lighting Arresters.

Bus bars:

There is double bus bar scheme in grid station. The advantage of this is that,

when there is any fault or any maintenance issue, we can convert whole load on any

one bus bar without any interruption in supply of power. Also there is a bus coupler,

which couples both of the buses. In normal conditions, the load is distributed on both

of the buses, using this bus coupler. Although, each of bus has capacity to handle the

complete load. During maintenance or any fault on any of the bus, the isolator of bus

coupler is set such that the load is transferred on the healthy bus.

Sectionalizer:-

On 132kV side, the two buses are sectionalized in two section each by using

the SECTIONALIZER. These sections are made to increase the reliability of the buses

and the transformers. This is also used for parallel operation of transformer (which

will be discussed later) and coupling the transformers for the power to transformer,

of same rating, percentage impedance and ratio. The buses are divided in many

sections, if there is fault in any of the sections, this section can be isolated from the

system without interruption in other sections.

Explanation of Equipment used:

Transmission line:

There are three types of transmission line:

Short Transmission Line (up to 80 Km)

Medium Transmission Line (80-240 Km)

Long Transmission Line (above 240 Km)

Effects on the transmission line:

The transmission lines have following three major effects:-

Ferranti effect:

When the voltage at the receiving end is increased as compared to

sending end voltage at normal load, called as Ferranti Effect. This is due to the

capacitance between line to line and line to ground. As with increase in length

of transmission line, the capacitance of line increases and inductance reduces,

so due to this capacitance the Ferranti effect is more in Long and transmission

lines as compared to Short transmission line as capacitance is negligible in

short transmission lines.

Skin Effect:

Due to frequency in alternating current, current starts to flow on the

surface of conductor instead of using whole conductor area. Due to this used

area of conductor is reduced and resistance faced by current is increased. So

due to this effect our conductor is wasted as it is not fully utilized.

To eliminate this effect we use stranded conductors.

Corona Effect:

Electric power transmission practically deals in the bulk transfer of

electrical energy, from generating stations situated many kilometers away from

the main consumption centers or the cities. For this reason the long distance

transmission cables are of utmost necessity for effective power transfer, which

in-evidently results in huge losses across the system. Minimizing those has

been a major challenge for power engineers of late and to do that one should

have a clear understanding of the type and nature of losses. One of them being

the corona effect in power system.

For corona effect to occur effectively, two factors here are of prime importance as

mentioned below:-

1) Alternating electrical potential difference must be supplied across the line.

2) The spacing of the conductors, must be large enough compared to the line

diameter.

Insulators:

The overhead line conductors should be supported on the poles or towers in

such a way that currents from conductors do not flow to earth through supports i.e.

line conductors must be properly insulated from supports. The insulator provides

necessary insulation between line conductors and supports and thus prevent any

leakage current form conductor to earth.

The insulators are made up of Porcelain, Glass, Steatite and special types of

materials.

Types:

Pin type (up to 33kV)

Suspension type (above 33kV)

Strain Type insulator

Shackle type insulator

Terminal Tower:

The tower at the end of transmission line i.e. at the start of the grid station is

known as the terminal tower. This tower carry the power line entering the grid station

having ‘strain insulators’ on it. It’s a double circuit tower having two parallel power

lines on it with a ‘SKY WIRE’ or ‘EARTHED WIRE’ or ‘OPGW’ (optical ground wire)

which is used for the protection of the transmission system from lightning strokes and

any of lightning falls on the transmission system, is grounded through this wire as

this wire is on the top of the tower.

The insulators used for insulation of line with ground (tower) and with each

other, depend upon the potential in the line. The “string” of insulator discs (made of

porcelain, glass or such type of insulating material) are made for this purpose,

according to the potential of the lines. For a rough estimate of the voltage of line

passing can be made by counting the number of plates of the insulator on the tower,

as they are designed and installed according to voltage level. The rough estimate is

like- if you multiply the number of discs with number 15, the answer will provide the

voltage in kV.

Wave Trap:

For the communication between two grid station and between the main head

office and the grid station, NTDCL has its own communication system on the

transmission line. As the

communication requires very

high frequency signals

(>500kHz) so, to separate these

signals from the power signals

(low frequency) we need an

equipment known as Wave Trap,

which is a high pass filter and

allows to pass high frequency

signals only to the

communication equipment. The

equipment or the system used for

it is called as POWER LINE

CARRIER (PLC).

CCVT:

This is Capacitor Coupling Transformer. As the power signal is of low frequency

(50 Hz) so to pass this CCVT is used which is a low pass filter and always gives output

110 volts. Its working is same as the Potential Transformer with addition of low pass

filter circuit.

Isolator:

Isolators are provided for isolation from live parts for the purpose of maintenance.

Isolators are located at either side of the circuit breaker. Isolators are operated

under no load. If it is operated under load, there will be arc between the contacts

of the isolator. Isolator does not have any rating for current breaking or current

making. Isolators are interlocked with circuit breakers

Types of Isolators

are

1. Central rotating,

horizontal swing

2. Centre-Break

3. Vertical swing

4. Pantograph type

Earthing Switch:

Earthing switches are mounted on the base of mainly line side isolator. Earthing

switches are normally vertically break switches. Earthing arms (contact arm of

earthing switch) are normally aligned horizontally at off condition. During

switching on operation, these earthing arms

rotate and move to vertical position

and make contact with

earth female contacts fitted at

the top of the post insulator

stack of isolator at its outgoing

side. The earthing arms are so

interlocked with main isolator

moving contacts that it can be

closed only when the main

contacts of isolator are in open

position. Similarly the main

isolator contacts can be closed

only when the earthing arms

are in open position.

Current Transformer:

For the measurement of the current in the line, the

current transformer (C.T) is used. It is an Instrument

transformer which brings current in the range to be

measured. It comes in different primary to secondary ratio

(800/1 or 800/5).

For the safety precaution, the secondary of C.T must

not be opened as on the secondary side there are a large

amount of voltage as current is very low, so due to high

potential arc will produce and CT will burst out. Care must

be taken that the secondary of a current transformer is not

disconnected from its load while current is in the primary,

as the transformer secondary will attempt to continue

driving current across the effectively infinite impedance up

to its core saturation voltage. This may produce a high

voltage across the open secondary into the range of several

kilovolts, causing arcing, compromising operator and

equipment safety, or permanently affect the accuracy of the

transformer.

The accuracy of a CT is directly related to a number of factors including:

Burden

Burden class/saturation class

Rating factor

Load

External electromagnetic fields

Temperature and Physical configuration.

The selected tap, for multi-ratio CTs

Phase change

Maintenance Test:

For the maintenance of the current transformer, two tests are performed,

Capacitance test

The capacitance between HV and LV side of transformer is checked. From

the reading of the instrument we multiply it with the place of knob reading

to get the capacitance. Then percentage depreciation factor is

measured according to the temperature. If the temperature is different form

the ambient (20 degree C) then multiplied by the correction factor provide

in table with the instrument manual.

The DF must be below 1 (or may be up to 1.2) for the best operation.

Insulation test:

Insulation of transformer is checked using MEGGAR (mega ohm-meter).

The MEGGAR is connected between HV side and ground and the resistance

is measured.

Generally, for one kV, 1Mohm is suitable resistance. i.e. for 132kV the

insulation must be about 132 mega ohm.

The calculations were taken at atm temperature and after correction factor

for 20 degree Celsius were applied and result was 5000 MOHM. Which is

suitable for working.

Potential transformer (PT):-

Voltage transformers are used to step down the voltage for measurement,

protection and control. Voltage transformers are of two types.

1. Electromagnetic type

2. Capacitive VT located on the feeder side

of the Circuit Breaker.

The primary of potential transformer must not

be short as it is connected in parallel and there

will be a burst and PT could be damaged.

A voltage transformer theory or potential

transformer theory is just like a theory of

general purpose step down transformer.

Primary of this transformer is connected across

the phase and ground. Just like the transformer

used for stepping down purpose, potential

transformer i.e. PT has lower turns winding at

its secondary. The system voltage is applied

across the terminals of primary winding of that

transformer, and then proportionate secondary

voltage appears across the secondary

terminals of the PT.

The secondary voltage of the PT is generally 110 V. In an ideal potential transformer

or voltage transformer, when rated burden gets connected across the secondary;

the ratio of primary and secondary voltages of transformer is equal to the turn ratio

and furthermore, the two terminal voltages are in precise phase opposite to each other.

But in actual transformer, there must be an error in the voltage ratio as well as in the

phase angle between primary and secondary voltages.

Circuit Breaker:

Circuit breaker is device used to break the circuit in case of excess amount of

current passing (due to any fault or other). It is protective device for the protection

of all equipment in case of fault that can damage the other devices. Here, the circuit

breakers used are SF6 circuit

breakers.

Sulfur Hexafluoride is an

excellent gaseous dielectric for

high voltage power applications.

It has been used extensively in

high voltage circuit breakers and

other switchgears employed by

the power industry. Application

for SF6 include gas insulated

transmission lines and gas

insulated power distributions.

The combined electrical,

physical, chemical and thermal

properties offer many

advantages when used in power

switchgears. Some of the

outstanding properties of this

are below:-

Very high dielectric

strength.

Very unique arc-quenching ability.

Very excellent thermal stability.

Very good thermal conductivity.

There is a tank of SF6 having specific pressure for the operation of circuit breaker

below which CB is unable to operate and there is pressure gauge for it showing and

monitoring the pressure in tank. When the pressure goes down, it causes a specific

relay to operate for alarm purposes.

MAINTAINANCE TEST:

For the checking the health of circuit breakers, three following tests are

performed annually:-

1. SF6 purity test For the best working of circuit breaker the gas should be pure. For this

purpose, test is performed at regular intervals to check the purity.

2. Contact resistance test :

Due to operation of circuit breaker, contacts of breaker become rough

due to arc between them. So, the resistance of contacts increases above limit

resulting very increase in I2R losses. So, the resistance of contacts are checked

annually and it must be in micro ohms and if exceed, these must be replaced.

This is off load test.

3. Timing test.

The opening and closing the contacts of circuit breaker matters a lot. So,

there must be a little time for circuit breaker to sense and break the circuit.

For this timing test is performed using a test set model number TM1600. This

test is off load test i.e. the circuit breaker is disconnected from the circuit and

then test is performed. For the best operation of circuit breaker, following must

be full filled:

Opening time must be between 28-30 msec.

Closing time must be about 60 msec.

AUTO-TRANSFORMER:-

An autotransformer is an electrical transformer with only one winding. The

"auto" (Greek for "self") prefix refers to the single coil acting on itself and not to any

kind of automatic mechanism. In an autotransformer, portions of the same winding

act as both the primary and secondary sides

of the transformer. The winding has at least

three taps where electrical connections are

made. Autotransformers have the

advantages of often being smaller, lighter,

and cheaper than typical dual-winding

transformers, but the disadvantage of not

providing electrical isolation. Other

advantages of autotransformers include

lower leakage reactance, lower losses, lower

excitation current, and increased KVA rating.

Autotransformers are often used to step up or step down voltages in the 110-115-

120 V range and voltages in the 220-230-240 volt range—for example. Providing 110

V or 120 V (with taps) from 230 V input, allowing equipment designed for 100 or

120 volts to be used with a 230 volt supply.

OPERATION:-

An autotransformer has a single winding with two end terminals, and one or more

terminals at intermediate tap points, or a transformer in which the primary and

secondary coils have part or all of their turns in common. The primary voltage is

applied across two of the terminals, and the secondary voltage taken from two

terminals, almost always having one terminal in common with the primary voltage.

The primary and secondary circuits therefore have a number of windings turns in

common. Since the volts-per-turn is the same in both windings, each develops a

voltage in proportion to its number of turns. In an autotransformer part of the current

flows directly from the input to the output, and only part is transferred inductively,

allowing a smaller, lighter, cheaper core to be used as well as requiring only a single

winding. However the voltage and current ratio of autotransformers can be

formulated the same as other two-winding transformers.

As in a two-winding transformer, the ratio of secondary to primary voltages is equal

to the ratio of the number of turns of the winding they connect to. For example,

connecting the load between the middle and bottom of the autotransformer will

reduce the voltage by 50%. Depending on the application, that portion of the winding

used solely in the higher-voltage (lower current) portion may be wound with wire of

a smaller gauge, though the entire winding is directly connected.

LIMITAION:-

An autotransformer does not provide electrical isolation between its windings as an

ordinary transformer does; if the neutral side of the input is not at ground voltage,

the neutral side of the output will not be either. A failure of the insulation of the

windings of an autotransformer can result in full input voltage applied to the output.

Also, a break in the part of the winding that is used as both primary and secondary

will result in the transformer acting as an inductor in series with the load (which under

light load conditions may result in near full input voltage being applied to the output).

These are important safety considerations when deciding to use an autotransformer

in a given application.

Because it requires both fewer windings and a smaller core, an autotransformer for

power applications is typically lighter and less costly than a two-winding transformer,

up to a voltage ratio of about 3:1; beyond that range, a two-winding transformer is

usually more economical.

In three phase power transmission applications, autotransformers have the limitations

of not suppressing harmonic currents and as acting as another source of ground

fault currents. A large three-phase autotransformer may have a "buried" delta

winding, not connected to the outside of the tank, to absorb some harmonic currents.

In practice, losses mean that both standard transformers and autotransformers are

not perfectly reversible; one designed for stepping down a voltage will deliver slightly

less voltage than required if it is used to step up. The difference is usually slight

enough to allow reversal where the actual voltage level is not critical.

Like multiple-winding transformers, autotransformers use time-varying magnetic

fields to transfer power. They require alternating currents to operate properly and will

not function on direct current.

Parallel Operation of transformer

To connect two or more transformers with each other, the operation is followed

is called as the parallel operation of transformer. Parallel operation is carried out to

increase the reliability of the system in such a way that the total load on both is equal

to the maximum rating of one transformer. That is, in case one transformer goes

down for some reason, the one can handle the load without interruption. In normal

conditions, both transformers are sharing the load and hence the reliability increases

and the life of transformer.

For the parallel operation of transformers, three following conditions must be

satisfied:

All transformers should have

1. Same impedance drops.

2. Same turn ratios.

3. Same ratings.

Transformers connected in parallel have the same voltage on each primary and

the same voltage on each secondary. The difference in the voltage between the

primary and secondary windings is the turn ratios. For these terminal voltages to be

the same for the paralleled transformers, their impedance drops must be identical.

Therefore, under any condition of load, the current will be divided such that the

product of impedance and current in one transformer is equal to the product of

impedance and current in the other. Also, if the turn ratios of the transformers are

different, but the primary and secondary terminal voltages are the same in both

transformers, then circulating currents must flow between the transformers, even at

no load.

Typically, transformers should not be operated in parallel when:

• The division of load is such that, with the total load current equal to the combined

kVA rating of the transformers, one of the transformers is overloaded.

• The no-load circulating currents in any transformer exceed 10% of the full load

rating

• The combination of the circulating currents and full load current exceed the full load

rating of either transformer.

Following table (1) shows conditions for transformers under which we can use

transformers in parallel operation:-

Vector group of transformer:-

In electrical engineering, a vector group is the International Electro technical

Commission (IEC) method of categorizing the high voltage (HV) windings and low

voltage (LV) winding configurations of three-phase transformers. The vector group

designation also indicates the windings configurations and the difference in phase

angle between them.

Symbol designation:-

Y = HV side connected in star

y= LV side connected in star

D= HV side connected in delta

d= LV side connected in delta

N= neutral connected to HV side

n= neural connected to LV side

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At the end there may be integers like 1, 2, 3… 12 for phase displacement between

LV and HV side. E.g.

1=300 , 2=600 ,3=900,…… 12=3600or 00

For example the vector group of auto transformer is Yyan0

This shows that this transformer is connected in star in HV and LV side and ‘a’ for

auto transformer and ‘n’ shows that neutral grounded to LV side and ‘0’ shows that

phase difference between HV and LV is zero degree.

PROTECTION OF GRID:-

Transformer protection:-

Transformer protection mainly divided into two major groups:

1. Mechanical Protection

2. Electrical Protection

Mechanical Protection:

Mechanically operated protection is called as mechanical protection which do

not have any concern with electric signal to operate. Some are following:

A) MAIN BUCHHOLZ RELAY:

It is gas operated, gas actuated relay. When there is any fault with the

windings of transformer. i.e. it is short from any part, spark will be produced

and it will decompose the oil in the transformer due to which gases will

produced and goes upward to the conservator tank where, there is buchholz

relay main and conservator tank and due to pressure of gases, relay operated.

B) Pressure relief relay:

This relay is operated due to sudden change of pressure of gases within

the tank. When winding is short from more than one place and more arc is

produced and large pressure is created in the tank.

C) ON LOAD TAP CHANGER RELAY:

This is for the protection of the tap changer. If there is any short circuit

or any fault in tap changer this relay operated.

18

D) WINDING TEMPERATURE RELAY:

This relay is used to maintain the temperature of winding of transformer

at safe level. When more current passes from the winding, it causes to increase

the temperature of winding hence, this current is used to maintain the

temperature of winding. From the current, we can find the voltage drop of it:

V=IR

This voltage is applied to the relay and used to operate it. When the voltage

drop increase form set level it will operate.

This relay has four taps during working. At first step when temperature

rises to about 60 percent of total it activate FAN group 1 if temperature further

rises, it activate FAN GROUP 2 on further increase in temperature, alarm will

be activated and at last relay is operated to trip circuit breakers of both sides.

E) OIL TEMERATURE AND OIL LEVEL RELAY:

These relays are used to check and maintain the temperature and level

of in transformer. These relays use some kind of sensors to sense the

temperature and oil level of main tank of transformer and operate after a set

level.

Electrical Protection:

A) Main Differential Relay:

Differential relay takes the difference of currents between primary and

secondary side of transformer- through the CT of both sides. If there is some

kind of difference between them, it operates the circuit breaker taking it as

fault. As when fault occur on either side of transformer, current increases and

hence the difference of current increases so relay will operate the circuit

breakers.

Ispill = IHV - ILV

To make zero the difference between them, Matching CTs are also placed after

them which make both currents equal for difference. When about 0.1A

difference occurs, main differential relay trip the circuit.

B) HV and LV Over Current Relay:

These relays are used to protect the HV and LV side of transformer in

case of high current. By the use of CT on both sides, it senses the current and

operates accordingly.

19

C) Tertiary Over Current Relay:

This relay is for the protection of the tertiary windings of transformer-

which is used to remove the harmonics in windings. When there is over current

flows in tertiary winding of transformer, this operates the circuit breakers of

transformer.

D) Breaker Failure Relay:

This relay operates when there is fault in breaker or when breaker is not

working properly. This checks the connection of circuit breaker and in case of

fault there, it operates.

E) Pole Discrepancy Relay:

For circuit breaker to be operated properly, it is necessary condition that

all the three poles must be operated at the same time. If all the three poles

fail to cut off or come to circuit at same time, fault occurs. This relay ensures

that, all the poles are operating, during circuit breaker operation. It operates

with the time delay of less than 300msec. This relay avoid that only one or two

phases are open during steady state operation.

F) Over Excitation Relay:

Over excitation of transformer can occur whenever ratio of per unit

voltage to per unit frequency at secondary terminals exceeds its rating. That

is, as we know that the core of transformer is excited and de-excited on each

cycle and due to the voltage applied. This excitation is given by formula

E =4.44 f N Φ

From this, it can be seen that Φ is directly related to the ratio of (V/f). So, to

avoid to exceed this ratio for specific transformer, a relay is set at point above

which it operates and trips the circuit.

DC Protection:- The protection of grid is done by the DC source. The 220kV protection

is done at 220 volts while 132kV protection is carried out at 110 volts. For these

voltages, there is a battery room where batteries are placed. Each cell is of

2volts and each plate of cell has capacity of 300Ah. So, for 220 volts, 110 such

cells are placed in series and for 110 volts, 55 such cells are placed. These are

lead-acid batteries and use electrolyte in them.

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For the maintenance of these batteries, the specific gravity of electrolyte

is checked regularly by the instrument called as ‘Hydrometer’. Another thing

for their maintenance is that they are charged properly from a panel providing

suitable voltage for charging. Normally, float charging is done in normal days

but once a week, boast charging also applied on these batteries for their good

health.

Fig (battery room)

Fig (hydrometer)

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