CHAPTER : 1

INTRODUCTION

The 220KV GSS is situated in VKIA ,Jaipur . In the GSS single main and auxiliary

scheme is used .there are total four feeders in which one for incoming from Heerapura ,one

interconnection for 220KV Kukas and two 132KV out going feeders from 100MVA capacity

transformer.

This GSS step down the voltage level from 220KV to 132KV .there are two bus bar

arrangement .main bus bar used at a time and auxiliary bus bar are connected through bus

coupler .in case of main bus or other equipment are out of order or other maintenance

purpose than supply should be given through auxiliary bus bar without interruption in supply.

1.1 Defination-

A substation is a part of an electrical generation, transmission, and distribution

system. Substations transform voltage from high to low, or the reverse, or perform any of

several other important functions. Electric power may flow through several substations

between generating plant and consumer, and its voltage may change in several steps.

A substation that has a step-up transformer increases the voltage while decreasing the

current, while a step-down transformer decreases the voltage while increasing the current for

domestic and commercial distribution. The word substation comes from the days before the

distribution system became a grid. The first substations were connected to only one power

station, where the generators were housed.

1.2 History of G.S.S.

A substation is a part of an electrical generation, transmission, and distribution

system. Substations transform voltage from high to low, or the reverse, or perform any of

several other important functions. Electric power may flow through several substations

between generating plant and consumer, and its voltage may change in several steps.

1

A substation that has a step-up transformer increases the voltage while decreasing the current,

while a step-down transformer decreases the voltage while increasing the current for

domestic and commercial distribution. The word substation comes from the days before the

distribution system became a grid. The first substations were connected to only one power

station, where the generators were housed, and were subsidiaries of that power station.

1.3 Elements of a substation

Substations generally have switching, protection and control equipment, and

transformers. In a large substation, circuit breakers are used to interrupt any short circuits or

overload currents that may occur on the network. Smaller distribution stations may use

recloser circuit breakers or fuses for protection of distribution circuits. Substations

themselves do not usually have generators, although a power plant may have a substation

nearby. Other devices such as capacitors and voltage regulators may also be located at a

substation

Fig- 1.1: substation

Substations may be on the surface in fenced enclosures, underground, or located in

special-purpose buildings. High-rise buildings may have several indoor substations. Indoor

substations are usually found in urban areas to reduce the noise from the transformers, for

reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.

2

Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to

protect people from high voltages that may occur during a fault in the network. Earth faults at

a substation can cause a ground potential rise. Currents flowing in the Earth's surface during a

fault can cause metal objects to have a significantly different voltage than the ground under a

person's feet; this touch potential presents a hazard of electrocution.

1.3.1 Transmission substation

A transmission substation connects two or more transmission lines. The simplest case

is where all transmission lines have the same voltage. In such cases, the substation contains

high-voltage switches that allow lines to be connected or isolated for fault clearance or

maintenance. A transmission station may have transformers to convert between two

transmission voltages, voltage control/power factor correction devices such as capacitors,

reactors or static VAr compensators and equipment such as phase shifting transformers to

control power flow between two adjacent power systems.

Transmission substations can range from simple to complex. A small "switching

station" may be little more than a bus plus some circuit breakers. The largest transmission

substations can cover a large area (several acres/hectares) with multiple voltage levels, many

circuit breakers and a large amount of protection and control equipment (voltage and current

transformers, relays and SCADA systems). Modern substations may be implemented using

International Standards such as IEC61850.

1.3.2 Distribution substation

fig- 1.2 Disribution substation

3

A distribution substation in Scarborough, Ontario disguised as a house, complete with

a driveway, front walk and a mown lawn and shrubs in the front yard. A warning notice can

be clearly seen on the "front door".

A distribution substation transfers power from the transmission system to the

distribution system of an area. It is uneconomical to directly connect electricity consumers to

the main transmission network, unless they use large amounts of power, so the distribution

station reduces voltage to a value suitable for local distribution.

The input for a distribution substation is typically at least two transmission or

subtransmission lines. Input voltage may be, for example, 115 kV, or whatever is common in

the area. The output is a number of feeders. Distribution voltages are typically medium

voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of

the local utility.

The feeders run along streets overhead (or underground, in some cases) and power the

distribution transformers at or near the customer premises.

In addition to transforming voltage, distribution substations also isolate faults in either

the transmission or distribution systems. Distribution substations are typically the points of

voltage regulation, although on long distribution circuits (of several miles/kilometers),

voltage regulation equipment may also be installed along the line.

The downtown areas of large cities feature complicated distribution substations, with

high-voltage switching, and switching and backup systems on the low-voltage side. More

typical distribution substations have a switch, one transformer, and minimal facilities on the

low-voltage side.

1.3.3 Collector substation

In distributed generation projects such as a wind farm, a collector substation may be

required. It somewhat resembles a distribution substation although power flow is in the

opposite direction, from many wind turbines up into the transmission grid. Usually for

economy of construction the collector system operates around 35 kV, and the collector

4

substation steps up voltage to a transmission voltage for the grid. The collector substation can

also provide power factor correction if it is needed, metering and control of the wind farm. In

some special cases a collector substation can also contain an HVDC static inverter plant.

Collector substations also exist where multiple thermal or hydroelectric power plants

of comparable output power are in proximity. Examples for such substations are Brauweiler

in Germany and Hradec in the Czech Republic, where power is collected from nearby lignite-

fired power plants. If no transformers are installed for increase of voltage to transmission

level, the substation is a switching station.

1.3.4 Switching substation

A switching substation is a substation which does not contain transformers and

operates only at a single voltage level. Switching substations are sometimes used as collector

and distribution stations. Sometimes they are used for switching the current to back-up lines

or for parallelizing circuits in case of failure. Example herefore are the switching stations at

HVDC Inga-Shaba.

1.3.5 Layout

The first step in planning a substation layout is the preparation of a one-line diagram

which shows in simplified form the switching and protection arrangement required, as well as

the incoming supply lines and outgoing feeders or transmission lines. It is a usual practice by

many electrical utilities to prepare one-line diagrams with principal elements (lines, switches,

circuit breakers, transformers) arranged on the page similarly to the way the apparatus would

be laid out in the actual station.

5

Fig -1.3: single line Diagram

In a common design, incoming lines have a disconnect switch and a circuit breaker.

In some cases, the lines will not have both, with either a switch or a circuit breaker being all

that is considered necessary. A disconnect switch is used to provide isolation, since it cannot

interrupt load current. A circuit breaker is used as a protection device to interrupt fault

currents automatically, and may be used to switch loads on and off, or to cut off a line when

power is flowing in the 'wrong' direction. When a large fault current flows through the circuit

breaker, this is detected through the use of current transformers. The magnitude of the current

transformer outputs may be used to trip the circuit breaker resulting in a disconnection of the

load supplied by the circuit break from the feeding point. This seeks to isolate the fault point

from the rest of the system, and allow the rest of the system to continue operating with

minimal impact. Both switches and circuit breakers may be operated locally (within the

substation) or remotely from a supervisory control center.

6

Once past the switching components, the lines of a given voltage connect to one or

more buses. These are sets of bus bars, usually in multiples of three, since three-phase

electrical power distribution is largely universal around the world.

The arrangement of switches, circuit breakers and buses used affects the cost and

reliability of the substation. For important substations a ring bus, double bus, or so-called

"breaker and a half" setup can be used, so that the failure of any one circuit breaker does not

interrupt power to other circuits, and so that parts of the substation may be de-energized for

maintenance and repairs. Substations feeding only a single industrial load may have minimal

switching provisions, especially for small installations.

Once having established buses for the various voltage levels, transformers may be

connected between the voltage levels. These will again have a circuit breaker, much like

transmission lines, in case a transformer has a fault (commonly called a "short circuit").

.

1.4 Switching function

An important function performed by a substation is switching, which is the

connecting and disconnecting of transmission lines or other components to and from the

system. Switching events may be "planned" or "unplanned".

A transmission line or other component may need to be deenergized for maintenance

or for new construction, for example, adding or removing a transmission line or a

transformer. To maintain reliability of supply, no company ever brings down its whole

system for maintenance.

7

CHAPTER : 2

POWER TRANSFORMER

The most common purpose of a power transformer is to convert alternating current

(A.C.) power from one A.C. voltage (or current) to another A.C. voltage (or current). Another

common purpose is to provide electrical isolation between electrical circuits. Power is the

product of voltage times current. Power transformers do not change power levels except for

parasitic losses. Input power minus parasitic power losses equals output power. Ideal power

transformers have no losses, hence output power equals input power. Increasing the output

voltage will decrease the output current. Electric utilities prefer to transmit electricity at low

current values to reduce resistive losses in the power transmission lines. Lower currents also

permit smaller size transmission cables. A power transformer is used between the generating

equipment and the power line(s) to step-up (increase) the transmission voltage (to high

voltage) and decrease the transmission current also again used to step-down (decrease) the

voltage to voltage levels needed for their purpose.

Power transformers may be classified by their power ratings (fractional VA to mega-

VA), 2their type of construction, and/or by their intended application. The same basic power

transformer may be suitable for multiple applications hence the same power transformer may

be classified under several overlapping category types. The common person associates power

transformers with the electric utilities.

The power transformer are classified as

1.Two winding Transformer

2.Three winding Transformer

3. Auto Transformer

The transformer used at 220KV GSS are Auto Transformer of Rating 100.

8

Fig2.1 Power trasnformer

2.1 POWER TRANSFORMER ACCESSORIES

2.1.1 Buchholz relay

Buchholz relay is a gas- actuated relay installed in oil-immersed transformers for

protection against all kind of internal faults. It is used to gives an alarm in case of slow

developing faults or incipient faults in the transformer and to disconnect the transformer from

the supply in the event of severe internal faults. It is installed in the pipe between the

conservator and main tank as shown in fig. below. This relay is used in oil-immersed

transformers of rating above 750 kVA.

9

Fig- 2.2 : Buchholz relay

2.2 CONSTRUCTION:-

Fig shows the constructional details of buchholz relay. It consists of a domed vessel

placed in the pipe between the conservator and main tank of the transformer. The device has

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two elements. The upper element consists of a mercury type switch attached to a float. The

lower element contains a mercury switch mounted on a hinged type flap located on the direct

path of flow of oil from the transformer to the conservator. The upper element closes an

alarm circuit during slow developing faults whereas the lower element is arranged to trip the

circuit breaker in case of severe internal faults.

2.3 OPERATION:-

The operation of buchholz relay is as follows:-

In case of slow developing faults within the transformer, the heat due to the fault causes

decomposition of some transformer oil in the main tank. The products of decomposition

mainly contain 70 % of hydrogen gas. The hydrogen gas being light tries to go into the

conservator and in the process gets trapped in the upper part of the relay chamber. When a

predetermined amount of gas gets accumulated, it exerts sufficient pressure on the float to

cause it to tilt and close the contacts of mercury switch attached to it. This completes the

alarm circuit to sound an alarm.If serious fault occur in the transformer, an enormous amount

of gas is generated in the main tank. The oil in the main tank rushes towards the conservator

via the buchholz relay and in doing so it tilts the flap to close the contacts of mercury switch.

This completes the trip circuit to open the circuit breaker controlling the transformer.

2.4 Merits And Demerits

2.4.1 ADVANTAGES:

• It is the simplest form of transformer protection

• It detects the slow developing faults at a stage much earlier than other forms of

protection.

2.4.2 DISADVANTAGES:

1. It can only be used with oil immersed transformers equipped with conservators

2. The device can detect only faults below oil leveling the transformer. Therefore separate

protection is needed for connecting cables.

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2.5 OIL SURGE RELAY

2.5.1 GENERAL

The protective relay serves for signalling a fault in the diverter switch / selector

switch oil compartment and is to limit the damage to the on-load tap changer in case of a

failure.It is, therefore, part of our delivery as a standard with all OLTC supplies.

Fig- 2.3: oil surge relay

2.5.2 Design

The housing is provided with 2 flanges for connecting pipes leading to the tap

changer head and to the oil conservator. The flap valve position can be checked through the

inspection glass on front of the housing. The terminals are housed in a terminal box sealed

against the oil of the relay. For checking the tripping function of the relay as well as for

subsequently resting the flap valve, two test push buttons are installed in the terminal box.

2.5.3 Relay

The relay drive element consists of the flap valve with attached permanent magnet.

The magnet actuates thereed- contact and serves for fixing the flap valve in position “

Transformer in service” An mintermediate position of the flap valve is not possible.

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2.5.4 FUNCTION

The protective relay is energized by an oil flow fromthe tap changer head to the oil

conservator only.The oil flow operates the flap valve , being tripped into the “OFF” position.

At that moment the reedcontactis actuated, the circuit breakers areoperated and the

transformer is switched off the line.The protective relay is not energised by thetapchanger

being subject to nominal load orpermissible overload.

2.6 Oil Temperature Indicator

The Oil Temperature Indicator (OTI) measures the Top oil Temperature. It is used

for control and protection for all transformers.

2.7 Winding Temperature Indicator

The Winding is the component with highest temperature within the transformer and,

above all, the one subject to the fastest temperature increase as the load increases. Thus to

have total control of the temperature parameter within transformer, the temperature of the

winding as well as top oil, must be measured. An indirect system is used to measure winding

temperature, since it is dangerous to place a sensor close to winding due to the high voltage.

The indirect measurement is done by means of a Built-in Thermal Image.

Winding Temperature Indicator is equipped with a specially designed Heater which is

placed around the operating bellows through which passes a current proportional to the

current passing through the transformer winding subject to a given load. Winding

Temperature is measured by connecting the CT Secondary of the Transformer through a

shunt resistor inside the Winding Temperature Indicator to the Heater Coil around the

operating Bellows. It is possible to adjust gradient by means of Shunt Resistor.

In this way the value of the winding temperature indicated by the instrument will be equal to

the one planned by the transformer manufacturer for a given transformer load.

2.8 Conservator Tank

Breather:-Breather are are provided in the main conservator and OLTC conservator of the

transformer to breathe the atmospheric air .

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Cooling Fans:-Cooling fans are installed on the radiator bank to push the atmospheric air on

the radiator to cool them.

Explosion Vent /PRV:-It is used for releasing the pressure built up in the transformer due to

arching and other internal faults.

MOCG:-Magnetic oil level gauge is provided on the conservator tank to indicate the oil level

in the transformer.

2.9 Other protection of Transformer:-

1.Differential Protection:-This is protection in which both HV and LV side current are

measured by CT and are feed to differential Relay,if there is some mismatching in the current

the relay gives trip signal to the controlling circuit breaker.

2.Back up Protection:-when main protection system is unable to trip the CB due to some

reason under abnormal condition then Back up Protection is used.

3.Over Flux Protection:-The voltage and frequency of the system are measured and both are

fed to a relay which is called over flux relay in which ratio of voltage and frequency is

calculated and if it is exceeds a pre set value ,the relay generate a trip signal.

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CHAPTER : 3

LIGHTENING ARRESTOR

A protective device for limiting surge voltages by discharging or bypassing surge

current, and it also prevents the flow of follow current while remaining capable of repeating

these functions.Surge protection has been a primary concern when connecting devices and

equipment to low-, medium-, or high-voltage electrical systems. As the use of products and

equipment with components and insulation systems vulnerable to voltage surges and spikes

continues to increase, the requirement for surge arresters to protect against the effects due to

lightning strikes, switching phenomenon, etc., continues to increase as well. From personal

computers to HV transmission and distribution systems, everything is susceptible to these

surges and their destructive effects. This subject is very broad with numerous conditions to

address, such that it is possible to treat only the basic aspects of selection and application in a

single article. Therefore, this article will concentrate on circuits/systems 1000V and greater

and is intended to provide the reader with some general guidelines for the appropriate

selection and application of lightning/surge arresters.

3.1 Types/Classifications

Originally, there were three types of surge arresters. They are:

• Expulsion type

• Nonlinear resistor type with gaps (currently silicone-carbide gap type)

• Gapless metal-oxide type.

There are four (4) classifications of surge arresters. They are:

• Station class

• Intermediate class

• Distribution class (heavy, normal, and light duty)

• Secondary class (for voltages 999V or less)

Of the three types noted above, the expulsion types are no longer being used. The

nonlinear resistor type with gaps was utilized through the middle of the 1970s and is

currently being phased out. The conventional gap type with silicone-carbide blocks/discs are

15

still being used and the gapless metal-oxide type are the most widely used today. We are not

addressing the secondary class in this article.

With respect to the four classes of surge arresters we are addressing as part of this article,

the station class surge arrester is the best because of its cost and overall protective quality and

durability. It has the lowest (best) available protection level and energy discharging capability

with successively higher (poorer) protection levels for the other classifications. As noted

above, the distribution class has several duty ratings, which are dependent upon the test

severity. Heavy-duty arresters are more durable and have lower protective characteristics.

The housing/enclosure construction of surge arresters can be of either polymer or porcelain.

Fig. 3.1: lighting arrester

Our focus will be on the gapless metal-oxide surge arrester (MOSA), since it provides

the best performance and reliability. Please note that both the gap and gapless type arresters

do the same job and the selection and application process of both types are similar. However,

the need to select higher voltage levels for the silicone-carbide gap type and the possibility of

contamination of the gap means the protection and reliability is slightly less. When gapped

type arresters fail, the reader should consider or recommend replacing them with the metal-

oxide gapless type.

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3.2 Selection and Application

The primary objective in arrester application is to select the lowest rated surge

arrester that will provide adequate protection of the equipment insulation and be rated such

that it will have a satisfactory service life when connected to the power system. An arrester of

the minimum practical rating is preferred because it provides the highest margin of protection

for the equipment insulation system. There is a fine line between protection and service life

of a surge arrester. Higher arrester ratings will increase the capability of the arrester to

survive on a specific power system but reduce the margin of protection provided for the

insulation level of the equipment it is protecting. Therefore, the reader should consider both

issues of arrester survival and equipment protection when selecting surge arresters.

The best location for installation of a surge arrester is as close as possible to

the equipment it is protecting, preferably at the terminals where the service is connected to

the equipment. This is based on the mathematics of wave theory addressing incident and

reflected waves at a junction (or protected equipment terminal). Lead length for the

connection of the surge arrester to the equipment terminals and to ground should be

minimized and installed as straight, minimizing bends in the leads, as possible. This will

ensure that the surge energies are shunted to ground by the most direct path. Increases in the

lead length will reduce the protection capabilities of the surge arrester, due to the additional

increase of impedance in the lead.

There are some basic considerations when selecting the appropriate surge arrester for a

particular application:

• Continuous system voltage

• Temporary overvoltages

• Switching surges (more often considered for transmission voltages of 345KV and

higher, capacitor banks, and cable applications)

• Lightning surges

• *System configuration (grounded or ungrounded/effectively ungrounded)

3.3 Continuous System Voltage

When arresters are connected to an electrical system, they are continually exposed to

the system operating voltage. For each arrester rating, there is a recommended limit to the

17

magnitude of voltage that may be applied continuously. This is termed the Maximum

Continuous Operating Voltage (MCOV) of the arrester. The arrester rating must be selected

such that the maximum continuous power system voltage applied to the arrester is less than,

or equal to, the arrester’s MCOV rating. Consideration should be given to both the circuit

configuration (wye or delta) and arrester connection (Line-to-Ground or Line-to-Line). In

most cases the arresters are connected line-to-ground. If arresters are connected line-to-line,

then phase-to-phase voltage must be considered. In addition, in determining the arrester

rating, attention should be given to the grounding configuration of the system, either solidly

grounded or effectively ungrounded (impedance/resistance grounded, ungrounded, or

temporarily ungrounded). This is a key factor in the selection and application of an arrester. If

the system grounding configuration is unknown, the reader should assume the system is

ungrounded. This will result in choosing an arrester with a higher continuous system voltage

and/or MCOV rating. Also, attention should be given to special arrester applications such as

that on the delta tertiary winding of a transformer where one corner of the delta is

permanently grounded. In this instance, the normal voltage continuously applied to the

Fig-3.2 : surge arrester

3.4 Switching Surges

The arrester’s ability to dissipate switching surges can be quantified to a large degree

in terms of energy. The unit used in quantifying the energy capability of metal-oxide arresters

is kilojoules/kilovolt (kj/kv).

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The maximum amount of energy that may be dissipated in GE TRANQUELL®

arresters is noted. These capabilities are defined assuming multiple discharges distributed

over a one-minute period. In applications where the discharges are distributed over a longer

period of time, the GE TRANQUELL® arresters will have considerably more capability. As

noted previously, arresters applied correctly can repeat these capabilities; therefore, after a

one-minute rest period the above discharges may be repeated. The one-minute rest period

allows the disk(s) temperature distribution to reach equilibrium and become uniform. These

energy ratings assume that the switching surges occur in a system having surge impedances

of several hundred ohms, which would be typical for overhead transmission lines. In low

impedance circuits having cables or shunt capacitors as elements, the energy capability

metal-oxide arresters may be reduced because currents can exceed the values noted .

3.5 Arrester Failure & Pressure Relief

If the capability of an arrester is exceeded, the metal-oxide disk(s) may crack or

puncture. Such damage will reduce the arrester internal electrical resistance. This condition

will limit the arrester’s ability to survive future system conditions; it does not jeopardize the

insulation protection provided by the arrester.

In the unlikely case of complete failure of an arrester, a line-ground arc will

develop and pressure will build up inside the housing. This pressure will be safely vented to

the outside and an external arc will be established provided the fault current is within the

pressure relief fault current capability of the arrester. This low-voltage arc maintains

equipment protection. Once an arrester has safely vented, it no longer possesses its pressure

relief/fault current capability and should be replaced immediately. For a given application,

the arrester selected should have a pressure/fault current .

3.6 Arrester Selection and Application Summary

The arrester selection and application process should include a review of all system

stresses, service conditions expected, and system-grounding configuration (grounded or

effectively ungrounded) at the arrester installation location.

19

CHAPTER : 4

CIRCUIT BREAKER

Sulfur Hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power

applications. It has been usedextensively in high voltage circuit breakers and other

switchgears employed by the power industry. Applicationsfor 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 theoutstanding properties of SF6 making it desirable to use in power applications

are:

i) High dielectric strength

ii) Unique arc-quenching ability

iii) Excellent thermal stability

iv) Good thermal conductivity

4.1 General Information

SF6 circuit breaker is equipped with separated poles each having its own gas. In all

types of the circuitbreakers, gas pressure is 2 bars (absolute 3 bars). Even if the pressure

drops to I bar, there will not be any changein the breaking properties of the circuit breaker

due to the superior features of SF6 and Elimsan's high safety factorfor the poles. During

arcing, the circuit breaker maintains a relatively low pressure (max 5-6 bars) inside the

chamberand there will be no danger of explosion and spilling of the gas around. Any leakage

from thechamber will not create a problem since SF6 can undergo considerable

decomposition, in which some of toxicproducts may stay inside the chamber in the form of

white dust. If the poles are dismantled for maintenance, it needs special attention during

removal of the parts of the pole. This type of maintenance should be carried out only by the

expertsof the manufacturer. (According to ELIMSAN Arcing Products and Safety Instruction

for Working on SF6 Circuit Breakers).

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4.2 Operation of Circuit Breaker

In general, the circuit breakers consist of two main parts, the poles and the

mechanism. The poles consist of contactand arc-extinguishing devices. The mechanism is the

part to open or close the contacts in the poles at the sametime instantaneously (with max. 5

milisec. Tolerance). The closing and opening procedures are performed through

springs which are charged by a servomotor and a driving lever. In the system, the closing

springs are first charged.If "close" button is pressed the opening springs get charged while the

contacts get closed. Thus, circuit breaker willbe ready for opening. The mechanical operating

cycle of the circuit breaker is (OPEN-3 Min CLOSE/OPEN-3 Min-CLOSE/OPEN) or

(OPEN-0.3 sec-CLOSE/OPEN-3 Min CLOSE/OPEN). The second cycle is valid when the

circuitbreaker is used with re-closing relay. In that case, after the closing operation, the

closing springs are charged bythe driving lever or by driving motor (if equipped). Thus, the

circuit breaker will be ready for opening and re-closing.

Fig-4.1: circuit breaker

4.2.1 Auxiliary Switch

The auxiliary switch mounted on the circuit breaker has 12 contacts. One of them is

for antipumping circuit, fourof them are allocated for opening and closing coils. The

remaining 7 contacts are spare. Three of them are normallyopened and four are normally

closed. When it is necessary, the number of the contacts can be increased.

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4.3 RANGE OF TYPES AND TECNICAL FEATURES

4.3.1 Closing and Opening Operation Of the Circuit Breaker

When manual or motor-drive is used, the circuit breaker will be ready to close. The

closure can be actuated pressingthe closing button located on the circuit breaker. It is

recommended to close it using remote control system forsecure operations. The opening can

be performed either by opening button or remote controlled opening coil. In

case of a fault, the relay signal actuates the opening coil and circuit breaker opens. (This is

mechanically a primaryprotection system). In addition, there is an anti-pumping relay for

preventing the re-closing and opening of the circuitbreaker more than one cycle (O - C - O)

and for preventing possible troubles created by remote closing button.

Fig-4.2: circuit breaker

4.4 Comparison Between Vacuum and SF6 Circuit Breaker

Until recently oil circuit breakers were used in large numbers for Medium voltage

Distribution system in many medium voltage switchgears. There are number of disadvantages

of using oil as quenching media in circuit breakers. Flammability and high maintenance cost

are two such disadvantages! Manufacturers and Users were forced to search for different

22

medium of quenching. Air blast and Magnetic air circuit breakers were developed but could

not sustain in the market due to other disadvantages associated with such circuit breakers.

These new types of breakers are bulky and cumbersome. Further research were done and

simultaneously two types of breakers were developed with SF6 as quenching media in one

type and Vacuum as quenching media in the other. These two new types of breakgasers will

ultimately replace the other previous types completely shortly. There are a few disadvantages

in this type of breakers also. One major problem is that the user of the breakers are biased in

favour .

4.5 Reliability

In practice, an aspect of the utmost importance in the choice of a circuit-breaker is

reliability.

The reliability of a piece of equipment is defined by its mean time to failure (MTF),

i.e. the average interval of time between failures. Today, the SF6 and vacuum circuit-breakers

made use of the same operating mechanisms, so in this regard they can be considered

identical.

However, in relation to their interrupters the two circuit breakers exhibit a marked

difference. The number of moving parts is higher for the SF6 circuit-breaker than that for the

vacuum unit. However, a reliability comparison of the two technologies on the basis of an

analysis of the number of components are completely different in regards design, material

and function due to the different media. Reliability is dependent upon far too many factors,

amongst others, dimensioning, design, base material, manufacturing methods, testing and

quality control procedures, that it can be so simply analyzed.

In the meantime, sufficient service experience is available for both types of circuit-

breakers to allow a valid practical comparison to be made.

23

CHAPTER : 5

OVERCURRENT & EARTH FAULT RELAYS

.

5.1 Theory

The function of a relay is to detect abnormal conditions in the system and to

initiatethrough appropriate circuit breakers the disconnection of faulty circuits so

thatinterference with the general supply is minimised. Relays are of many types. Somedepend

on the operation of an armature by some form of electromagnet. A verylarge number of

relays operate on the induction principle. When a relay operates itcloses contacts in the trip

circuit which is normally connected across 110 V D.C.supply from a battery. The passage of

current in the coil of the trip circuit actuatesthe plunger, which causes operation of the circuit

breaker, disconnecting the faultysystem.In the laboratory, a 3-phase contactor simulates the

operation of the circuit breaker The closure of the relay contacts short-circuits the 'no-volt '

coil of the contactor,which, in turn, disconnects the faulty system.

The protective relaying which responds to a rise in current flowing through

theprotected element over a pre-determined value is called 'overcurrent protection' andthe

relays used for this purpose are known as overcurrent relays. Earth faultprotection can be

provided with normal overcurrent relays, if the minimum earthfault current is sufficient in

magnitude. The design of a comprehensive protectionscheme in a power system requires the

detailed study of time-current characteristicsof the various relays used in the scheme. Thus it

is necessary to obtain the timecurrentcharacteristics of these relays.

The overcurrent relay works on the induction principle. The moving system consistsof

an aluminium disc fixed on a vertical shaft and rotating on two jewelled bearingsbetween the

poles of an electromagnet and a damping magnet. The winding of theelectromagnet is

provided with seven taps (generally0, which are brought on thefront panel, and the required

tap is selected by a push-in -type plug. The pick-upcurrent setting can thus be varied by the

use of such plug multiplier setting. Thepick-up current values of earth fault relays are

normally quite low.The operating time of all overcurrent relays tends to become asymptotic

to a definiteminimum value with increase in the value of current. This is an inherent property

ofhe electromagnetic relays due to saturation of the magnetic circuit. By varying thepoint of

saturation, different characteristics can be obtained and these are

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1. Definite time

2. Inverse Definite Minimum Time (IDMT)

3. Very Inverse

4. Extremely Inverse

The torque of these relays is proportional to f1f2 Sina, where f1 and f2 are thetwo

fluxes and a is the angle between them. Where both the fluxes are produced bythe same

quantity (single quantity relays) as in the case of current or voltageoperated, the torque T is

proportional to I2, or T = K I2, for coil current belowsaturation. If the core is made to saturate

at very early stages such that with increaseof I, K decreases so that the time of operation

remains the same over the workingrange. The time -current characteristic obtained is known

as definite –time characteristic. If the core is made to saturate at a later stage, the

characteristic obtained is known asIDMT. The time-current characteristic is inverse over

some range and then aftersaturation assumes the definite time form. In order to ensure

selectivity, it isessential that the time of operation of the relays should be dependent on the

severityof the fault in such a way that more severe the fault, the less is the time to operate,this

being called the inverse-time characteristic. This will also ensure that a relayshall not operate

under normal overload conditions of short duration.It is essential also that there shall be a

definite minimum time of operation, whichcan be adjusted to suit the requirements of the

particular installation. At low valuesof operating current the shape of the curve is determined

by the effect of therestraining force of the control spring.

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CHAPTER : 6

CAPACITOR BANK

Shunt capacitor banks are used to improve the quality of the electrical supply and

the efficient operation of the power system. Studies show that a flat voltage profile on the

system can significantly reduce line losses. Shunt capacitor banks are relatively inexpensive

and can be easily installed anywhere on the network. This paper reviews principles of shunt

capacitor bank design for substation installation and basic protection techniques. The

protection of shunt capacitor bank includes: a) protection against internal bank faults and

faults that occur inside the capacitor unit; and, b) protection of the bank against system

disturbances. Section 2 of the paper describes the capacitor unit and how they are connected

for different bank configurations. Section 3 discusses bank designs and grounding

connections. Bank protection schemes that initiate a shutdown of the bank in case of faults

within the bank that may lead to catastrophic failures are presented in Section 4. The paper

does not address the means (fuses)and strategies to protect individual elements or capacitor

units, nor the protection of capacitor filter banks.

.

Fig6.1: Capacitot Bank

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6.1 THE CAPACITOR UNIT AND BANK CONFIGURATIONS

6.1.1 The Capacitor Unit

The capacitor unit, is the building block of a shunt capacitor bank. The capacitor unit

is made up of individual capacitor elements, arranged in parallel/ series connected groups,

within a steel enclosure. The internal discharge device is a resistor that reduces the unit

residual voltage to 50V or less in 5 min. Capacitor units are available in a variety of voltage

ratings (240 V to 24940V) and sizes (2.5 kvar to about 1000 kvar).

6.1.2 Capacitor unit capabilities

Relay protection of shunt capacitor banks requires some knowledge of the capabilities

and limitations of the capacitor unit and associated electrical equipment including: individual

capacitor unit, bank switching devices, fuses, voltage and current sensing devices.

Capacitors are intended to be operated at or below their rated voltage and frequency as they

are very sensitive to these values; the reactive power generated by a capacitor is proportional

to both of them (kVar ≈ 2ð f V 2). The IEEE Std 18-1992 and Std 1036-1992 specify the

standard ratings of the capacitors designed for shunt connection to ac systems and also

provide application guidelines.

These standards stipulate that:

a) Capacitor units should be capable of continuous operation up to 110% of rated terminal

rms voltage and a crest voltage not exceeding 1.2 x √2 of rated rms voltage, including

harmonics but excluding transients. The capacitor should also be able to carry 135% of

nominal current.

b) Capacitors units should not give less than 100% nor more than 115% of rated reactive

power at rated sinusoidal voltage and frequency.

c) Capacitor units should be suitable for continuous operation at up to 135%of rated reactive

power caused by the combined effects of:

• Voltage in excess of the nameplate rating at fundamental frequency, but not over

110% of rated rms voltage.

• Harmonic voltages superimposed on the fundamental frequency.

• Reactive power manufacturing tolerance of up to 115% of rated reactive power.

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6.2 CAPACITOR BANK DESIGN

The protection of shunt capacitor banks requires understanding the basics of capacitor

bank design and capacitor unit connections. Shunt capacitors banks are arrangements of

series/ paralleled connected units. Capacitor units connected in paralleled make up a group

and series connected groups form a single-phase capacitor bank. As a general rule, the

minimum number of units connected in parallel is such that isolation of one capacitor unit in

a group should not cause a voltage unbalance sufficient to place more than 110% of rated

voltage on the remaining capacitors of the group. Equally, the minimum number of series

connected groups is that in which the complete bypass of the group does not subject the

others remaining in service to a permanent overvoltage of more than 110%. The maximum

number of capacitor units that may be placed in parallel per group is governed by a different

consideration. When a capacitor bank unit fails, other capacitors in the same parallel group

contain some amount of charge. This charge will drain off as a high frequency transient

current that flows through the failed capacitor unit and its fuse. The fuse holder and the failed

capacitor unit should withstand this discharge transient. The discharge transient from a large

number of paralleled capacitors can be severe enough to rupture the failed capacitor unit or

the expulsion fuse holder, which may result in damage to adjacent units or cause a major bus

fault within the bank. To minimize the probability of failure of the expulsion fuse holder, or

rupture of the capacitor case, or both, the standards impose a limit to the total maximum

energy stored in a paralleled connected group to 4659 kVar. In order not to violate this limit,

more capacitor groups of a lower voltage rating connected in series with fewer units in

parallel per group may be a suitable solution. However, this may reduce the sensitivity of the

unbalance detection scheme.

6.3 CONCLUSIONS

The protection of shunt capacitor banks uses simple, well known relaying principles

such as overvoltage, overcurrents. However, it requires the protection engineer to have a

good Shunt Capacitor Bank Fundamentals and Protection 17 understanding of the capacitor

unit, its arrangement and bank design issues before embarking in its protection.

Unbalance is the most important protection in a shunt capacitor bank, as it provides

fast and effective protection to assure a long and reliable life for the bank.

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CHAPTER : 7

CAPACITOR VOLTAGE TRANSFORMER

Fig- 7.1: capacitor voltage transformer

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer

(CCVT) is a transformer used in power systems to step down extra high voltage signals and

provide a low voltage signal, for measurement or to operate a protective relay. In its most

basic form the device consists of three parts: two capacitors across which the transmission

line signal is split, an inductive element to tune the device to the line frequency, and a

transformer to isolate and further step down the voltage for the instrumentation or protective

relay. The device has at least four terminals: a terminal for connection to the high voltage

signal, a ground terminal, and two secondary terminals which connect to the instrumentation

or protective relay. CVTs are typically single-phase devices used for measuring voltages in

excess of one hundred kilovolts where the use of voltage transformers would be

uneconomical. In practice, capacitor C1 is often constructed as a stack of smaller capacitors

connected in series. This provides a large voltage drop across C1 and a relatively small

voltage drop across C2.

The CVT is also useful in communication systems. CVTs in combination with wave

traps are used for filtering high frequency communication signals from power frequency.

This forms a carrier communication network throughout the transmission network.

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CHAPTER : 8

CURRENT TRANSFORMER

A current transformer (CT) is a measurement device designed to provide a current in its

secondary coil proportional to the current flowing in its primary.

• can safely isolate measurement and control circuitry from the high voltages typically

present on the circuit being measured.

• are often constructed by passing a single primary turn (either an insulated cable or an

uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns

of wire.

• may be designed to connect directly to an ac ammeter or a voltmeter using a burden

resistor across its terminals.

Current transformers are commonly used in metering and protective relaying in the electrical

power industry where they facilitate the safe measurement of large currents, often in the

presence of high voltages.

Design specifications and upper limits of current transformers:

• The CT is typically described by its current ratio from primary to secondary. For

example, a 400:5 CT would provide an output current of 5 amperes when the primary

was passing 400 amperes.

• The secondary winding can be single ratio or have several tap points to provide a

range of ratios.

• The Volt-Ampere, (VA) rating is needed as a design specification.

• Care must be taken that the secondary winding is not disconnected from its load while

current flows in the primary, as this will produce a dangerously high voltage across

the open secondary and may permanently affect the accuracy of the transformer.

8.1 What is the purpose of a current transformer?

30

To measure alternating current flowing through a conductor, and a current transformer

is often classified as a type of instrument transformer. The voltage drop across a known

resistor can be measured, and this is okay for low current applications but is often impractical

for high current applications. The resistor consumes a lot of power (lowering efficiency)

unless the resistor is very low in value, in which case there may be very little voltage to

measure. The resistor could be excessively large. The resistor’s heat may affect the resistor

value, thereby reducing the accuracy of the measurement. A current transformer can

accurately measure the alternating current and put out a reasonable voltage, which is

proportional to the current, but without as much heat and size that an appropriate resistor

would require. The current transformer can perform its function with very little insertion loss

into the conductor current being measured. The current transformer also provides voltage

isolation between the conductor and the measuring circuitry. Proper function of a current

transformer requires use of a load resistor. The load resistor is often referred to as a "burden

resistor".

8.2 What is the core structure of a current transformer?

The best core structure for a current transformer in terms of electrical performance

is a toroidal coil. Many toroidal current transformers have only one winding. This winding is

usually a "high turns" winding which functions as the secondary winding. In application, the

toroidal current transformer is slipped over an end of a high current wire or buss bar, which

conducts the primary current. Said wire or buss bar constitutes a one turn primary winding.

Split core current transformers are designed so that they can be assembled around a buss bar

without disconnecting the buss bar. "C"- cores and "U" core structures are commonly used

for split-core current transformers because they are relatively easy to take apart and put back

together around the buss bar. Historically, this has not been practical for toroidal coils, but

there are now some flexible toroids, which permit the "split-core" feature of installing it

around a buss bar. They have limited application. Some printed circuit board applications will

utilize bobbin wound current transformers with two or more windings. One winding is an

integral part of the circuitry, while the other winding acts the secondary.

8.3 Safety precautions

Care must be taken that the secondary of a current transformer is not disconnected

from its load while current is flowing in the primary, as the transformer secondary will

31

attempt to continue driving current across the effectively infinite impedance. This will

produce a high voltage across the open secondary (into the range of several kilovolts in some

cases).

fig- 8.1: current transformer

8.4 Accuracy

32

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

For the IEC standard, accuracy classes for various types of measurement are set out in IEC

60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate

measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT

is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are

also important especially in power measuring circuits, and each class has an allowable

maximum phase error for a specified load impedance. Current transformers used for

protective relaying also have accuracy requirements at overload currents in excess of the

normal rating to ensure accurate performance of relays during system faults.

8.5 Burden

The load, or burden, in a CT metering circuit is the (largely resistive) impedance

presented to its secondary winding. Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5

VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA & 60 VA. As for ANSI/IEEE burden ratings are

B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2

can tolerate up to 0.2 Ω of impedance in the metering circuit before its output current is no

longer a fixed ratio to the primary current. Items that contribute to the burden of a current

measurement circuit are switch-blocks, meters and intermediate conductors. The most

common source of excess burden in a current measurement circuit is the conductor between

the meter and the CT. Often, substation meters are located significant distances from the

meter cabinets and the excessive length of small gauge conductor creates a large resistance.

This problem can be solved by using CT with 1 ampere secondaries which will produce less

voltage drop between a CT and its metering devices.

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8.6 Knee-point voltage

The knee-point voltage of a current transformer is the magnitude of the secondary

voltage after which the output current ceases to follow linearly the input current. In testing, if

a voltage is applied across the secondary terminals the magnitising current will increase in

proportion to the applied voltage up until the knee point. The knee point is defined as the

point at which an increase of applied voltage of 10% results in an increase in magnitising

current of 50%. From the knee point upwards, the magnitising current increases abruptly

even with small increments in the voltage across the secondary terminals. The knee-point

voltage is less applicable for metering current transformers as their accuracy is generally

much tighter but constrained within a very small bandwidth of the current transformer rating,

typically 1.2 to 1.5 times rated current.

CHAPTER:9

POWER LINE COMMUNICATION

The main purpose of power line communication is to transmit speech or to convey message from one substation to the other through transmission lines at higher frequencies. However power line communication serves also other purpose like tele-metering, tele-printing, tele-control, tele-indication and tele-protection. All these signals are being communicated in carrier frequency range 35 KHz to 500KHz. Thus by PLC system, the various electrical quantities are measured, message conveyed can be printed on the paper and protective devices can be operated very quickly in mili-seconds.

9.1 Principle of operation of PLC: -

In power line communication a speech signal is modulated with the carrier frequency ranging from 35 kHz to 500 kHz before modulation the speech band is limited to 300 to 2400 Hz whereas 2.4 to 4.0 kHz frequency band is used for tele-metering tele-printing tele-indication and tele-protection. The modulation signal is filters and amplified the it is transmitted over the power lines through line matching unit protective devices and coupling capacitor. At receiving end the HF carrier signal is protected from the HV power frequency with the help of line trap and coupling capacitor through line matching unit carrier frequency signal is sent to the power line communication terminal. Where the speech signal is separated from the carrier frequency and is sent to subscriber.

Equipment used in power line communication:

The following are the main equipment used in the power line carrier communication:

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9.1.1 Wave Trap:-Wave trap contains main coils lightning arrestor and a tuning device. All are

connected in parallel; the main coil has an inductance of 0.2 mH to 2.0 mH. This inductance offer high impedance to the high frequency carrier signals and block them here.

FIG 9.1 IN-LINE WAVE TRAP

9.2 Coupling Capacitor:-The coupling capacitor used for power line carrier communication has capacitance

ranging from 2200 pF to 1000 pF. It allows a low impedance path to high frequency carrier signals and allows them to enter the line matching unit however it offers a high impedance path to low frequency signals or wave and block it.

FIG 9.2 Coupling Capacitor

9.3 Line matching unit:-The carrier signal received from the high voltage transmission line through coupling

capacitor is given to the line matching unit 2 form here the signal is transferred to power line communication terminal. On other hand the carrier signal generated in a high frequency cubicle is transmitted via terminal 6 through a line matching unit 2.

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9.4 Power line carrier communication terminal:-It is just a cabinate which contain number of electronic circuit.

FIG 9.3 Power Line Carrier FIG 9.4 Power Line Communication

9.5 Switching equipment:-

The signal is fed to the switching system where it is used for any one operation i.e. tele-metering, tele-printing, tele-control .300Hz to 2.4Hz band is used for tele-voice whereas 2.4 kHz to 4 kHz band is used for remaining operations.

FIG 9.5 Switching Equipment

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CHAPTER : 10

ISOLATOR

Centre rotating type High Tension Triple pole out door disconnecting switches is

adaptable to suit ANY switchyard. They provide DOUBLE BREAKS per pole, with centre

insulator support rotating the moving contacts assembly in horizontal plane. This design has

proved its worth over the conventional vertical single break, by:

• Higher line current breaking capacity

• Absence of hinged connections & consequent hot spots

• Balance symmetrical operation, thereby reducing maintenance

• Involving only one variety of standard insulators they are therefore ideally suitable for

Line - isolators as well as Bus Bar Isolators and /or couplers in High Voltage.

Isolators are having fixed & moving contacts made of HDHC copper & die forged fingers of

phosphor bronze having an inherent non-deteriorating spring quality at high temperature &

corrosion proof . The assembly of fixed & moving contacts is based on self compensating

principle to withstand both the thermal & dynamic stresses of short circuit currents, & still

carry the continuous rated current, with plenty of margin under the maximum specified

temperature. contacts for higher These isolators are manufactured in a range covering

6.6KV to 400 KV. Operating handle is easy to operate by single person & conveniently

37

located for local operation in the switchyard. All ferrous parts are hot dip galvanised & non-

ferrous parts are heavily tin plated to provide protection against corrosion, current ratings are

silver plated.

Contact assemblies can be mounted on ANY standard or special insulators with suitable

CREEPAGE distance to take care of local pollution. Terminal connectors suitable for

standard cables can be provided according to user's specifications.

Fig. 10.1 Islator

The center support rotates with bush & thrust ball bearings, providing smooth & fast

actions, with little operating effort. All isolators are supplied complete with supporting bases,

phase-coupling galvanized pipe, manual operating mechanism. Provision is made for

padlocking in "on" & "off" position. The phase units can be mounted independently & then

finally coupled together after adjustment for alignment has been completed.

38

CHAPTER : 11

CONTROL ROOM

Control room is heart of any plant .control room is the palace where various

instruments for indicating, alarming, controlling and observation purpose are installed. Our

motive behind room is to protect personal form vicinity of high voltage.

Control room provides us opportunity to handle a larger plant alone place with less

human interaction thus control room benefits us in terms of labor and efficiency.

In control room of GSS lleerapura there are various cabinets which are setup on a

wooden floor. Faces of cabinet are provided with different indicating instrument and controls.

All controls shown with single line diagram, to showed the position of breaker and isolator. I

here are indieating instruments known as “Semafore”. Semafor is an indicating type device

provided to show the isolator or breaker. If isolator or breaker is on then related semafor

completes the line diagram.

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Fig. 11.1 control room of GSS 220KV

CONCLUSION

Practical training to technical aspirant has a great significance .as an engineering

student the short span of a time delivered for training teach a great a great knowledge of real

problem of the system .in the college students gets theoretical knowledge and its practical

application up to limit but in the training period, we learns all about installation, operation

and maintenance with economical issues related with it.

Electrical power system has three partitions that is generation, transmission

and distribution and last utilization. Transmission and distribution have a great significance

generally due to the great distance between loads and generating plants.

At last I would like to that the practical training taken at 220 KV G.S.S. teach

me a great and open the door of my thinking as a professional. Finally, wishing all the best to

220 KV G.S.S., VKIA, JAIPUR, their staff and power revolution in India.

40

Control panels have alarm system and indicating panel to show the occurrence of

error. Whenever an error occurs then particular lights in dictates. There press alarm-reset

button so that alarm and spots and hight is on until the error is removed.

41

REFERENCES

R. L. Witzke and J. S. William :Electrical Transmission and Distribution E. L. Harder and J. C. Cunningham Electrical Transmission and Distribution Badriram: Switch gear protection system B. R. Gupta: Transmission & Distribution system

OTHERS:-Wikipedia for Transmission & Distribution system

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