CHAPTER 1

INTRODUCTION TO 220 KV GSS, SANGANER, JAIPUR

1.1 Introduction

When India became independent its overall installed capacity was hardly 1900MW. During

first five year plan (1951-1956) the capacity was 2300MW. The contribution of Rajasthan

state was negligible. Rajasthan state electricity board came into existence in July1957.

In India electrical power is generated at a voltage of 11 KV to 33 KV. This is taken stepped

up to the transmission level in the range of 66 KV to 400 KV.

Member of transmission and switching have to be created. These are known as “SUB

STATION”.

A Sub Station is an assembly of apparatus, which transform the characteristics of electrical

energy from one form to another say from one voltage level to another level. Hence a

substation is an intermediate link between the generating station and consumer.

Electricity boards are set up in all states of India which are responsible for

1. Generation

2. Transmission

3. Distribution

They also construct, install and maintain all the station made for these purpose. In Rajasthan

R.R.V.P.N.L is responsible for transmission and distribution of electrical power all over

Rajasthan. It has it’s on generating station and it’s also get’s power from various other

stations also.

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It get’s power from following station:-

1. Badarpur Thermal Power Station Delhi

2. Bhakara Nangal Project (at Satlaj in Punjab)

3. Gandhi Sagar Dam Kota

4. Jawahar Dam Kota

5. Rana Pratap Sagar Dam Kota

6. Rajasthan Atomic Power Plant (RAPP) Kota

7. Kota Super Thermal Power Station (KSTPS) Kota

8. Anta Gas Power Plant (NTPC) Anta

9. Rajasthan Share in Bhakra Beas Management Board (BBMB)

Power obtain from these stations is transmitted all over Rajasthan with the help of grid

stations. Depending on the purpose, substations may be classified as :-

1. Step up substation

2. Primary grid substation

3. Secondary substation

4 .Distribution substation

5. Bulky supply and industrial substation

6. Mining substation

7. Mobile substation

8. Cinematograph substation

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Depending on constructional feature substation are classified as:-

1. Outdoor type

2. Indoor type

3. Basement or Underground type

4. Pole mounting open or kilos type

INCOMING FEEDER:-

1 400 KV Heerapura-Sanganer

2. 220KV Heerapura-Sanganer

3. 220KV Kota-Sanganer

OUTING FEEDER:-

A.132KV

1. Balawala

2. Heerapura

3. Mansarowar

4. SMS Stadium

5. Chaksu

6. Sitapura

B.33 KV

1. Durgapura I & II

2. Sanganer

3. Sitapura

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4. Muhana Mandi

5. Malpura Gate

6. Vatika

7. Phagi

8. IOC

C. 11 KV

1. Tajawala

2. Muhana Mandi

3. Prem nagar

4. Insdustrial

Any substation has many types of civil and electrical works. Main components

are :-

1. Bus bar

2. Power Transformers

3. Isolators

4. Circuit Breaker

5. Lightening Arrester

6. Insulators

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1.2 Introduction Of R.S.E.B.

Electrical energy occupies the top grade in energy hierarchy. It find innumerable used in

home, industry, Agriculture etc and even in the transport.

Rajasthan state electric board started functioning from 1st July 1957. But it has been divided

into 5 companies namely.

1. RAJASTHAN RAJYA VIDYUT PRASARAN NIGAM LTD (RRVPNL)

2. RAJASTHAN VIDYUT PRASARAN NIGAM LTD (RVPNL)

3. JAIPUR VIDYUT VITRAN NIGAM LTD (JVVNL)

4. AJMER VIDYUT VITRAN NIGAM LTD (AVVNL)

5. JODHPUR VIDYUT VITRAN NIGAM LTD (Jd.VVNL)

R.S.E.B. is the big public organization, which is responsible for the supply of electricity to

whole of the state. It is a organization of public having a business of unique quality the

electricity must be use at the instant of its generation because electricity cannot be stored at

large amount.

1.2.1 Duties: Following are the duties of all 5 companies.

1. Must supply is maximum at demand and should be prepared to increase it in future if

asked.

2. Must provide the service line to consumer, which must carry the consumer’s load safely.

3. Must not discriminate between consumers of some category.

1.2.2 Right: Following are the rights of all 5 companies.

1. It can charge from the consumer for the electricity at responsible rate.

2. It may acquire land for building a sub-station etc out of 5 companies are have their own

duties Jaipur Discom (RRVPNL), Take the distribution Part of Jaipur state and areas its

limits, RRVPNL Take over the distribution part of Ajmer region and Jodhpur vidyut vitran

nigam Ltd Take over the Jodhpur region.

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CHAPTER 2

TRANSMISSION LINES

2.1 Transmission Lines

In this category the EHV lines viz. Extra high voltage lines of 400 kV, 220 kV, 132 kV and

66 kV are considered/are used voltage from one grid sub-station to other sub-station through

six various types conductors.

2.1.1 The Conductors Used For:-

i. For 400 KV line: Taran Tulla and Marculla conductor.

ii. For 220 KV line: Zebra conductor is used composite of aluminums strands and steel

wires.

iii. For 132 KV line: Panther conductor is used composite of aluminum strands and steel

wires.

The material used in these conductors is generally aluminum conductor steel reinforced

(ACSR).  The conductors run over the tower cross arms of sufficient height with the

consideration to keep safe clearance of sagged conductor from ground level and from the

objects (trees, buildings etc.) either side also.

2.2 Substation:-

A sub-station is a intermediate link between the generating station and consumer. It may be

defined as the assemblies of apparatus which transfer the characteristic of electrical energy

from one form to another for example one voltage to another.

The electrical energy is generated at low voltage link 6.6 or 11 KV, through higher voltage to

33 KV are also possible due to economic considerations, low voltage is converted to high

voltage like 66 KV, 132 KV, 220 KV, 400 KV, 765 KV for transmission purpose. This can

be done with the help of the transformer.

The consumers’ apparatus are made for low voltage so this voltage is again to be stepped

down to the required voltage at the sub-station.

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The figure of transmission line is shown in figure below:-

Figure 2.1- View Of Transmission Line

2.2.1 Grid:-

Grid is a technical word use for the term for interconnection of power received from more

than one place. It is network of main power lines for distribution of electricity.

Now Grid sub-station (G.S.S.) means a sub-station where power from more than one place is

interconnected through equipment.

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The figure of View Of Grid & Substation is shown as below:-

Figure- 2.2- View Of Grid & Substation

2.2.2 Aspects Of A Grid Sub-Station:-

The sub-station may compared of the following.

a. A switch yard for locating various equipment and instruments protective CT & PT and

Lighting arrester etc. if the substation is near to load center it will have L.T. distribution &

sub transmission equipment and if the load is far from switch yard, will have transformers

breakers and other H.T. control equipment.

b. Transmission lines, towers, poles etc.

c. Sub- Station (distribution) from where the feeders are run for consumers.

d. Workshop for the repairs and maintains every sub-station is designed keeping in of the

following aspects.

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1. Reliability

2. Minimum capital cost and operating cost.

3. Maximum demand of consumers groups.

Total No. Of the Towers are:-

a. Transmission Towers

b. Dead end towers

c. Tangent towers

d. Angle towers

e. Extension towers

f. Spiral towers

g. Section towers

h. Narrow base towers (used in 33 KV lines)

i.  Board base towers.

Fig. 2.3- view of transmission line

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CHAPTER 3

SINGLE LINE DIAGRAM

A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of the concerned

Electrical System which includes all the required ELECTRICAL EQUIPMENT connection

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

mentioned Work.

As in the case of 220 KV Substation, the SLD shall show Lightening Arrestor, State

Electricity Board's C.T/P.T Unit, Isolators, Protection and Metering P.T & C.T. Circuit

Breakers, again Isolators and circuit Breakers, Main Power Transformer, all protective

devices/relays and other special equipment like NGR, CVT, GUARD RINGS, SDR etc as per

design criteria.

As these feeders enter the station they are to pass through various instruments. The

instruments have their usual functioning. They are as follows in the single line diagram-

1. Lightening arrestors,

2. C V T

3. Wave trap

4. Current transformer

5. Isolators with earth switch

6. Circuit breaker

7. Line isolator

8. Bus Bar

9. Potential transformer in the bus with a bus isolator

10. Isolator

11. Lightening arrestors with earth switch

12. A capacitor bank attached to the bus.

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3.1 Brief Descriptions Of The Instruments In The Line Diagram Are:-

3.1.1. Lightening Arrestors:-

Lightening arrestors are the instrument that are used in the incoming feeders so that to

prevent the high voltage entering the main station. This high voltage is very dangerous to the

instruments used in the substation. Even the instruments are very costly, so to prevent any

damage lightening arrestors are used. The lightening arrestors do not let the lightening to fall

on the station. If some lightening occurs the arrestors pull the lightening and ground it to the

earth. In any substation the main important is of protection which is firstly done by these

lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the

lightening to the ground. The lightening arrestor works with an angle of 30° to 45° making a

cone.

3.1.2 Capacitive Voltage Transformer:-

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down

extra high voltage signals and provide low voltage signals either for measurement or to

operate a protective relay. In its most basic form the device consists of three parts: two

capacitors across which the voltage signal is split, an inductive element used to tune the

device to the supply frequency and a transformer used to isolate and further step-down the

voltage for the instrumentation or protective relay. The device has at least four terminals, a

high-voltage terminal for connection to the high voltage signal, a ground terminal and at least

one set of secondary terminals for connection 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

the first capacitor, C1, is often replaced by a stack of capacitors connected in series. This

results in a large voltage drop across the stack of capacitors that replaced the first capacitor

and a comparatively small voltage drop across the second capacitor, C2, and hence the

secondary terminals.

3.1.3. Wave Trap:-

Wave trap is an instrument using for tripping of the wave. The function of this trap is that it

traps the unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is

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connected to the main incoming feeder so that it can trap the waves which may be dangerous

to the instruments here in the substation.

Figure:- 3.1 view of wave trap

3.1.4. Current Transformer:-

Current transformers are basically used to take the readings of the currents entering the

substation. This transformer steps down the current from 800 amps to 1 amp. This is done

because we have no instrument for measuring of such a large current. The main use of this

transformer is (a) distance protection; (b) backup protection; (c) measurement.

3.1.5 Lightening Arrestors With Earth Switch:-

Lightening arrestors after the current transformer are used so as to protect it from lightening

i.e. from high voltage entering into it. This lightening arrestor has an earth switch, which can

directly earth the lightening. The arrestor works at 30° to 45° angel of the lightening making

a cone. The earth switch can be operated manually, by pulling the switch towards ground.

This also helps in breaking the line entering the station. By doing so maintenance and repair

of any instrument can b performed.

3.1.6. Circuit Breaker:-

The circuit breakers are used to break the circuit if any fault occurs in any of the instrument.

These circuit breaker breaks for a fault which can damage other instrument in the station. For

any unwanted fault over the station we need to break the line current. This is only done

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automatically by the circuit breaker. There are mainly two types of circuit breakers used for

any substations. They are (a) SF6 circuit breakers; (b) spring circuit breakers.

The use of SF6 circuit breaker is mainly in the substations which are having high input kv

input, say above 220kv and more. The gas is put inside the circuit breaker by force ie under

high pressure. When if the gas gets decreases there is a motor connected to the circuit

breaker. The motor starts operating if the gas went lower than 20.8 bar. There is a meter

connected to the breaker so that it can be manually seen if the gas goes low. The circuit

breaker uses the SF6 gas to reduce the torque produce in it due to any fault in the line. The

circuit breaker has a direct link with the instruments in the station, when any fault occur

alarm bell rings.The spring type of circuit breakers is used for small kv stations. The spring

here reduces the torque produced so that the breaker can function again. The spring type is

used for step down side of 132kv to 33kv also in 33kv to 11kv and so on. They are only used

in low distribution side.

3.1.7. Line Isolator:-

The line isolators are used to isolate the high voltage from flow through the line into the bus.

This isolator prevents the instruments to get damaged. It also allows the only needed voltage

and rest is earthed by itself.

3.1.8. Bus:-

The bus is a line in which the incoming feeders come into and get into the instruments for

further step up or step down. The first bus is used for putting the incoming feeders in la single

line. There may be double line in the bus so that if any fault occurs in the one the other can

still have the current and the supply will not stop. The two lines in the bus are separated by a

little distance by a conductor having a connector between them. This is so that one can work

at a time and the other works only if the first is having any fault.

3.1.9. Potential Transformers With Bus Isolators:-

There are two potential transformers used in the bus connected both side of the bus. The

potential transformer uses a bus isolator to protect itself. The main use of this transformer is

to measure the voltage through the bus. This is done so as to get the detail information of the

voltage passing through the bus to the instrument. There are two main parts in it (a)

measurement; (b) protection.

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3.1.10. Isolators:-

The use of this isolator is to protect the transformer and the other instrument in the line. The

isolator isolates the extra voltage to the ground and thus any extra voltage cannot enter the

line. Thus an isolator is used after the bus also for protection.

3.1.11. Capacitor Bank Attached To The Bus:-

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

require place. By using capacitor bank we may improve power factor which provide

economic operation for transmission.

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CHAPTER 4

TRANSFORMERS

4.1 Introduction:-

A transformer is a static (or stationary) piece of apparatus by means of which electric power

of one circuit is transformed into electric power of same frequency in another circuit.

In brief, a transformer is a device that:

1. Transfer electric power from one circuit to another

2. It does so without change of frequency

3. It accomplishes this by electromagnetic induction

4. Where the two electric circuits are in mutual inductive influence of each other

A high voltage is desirable for transmitting large powers in order to decrease the IR losses

and reduce the amount of conductor material.  A very much lower voltage, on the other hand,

is required for distribution, for various reasons connected with safety and convenience.  The

transformers make this easily and economically possible.

There are single phase and three phase transformers which are used as requirement. The main

advantage of a 3-phase X-former is that 3-phase load tap changing mechanism could be used

further the installation of a single 3-phase transformer is simple than 3-phase transformers.

In 220 KV GSS , S.W.M. five power transformers are used. Two transformer’s rating 132/33

KV, 20/25 10/12.5 MVA & one rating 132/11 KV, 10/12.5 MVA. The station transformer

is 33/0.4 KV & 11/0.4 KV, 220/132 KV 100 MVA.

4.2 Power Transformer:-

The transformer is oil immersed with triple rating of 100 MVA auto under ON (natural

cooling) (oil immersed with natural air-cooling) or (oil immersed with forced oil cooling).

The tertiary is suitable for 11MVA continuous synchronous condenser loading. When the

tertiary is load the secondary load should be limited, such that no industrial individual

winding is over loaded and are also that total losses are not exceeds.

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It is ensured that the tertiary winding will also operate satisfactory with each other. The

transformer is provided with separate tank of radiators, fans, pumps and associated control

equipment. The control equipment is housed in a tank mounted marshalling commercial. It is

provided with on load tap charge. The diagram power transformer is shown as below:-

Figure-

4.1- Power Transformer

A transformer double wound or auto wound has minimum of two voltage, one corresponding

to the supply and second to the load side.  Many time a third winding is introduced in primary

and secondary, winding requires it because another voltage may be required at the place of

supply to load.  In either core the third winding is connected to delta formation and is

generally known as "tertiary" winding in the case of star/star methods of connection three

phase shell type transformer is used  which causes a serious problem of the third harmonic

components of the magnetic currents.  The tertiary delta provides a short circuit for flow of

the third harmonic currents therefore eliminating third harmonic multiple connection are

provided on primary and secondary winding.

The neutral point of such winding is therefore stable and can earthen without any effect to the

X-mer on the system

The territory winding helps-

1. To reduce the unbalancing in the phase of the primary side due to unbalanced three phase

loads;

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2. To redistribute the flow of faults currents;

3. To supply an auxiliary load in addition to the main load.  This could be consists of the

power factor improvement synchronous condensers or shunt capacitors. In such a case the

purchaser of a power X mer should always specify the voltage and power ratings of the

tertiary winding;

4. As compared with star/delta connection of fault current in the event of secondary form line

to neutral tertiary delta consists in the laminations.  This of course further depends upon the

impedance between tertiary and main winding.

5. The transformer having mixed cooling OFB(forced oil air blast cooling) (100 MVA)

OB(air blast) (70 MVA) and ON(natural cooling) (50 MVA) is provided with separately

mounted banked radiators.  There are eight radiators of elements 2920 mm. Long.  The

radiators are mounted on the top and bottom headers, which are supported by facilitated

frames.  Each radiator is connected at the top and bottom with respective header throughout

let and inlet valves.  The top and bottom headers are connected to the tank by 200 mm

diameter pipes and valves one each radiators as well as headers.

Its temperature operates on the principle of liquid expansion (mercury is in steel).  The

temperature indicator is provided with a max-pointer and two mercury switches.  The

mercury switches are adjustable to make contacts between 50oC and have a fixed difference

of 10oC.

This operates on the principle of liquid expansion (mercury in steel) provides in deal local

indication at the marshalling box.  Half of the sheet temperature irrespective of the suitable

conditions, the thermometer bulb is connected by capillary tube to the local indicator.

The marshalling box is a weather proof steel box mounted on the side of transformer tank, the

marshaling box is provided with heater for the prevention of moisture condensation, besides

this the inside of the box is provided with anti condensation point. 

The following equipment are mounted inside the marshalling box.

S. No. Name of equipment

1.         Temperature indicator

2.         Auxiliary gear for tap changer control

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3.         Cooler Control gear

4.        Heater Switches, illumination lamps

The transformer is in the yard has many tapping on ways every one about 17 tapping. When

the load on transformer increase due to regulation of the voltage 90 down to increase the

voltage on the secondary side by the changing their tapping to higher position.  For changing

tap simply we have to close the supply and taken change the tap by mechanically means.  In

GSS there is on load taps changer and it is totally remote control.  There are four panels at

control room for transformer and by pushing button, once we increase or decrease the tapping

we can see the number of trip at the panels, all four transformer must have the same number

of tapping.

The on load tap changer design is a part of transformer unit winding .The on load tap changer

consists load diverter and a selected switch, the desired winding is first selects currents legs

by a slow switching selector switch then follow the charge over by means of load diverter

connection, the neutral point has even and odd number selector and contact plates

alternatively.

4.3 Auto Transformers:-

Basically auto-transformer comprises of only one winding per phase, part of which is used by

both primary and secondary winding.  This arrangement results into an appreciable saving in

cost as well as higher operating efficiency is achieved, but their extensive use is not being

favored by power utilities due to certain inherent disadvantages which are as follows:

1. It has got low inherent reactance as such is subjected to severe short circuit conditions.

2. Since primary and secondary side uses same windings, there is always possibility of

imposition of higher voltage on secondary in case of fault.

3. Both the windings make use of common neutral, as such neutral is required to be

earthed or isolated on both sides.

4. Provision of additional insulation on secondary side and increased frame size when

adjustable taps are provided erodes the initial advantage of low cost.

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Power transformers are installed with various fittings and devices which are necessary for

their proper functioning.

4.3.1 Constructional Part:-

The chief elements of the construction are :

1. Magnetic Circuits:  Comprising limbs, yokes and clamping structures.

2. Electrical Circuits: The primary, secondary and (if any) tertiary windings, formers,

insulation and bracing devices.

3. Terminals: Tapings, tapping switches, terminal insulators and leads.

4. Tank: oil, cooling devices, conservators, dryers and auxiliary apparatus.

The figure of auto transformer is shown as below:-

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Figure- 4.2- Auto Transformer

4.3.2 Core Construction:-

Special alloy steel of high resistance and low hysteresis loss is used almost exclusively in

transformer's cores.  Induction densities up to 1.35 - 1.55 wb/m2    are possible.  The limit for

50 c/s is being the loss and the magnetizing current.

As the flux in the cores is a pulsating one, the magnetic circuit must be laminated and the

separate laminations insulated in order to retain the advantages of subdivision.  Paper, Japan,

Varnish, China clay or phosphate may be used.

Burring of edges of plates may cause a considerable increase in a core loss by providing paths

for eddy currents should the sharp edges cut through the insulation and establish contacts

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between adjacent plates.  Burrs are removed before core assembly.  Silicon alloy steel are

hard, and cause wearing of the punching tools, so that the removal of burrs needs special

attention.

Transformer shut sheet are cut as far as possible along the grain which is in the direction in

which the material has a higher permeability.

4.3.3 Constructional Frame Work:-

Considerable use is made of channel and angle section rolled steel in the framework of core

type transformers.  a typical construction is to clamp the top and bottom yokes between

channel sections, held firmly by tie-holts.  The bottom pair of channels has cross channels as

feet.  The upper pair carries clamps for the high and low voltage connections.

4.3.4 Windings :

Classification of windings maybe done as (a) Circular or rectangular & (b) Concentric or

sandwiched.

In core type circular or rectangular type of windings are used and in shell type generally

sandwiched type windings are used.

On account of easier insulation facilities, the low voltage winding is placed nearer to the

core.  In the case of core type and on the outside positions in the case of shell type

transformers.  The insulation spaces between low and high voltage coils also serve to

facilitate cooling.

4.3.5 Insulation:-

The insulation between the H.V. and I.V. windings, and between I.V. winding and core,

compresses Bakelite paper cylinders or elephantine wrap.

The insulation of the conductors may be of paper, cotton or glass tape being used for air

insulated transformers.  The paper is wrapped round the conductor in a suitable machine,

preferably without overlap of adjacent turns.  In the power transformers, owing to strain on

the insulation between turns t the line end of the high voltage winding, about 5 percent of the

turns are reinforced with the extra insulating material.

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4.3.6 Leads And Material:-

The connections to the windings are copper rods or bars, insulated wholly or in part, and

taken to the bus bars directly in the case of oil cooled transformers.  The shape and size of the

conductors are of importance in very high voltage systems, not on account of the current

carrying capacity, but because of dielectric stresses, corona, etc. at sharp bends corners with

such voltages.

4.3.7 Bushings:-

Up to voltages of 33 kV, ordinary porcelain insulators can be used. Above this voltage the

use of conductor and of oil filled terminal bushings, or, for certain cases, a combination of the

two has to be considered.  Of course, any conductor can be effectively insulated by air

provided that it is at a sufficient distance from other conducting bodies and sufficiently

proportioned to prevent corona phenomena.  Such conditions are naturally UN-obtainable

with transformers where the conductor has to be taken through the cover of the containing

tank, although common enough with over head transmission lines.

The oil filled bushing consists of a hollow porcelain cylinder of special shape with a

conductor (usually a hollow tube) through its centre.

The space between the conductor and the porcelain is filled with oil, the dielectric strength of

which is greater than that of air.  The dielectric field strength is greatest at the surface of the

conductor, and this breaks down at a much lower voltage in air than in oil. Oil is fed into the

bushing at the top, act as an expansion chamber for the oil when the bushing temperature

rises.  Under the influence of the electric field, foreign substances in the form of dust,

moisture or metallic particles have a tendency to arrange themselves in radial lines giving rise

to paths of low dielectric strength, with constant danger of breakdown.  To prevent such

action by unavoidable impurities in the oil Bakelite tubes are used to surround the conductor

concentrically.  The effect is to break up radial chains of semi-conducting particles.

4.3.8 Tanks:-

Small tanks are constructed from welded sheet steel, and larger ones from plain boilerplates. 

The lids may be cast iron, or waterproof gasket being used at the joints.  The fittings include

thermometer pockets, drain cock, rollers or wheels for moving the transformer into position,

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eye bolts for lifting, conservators and breathers, cooling tubes are welded in, but separate

radiators are individually welded and afterwards bolted on.

4.3.9 Conservator:-

Conservators are required to take up the expansion and contraction of the oil to come in

contact with the air, from which it is liable to take up moisture.  The conservator may consist

of an airtight cylindrical metal drum supported on the transformer lid or on a neighboring

wall, or of a flexible flat corrugated disc drum.  The tank is filled when cold and the

expansion is taken up in the conservator.

The figure conservator is as shown below:-

Figure- 4.3- Conservator

4.3.10 Transformer Oil:-

Oil in transformer construction serves the double purpose of cooling and insulating. In the

choice of oil for transformer use the following characteristics have to be considered.

Viscosity

Insulating property

Flash point

Fire point

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Purity

Slugging

Audity

4.3.11 Synthetic Transformer Oil:-

These have been developed to avoid the risk of fire and explosion, present always with

normal mineral oils.  Chlorinated di-phenyl, synthetic oil suitable for transformer is

chemically stable, non-oxidizing, rather volatile, and heavier than water.  Its dielectric

strength is higher than that of mineral oil, and moisture has a smaller tendency to migrate

through it.  The permittivity is 4.5, compared with about 2.5.

4.3.12 Marshaling Box

The marshaling box is of steel sheath. It is provided with a hinged door and winding are

connected to the position. For 132 KV Crompton Greaves made high speed resistor transistor

on load tap changer. The tap changer may be either manually operated or motor driven

mechanism is arranged for the following types of control-

1. Local Electrical Independent

2. Remote Electrical Independent

3. Remote Electrical Group Manually Control.

4.4 Instrument – Transformer:-

The transformer are used in a.c. system for the measurement of current , voltage, power and

energy, the actual measurements being done by measuring instruments. Transformer used in

conjunction with measuring instrument for measurement purposes are called as "instrument

transformer". The transformer used for the measurement of current is called "current

transformer". Transformer used for voltage measurement are called as "voltage transformer"

or" potential transformer".

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4.4.1 Current Transformer:-

In electrical engineering, a current transformer (CT) is used for measurement of electric

currents. Current transformers, together with voltage transformers (VT) (potential

transformers (PT)), are known as instrument transformers. When current in a circuit is too

high to directly apply to measuring instruments, a current transformer produces a reduced

current accurately proportional to the current in the circuit, which can be conveniently

connected to measuring and recording instruments. A current transformer also isolates the

measuring instruments from what may be very high voltage in the monitored circuit. Current

transformers are commonly used in metering and protective relays in the electrical power

industry.

The figure of current transformer is as shown below:-

Figure- 4.4- Current Transformer

4.4.2 Voltage Transformers:-

Voltage transformers (VT) or potential transformers (PT) are another type of instrument

transformer, used for metering and protection in high-voltage circuits. They are designed to

present negligible load to the supply being measured and to have a precise voltage ratio to

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accurately step down high voltages so that metering and protective relay equipment can be

operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69

V or 120 V at rated primary voltage, to match the input ratings of protection relays.

The transformer winding high-voltage connection points are typically labeled as H1,

H2 (sometimes H0 if it is internally grounded) and X1, X2 and sometimes an X3 tap may be

present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same

voltage transformer. The high side (primary) may be connected phase to ground or phase to

phase. The low side (secondary) is usually phase to ground.

The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to

current transformers as well. At any instant terminals with the same suffix numeral have the

same polarity and phase. Correct identification of terminals and wiring is essential for proper

operation of metering and protection relays.

Some meters operate directly on the secondary service voltages at or below 600 V. VTs are

typically used for higher voltages (for example, 765 kV for power transmission), or where

isolation is desired between the meter and the measured circuit.

The figure of voltage transformer is shown below:-

Figure- 4.5- Voltage Transformer

Voltage transformers which step-down systems voltages to sufficiently low values are

necessary on every system for:

a. Induction of the voltage conditions

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b. Metering of the supply (or exchange of energy)

c. Relaying and

d. Synchronizing

On account of cost and voltage the indicating instruments meters and relays are designed for

the voltage as obtainable from the secondary sides of the voltage transformer.  The

calibration of the indicating instruction and meters is however done accordingly to the

primary voltage of the V.Ts.

The voltage transformers are classified as under:

a Capacitive voltage transformer or Capacitive type

b. Magnetic type

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CHAPTER 5

ISOLATOR

5.1 Introduction:-

When carrying out inspection to disconnect reliably the unit or section on which the work to

be done from all other live parts on the in-station in order to ensure completely safety of the

working staff.

To afford against minute mistakes it is desirable that it should be done by an apparatus which

makes a visible break in the circuit such an apparatus is the isolating switch (for insulator).   It

may be defined as a device used to open (or use) a circuit either when negligible current is

interrupted (or established) or when no significant charge the voltage across the terminals of

each pole of the isolator will result from the operation.

Isolator may be classified as single pole and 3-pole isolator i.e. according to number of

poles.  According to the service type these are:

i. Indoor type and

ii  Outdoor type

The doubling break rotating centre part isolating switches has three isolator parts per phase

mounted on a base of fabricated construction.

The centre part carries the moving contacts arms tabular or fault with the intact assembles at

the extremities.  The moving contacts engage the fixed contacts on the outer fixed insulator

parts.  The designs of moving and fixed contacts vary from manufactures to the other.  The

variants are generally simple one of the contacts is the male contacts with the other is

contacts.

The rotating centre part of the three phases are inter connected by operating rods 50 that

simultaneous movement of each part, connected by the operating rods and driven form one

post by operating mechanism through an adjustable lever drive rod and torque shaft

supporting structures. The design of a contact could be different with different manufacturers

for closing or both the isolator parts rotate causing moment of contact arm.  The insulator

shown is pneumatic operates but is provided with emergency hand drive mechanism, also.

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The contact at extremely which enrages with the isolator contacts the line side.  The earthing

blade when provided are so inter locked with the main line blade that there can be closed only

when the main blade are in fully open positions.  Similarly it is possible to close the main

blades only when the earthing blades are fully in open position.  The earthing blade of a line

switch have a separate operating mechanism as well as gallery switch indicate on contact

room the open or close position of the earthing blades.

The figure of isolator is shown as below:-

Figure- 5.1- Isolator

5.2 Operation:-

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The operation of an isolator may be manual i.e. by hand without using any other supply or

storage of energy meter power operated isolates during the cause of operation utilize energy

which is not supplied by the operator.  The energy may be electrical pneumatic or the energy

previously stored in spring or counter weight.

5.3 Control:-

In case power operated isolators are purchased for any installation it may be worthwhile to

examine further weather control should be local in switchyard or remote in the control room.

The extra cost enrolled in the isolated is quite substantial particularly at voltage 132 kV and

below.  It should therefore considered in detail whether any installation really instifies the

procurement of remote operated isolators keeping in view the past that the frequency of

operation of isolators is rather low.

5.4 Auxiliary Switch:-

This is an operating and important accessory and is designed as a switching device working

in conjunction with an isolator for controlling a circuit for auxiliary device such as trip coils

indicators or indicating lamps. The number of normally closed and normally open contacts

should be specially worked out particularly if electrical interlocking between breakers and

isolators is chosen.

5.4.1 Make Before And Break After Contacts:-

These are provided in series with the main contacts so that in case of load isolators, the arcing

is taken and whenever necessary only the arcing contacts are replaced.

5.5 Arcing Horns:-

These are provided on each stack of post Insulator for the purpose of insulation co-ordination

some time confusion is created in the function of these arcing horns vis a vis (make before

and break after contacts. These may be fixed or adjustable types).

The use of arcing horns is avoided where insulation strength between poles or phases and

between higher than that of earth.  This is necessary for safety and security.  Any travelling

wave meeting an isolator is the closed position should causes of it must a flash over to earth

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rather than between phase or between terminal of the same pole where the design of the

isolator itself provides for this.  It is necessary to use arcing hours on the insulator stacks.

Figure:-5.2- view of arcing horn

5.6 Interlocking:-

In correct operation of an isolating switch may be accidentally harmful effects and may cause

distribution of part of the plant as well as costly service interruption for preventing such

incorrect operation inter locks are used i.e. isolating switches.  The mechanical interlocking

between isolating switches and it is earthing switch consists of a rod linkage between

isolating switch and its earthing switch shafts of the respective switches.

The mechanical interlocking between isolation switches and circuit breaker and different

isolating switches is generally in the form of lock and key arrangement.  There is usually a

common key for a number of locks mechanical interlocking is generally provided on hand

operated are isolating switch only. Electrical interlocking is achieved with blocking magnets;

these magnets are arranged on the isolating switch on the hand drive or in the value

controlled.  The pneumatic drive and are controlled by pilot switch contents. 

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CHAPTER 6

CIRCUIT BREAKER

6.1 Introduction:-

A circuit breaker is equipment, which can open or close circuit under all condition viz. No

load, full loads an fault conditions.  It is so designed that it can be operated manually under

normal conditions and automatically under fault conditions, for the later operation, relay

circuit is used.

Circuit breaker can be defined as an electrical device, which protects the system from short

circuits or overloads with the help of relays.  In case, circuit breaker is not of adequate

capacity, its failure may result into interruption of power, shut downs, injury to personals and

damage to property.  Installation of over rated circuit breakers or extra sensitive and costly

protective devices will mean un-warranted expenditure.  It is therefore necessary that

calculations in respect of short circuit currents for the concerned system be made before

correctly rated circuit breakers are selected or steps are taken to improve the existing system.

6.2 Operating Principle:-

A circuit breaker consists of fixed and moving contacts under normal operating conditions,

these contacts remain closed.

In this condition, the emf in the secondary winding of current transformer (CT) in sufficient

to operate the trip coil of the breakers but the contacts can be opened by manual or automatic

control.

When a fault occurs on any part of the system the resulting over current in the C.T. primary

winding increases the secondary winding EMF and hence the current through the relay

operating coils.  The relay contacts are closed and the trip coil (tripping coil) of the breaker is

energized.  The moving contacts are pulled apart by some mechanism thus operating the

circuit breaker. When the contacts of the circuit breaker are separated under fault conditions,

arc is produced between them (male and female contact).  The current is thus able to continue

until the arc ceases.  This arc generates enormous heat, which may cause damage to the

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system or to the breaker itself. Therefore, the main problem in a circuit breaker is to

extinguish the arc within the shortest time so that heat generates by it may not reach a

dangerous value.

6.3 Classification Of Circuit Breakers For Various Voltages:-

1. Bulk oil circuit breaker.

2. Air blast circuit breaker.

3. SF-6 circuit breaker.

4. Minimum oil circuit breaker.

5. Vacuums circuit breaker.

6.3.1 Bulk Oil Circuit Breaker :-

In such circuit breaker transformer oil is used for arc extinction. The contacts are opened

under oil, which absorbs the heat of arc, and decomposed into gases as hydrogen, which have

excellent cooling properties due to high heat conductivity.

Circuit breaker compresses of three-pole contact assembly housed in a circular welded steel

tank. Circuit breaker is mounted on an angle iron frame grounded in cement concrete base the

breaker is provided with spring or solenoid operating mechanism. The contacts are of but

type the stationary portion comprises of two contacts pivoted at the base of the explosion pot.

The cross jet assembly is made of blocks of insulating materials, which together form a

chamber of irregular shape. The throat block and single block have circular holes located

centrally through which moving contact passes. The barrier plates are shaped to form nozzle

outlets through which the oil and arc glasses are projected from the explosion pot. When the

breaker is tripped on load or on fault, the moving contact breaks circuit with the stationary

contacts and the resulting arc is drawn downward through the throat hole. The gas produced

from the oil by the arc accumulated in the expulsion a pot at high pressure. This pressure acts

downward on the end of the moving contact and so accelerates its movement. As the moving

contacts descends, the flush of oil and gas sweeps through the arc an passes out through

nozzle outlets thereby producing powerful quenching effect and causing disruption of the arc

before the contact leaves the cross jet assembly.

The disadvantages of oil as an arc excitation medium for an arc: -

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1. It is inflammable and there is a risk of fire.

2. The quality of oil deteriorates, due to increase of carbon in oil with the excessive use of

breaker. This needs periodic checking and replacement of oil.

3. In B.O.C.B. the increase in carbonization weakness the dielectric strength of the oil of

breaking strength of oil.

The figure of bulk oil circuit breaker is shown as below:-

Figure 6.1- Bulk Oil Circuit breaker

6.3.2 Air Blast Circuit Breaker:-

In such circuit breaker, high-pressure air blast is used for arc extinction.  The contacts are

opened in the flow of air blast.  The air blast cools the arc and removes the arcing products

(mainly composed of carbon) to the atmosphere.  This rapidly increases the dielectric strength

of the medium between contacts and prevents the arc restricting.  Consequently the arc is

extinguished a flow of current is interrupted.

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For the interruption of current on load, air blasts C.B. are used. In 220kv A.B.C.B. is used.

This type of circuit breaker has four interrupter terminals while in 132kv there are three

interrupter terminal of A.B.C.B. A pole chiefly consists of number of identical column each

are standing on it compressed air receiver supporting interrupter.

The C.B. can be operated both electrically either from relay station or with a separate master

switch and a push button operate valve in the central controlling cabinet AB-5.

The controlling impulse are transmitted from the controlling cabinet AB-5 electrically or

pneumatically to the control again in the panel box and from these to the intermediate value

AB-5 which in turn determines the condition in air insulators and position of C.B.. The

controlling cabinet has alarm type pressure gauge for the pressure in the receiver of the pole

pressure switches and mechanically lode pressure blocking device.

Figure:-6.2 view of air blast circuit breakers

6.3.3 SF6 Circuit Breaker:-

The arc excitation process in SF6 gas removes the heat from the arc by axial convention and

radial dissipation. As a result the arc diameter reduces during the decreasing made of the

current zero and arc is extinguished due to its Electromagnetic and low arc time constant, the

gas remains its dielectric strength rapidly after the final current zero. The rate of rise of

dielectric strength is very high and time constant is very small. The arc extinguished

properties of SF6 gas has pointed out in 1983.

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The figure of SF6 Circuit Breaker

Figure- 6.3- SF6 Circuit Breaker

6.3.4 Minimum Oil Circuit Breaker:-

Oil circuit breaker uses dielectric oil (transformer oil) for the purpose of arc extinction. In

bulk oil circuit breaker the arc extinction takes place in the tank where as in M.O.C.B. the

current interruption takes inside interruption. The enclosure of the interpreter is made of

insulating material like porcelain. Hence clearance between the live part and the enclosure

can reduce and layer quality requires of internal insulation.

The figure of minimum oil circuit breaker is as shown below:-

Figure 6.4- Minimum oil Circuit Breaker

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6.3.5 Vacuum Circuit Breaker:- 

Principle:-

When the contacts of the breaker are opened in the vacuum (10 -7 toor and 10 -5 torr) an arc

is produced between the contacts by the ionisation of metal vapours of contact and it is

quickly extinguished in the vacuum because it has excellent suspension arc quenching

properties than any other medium.

The figure of indoor vacuum circuit breaker is shown as below:-

Figure- 6.5- Indoor Vacuum Circuit Breakers

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CHAPTER 7

LIGHTNING ARRESTER

7.1 Introduction:-

 Lightning Arresters are installed in power houses and sub-stations to safeguard the major

equipment like power-transformers, switch gear and to ensure the flow of power un-

interruptedly.  It is true that lightning arresters require minimum post-installation care, but

their importance as a critical equipment can hardly be disputed.

7.2 Lightning Strokes And Over-Voltages:-

The overhead transmission lines and connected electrical apparatus i.e. Power Transformers,

Switch gear etc. are subjected to over voltages on account of lightning discharges caused by

atmospheric disturbances and or by switching operations.  Abnormal voltages are caused by

atmospheric disturbances as a result of:

7.2.1   Direct Strokes

Direct stroke to the phase conductor or ground wire or to supporting structure results into

abnormal transient voltage, which gets super-imposed on the power net work.

7.2.2   Indirect Strokes

Direct stroke in the vicinity of the line or the equipment or charged cloud over the power line

induces abnormal voltages.

Abnormal transient over voltages super-imposed by direct or indirect strokes travel along the

conductor in both the directions with the speed of light i.e. 186,000 miles per second or 1000

feet per micro second.  These waves are steep fronted in case of direct strokes and travel till

the surge voltage is attenuated or neutralized by reflected waves of opposite polarity from the

earthed object.  E.H.V. transmission lines and sub-stations are designed to take care of direct

strokes by providing:

1. Higher impulse level

2 .Shielding and lower footing resistance

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3. Lightning arresters for draining undesirable voltage to the ground.

The figure of lightning arresters are as shown below:-

Figure- 7.1- Lightning Arrester

7.3 Types Of Lightning Arresters:-

Ground wires or shielding wires generally of steel are fixed over the phase conductors in case

of transmission lines and sub-stations and are solidly grounded.  The ground wire when

solidly grounded through a very small resistance reduces the magnitude of voltage induces

upon the line conductors due to electrostatic field produced by charging cloud.  The ground

wire is in a general sense is preventive device, but it does not entirely prevent the formation

of travelling waves on a line.  Surges produced by direct strokes or by induced strokes must

be drained to the ground through low impedance ground to protect power transformers and

other costly equipment and to reduce outages in the system.  Lightning arresters are the

devices to provide the necessary path to the ground for such surges.  An ideal arrester must

therefore have the following properties:

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i. It should be able to drain the surge energy from the line in a minimum time.

ii. Should offer high resistance to the flow of power current.

iii. Performance of the arresters should be such that no system disturbances are introduced by

its operation.

iv. Should be always in perfect form to perform the function assigned to it.

v. After allowing the surge to pass, it should close up so as not to permit power current to

flow to ground.

Lightning protective devices, which are in market, are of the following type:

7.3.1 Rod Gap or Sphere Gap:-

It is a very simple protective device i.e. gap is provided across the stack of insulators to

permit flash-over when undesirable voltages are impressed on the system.  It does not fulfill

the function of ideal lightning arresters i.e. it does not cut off power voltage after it has

flashed over by a surge, in other words a short circuit will be caused on the system every time

a surge causes a flash-over. Flash over conditions are also affected by rain, pollution,

humidity temperature and polarity of the incident waves.  In view of these disadvantages it

can be only used as "back up" protection in case main lightning arrester gets damaged.

Figure:-7.2 view of rod gap type lightening arrester

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7.3.2 Expulsion type Lightning arresters:-

Expulsion type lightning arresters are also called "expulsion protector tubes", "de-ion tubes"

and "line type expulsion arresters." Constructional details and salient features of expulsion

type lightning arresters are shown in fig.

It consists of an insulating tube, which has got an electrode at each end and discharge hole at

the lower end.  The length of the tube is such that spark over occurs in the tube between the

two electrodes.  While installing lightning arresters it is ensured that there is external series

gap between the cap and the line.  Series gap prevents constant application of system voltage

and thus leakage corona is avoided.  Whenever undesirable transient voltages occur, two gaps

i.e. external and internal breakdown due to flash over and provide a conducting path in the

form of arc for drainage of the voltage to the ground.  They are produced inside the tube by

"follow up current" produces gas which drives out ionized air (air is ionized by the arc)

through the bottom vent.  The "follow up current" at its zero finds the arc path de-ionized and

space between the electrodes fully insulated to prevent the flow of "follow up current."   The

rapid expulsion of the gases in the tube normlly interrupt the short circuit power follow

current within the first or second half cycle.

Figure:- 7.3 view of Expulsion type L.A.

7.3.3 Valve Type Lightning Arresters:-

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Valve type Lightning arrester consists of number of spark gaps in series with non-linear

resistors, the whole assembly being rigidly housed inside a hermetically sealed bushing.

Under normal conditions, power frequency system voltage does not cause break down of

series spark gaps and thereby insulate the line from ground for the highest system voltage. 

When undesirable transient voltages due to lightning are super-imposed over the system, the

series gap assemblies spark over at a pre-determined value.  After the breakdown of the gaps,

the non-linear resistors conduct the surge current to the ground offering very low resistance

and limit the power frequency current, to a value, the gaps can interrupt at the first current

zero.  During the flow of the discharge current the non-linear resistor limit the voltage drop

across the arrester to a value far below the BIL of the equipment.

The valve type lightning arresters are generally classified as station type and line type. 

Station type lightning arresters are very robust and efficient and are installed in sub-stations

and power houses.  Line type lightning arresters are similar to station type lightning arresters

but are smaller in cross-section and are less costly.  Line type arresters allow higher surge

voltages across their terminals and have low surge current capacity

Figure:-7.4 view of valve type L.A.

7.4 General Rating Recommendations Of Lightning Arresters:-

i. 10 KV rated lightning arresters:  Arresters of this rating are used in case of power

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stations and E.H.V. sub-stations.

ii. 5 kA rated lightning arrests: Arresters of this capacity normally are used in case of high

voltage sub-stations having system voltage as 66 KV or less.  These are also used in case

of small power houses.

iii. 2.5 kA rated lightning arresters:  Arresters of these ratings are used in case of system

up to 11 KV.

iv. 1.5 KA rated lightning arresters: arresters of these ratings are normally used in case of

distribution system.

7.5 Location of Lightning Arresters:-

In order to ensure effective protection of the equipment lightning arresters should be located:

a. Very close to the equipment to be protected and connected with shortest leads on both the

line and ground side to reduce the inductive effects of the leads while discharging large surge

currents.

b. In order to ensure the protection of transformer windings it is desirable to inter-connect the

ground lead of the arrester with the tank and also the neutral of secondary.  This

interconnection reduces the stress imposed on the transformer windings by the surge currents

to the extent of the drop across the earth resistance and the inductive drop across the ground

lead.

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CHAPTER 8

INSULATORS & RELAYS

In order to present the flow of current to the each earth from support the transmission line and

distribution lines are secured to the supporting tiers or pole with the help of insulators. Thus

the insulators play an important role in the successful operation of lines.

8.1 Requirements For Insulators:-

1. Mechanically Strong

2. High dielectric strength

3. High insulation resistance to the leakage current

4. Free from internal impurities

5. They should be porous

6. They should not be affected with change in temperature.

8.2 Types Of Insulators:-

8.2.1 Pin Type Insulators: - It is one of the earliest designs used for supporting lives

conductors. is used for low voltage up to 33 KV. The pin insulators are screwed on and

firmly attached to galvanized steel bolts.

The figure of pin type insulator is shown as below:-

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Figure- 8.1- Pin Type Insulator

8.2.2 Suspension Type Insulators:-

For higher voltage up to 132 KV suspension insulators are used, a number of them are

connected in series by metallic links to form a chain and the line conductor is carried by the

bottom most insulator.

The figure of suspension type insulator is shown as below:-

Figure- 8.2- Suspension Type Insulator

8.2.3 Shackle insulators:-

It is mostly used for low voltage distribution lines such insulators can either be used in a

horizontal position or in a vertical position. The conductor in the groove are fixed with the

help of soft binding wires.

The figure of shackle type insulator is shown as below:-

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Figure 8.3- Shackle Type Insulator

8.2.4 Strain Insulators:-

When there is a corner or a sharp curve or the lines crosses river etc., the line is to withstand

great strain.

The figure of strain type insulator is shown as below:-

Figure 8.4- Strain Type Insulator

8.3 Bus Bar

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It is a incoming 220kv feeder BUS from which the line is taken to the transformer for further

step down. There are two bus systems.

8.3.1 Double Main Bus & Transfer Bus System

Merits:-

1. Most flexible in operation.

2. Highly reliable.

3. Breaker failure on bus side breaker removes only one circuit. From service.

4. All switching done with breakers.

5. Simple operation, no isolator switching required.

6. Either main bus can be taken out of service at any time for maintenance.

7. Bus fault does not remove any feeder from the service.

Demerits:-

1. High cost due to three buses.

8.3.2 Mesh (Ring) Bus Bar System

Merits:-

1. Bus bars gave some operational flexibility.

Demerits:-

1. If fault occurs during bus maintenance, ring gets separated into two sections.

2. Auto-reclosing and protection complex.

3. Requires VT’s on all circuits because there is no definite voltage reference point. These

VT’s may be required in all cases for synchronizing live line or voltage indication.

4. Breaker failure during fault on one circuit causes loss of additional circuit because of

breaker failure.

The figure of bus bar overview is shown as below:-

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Figure 8.5- Bus Bar Overview

8.4 Control Room

8.4.1 Synchronizing – Panel:-

There is a hinged synchronized panel mounted at the end of a control board.   To take out new

supply on the bus bar supply so the panel handles put to cuts synchronizing and then see the

synchronies scope. There is also two voltmeter one-give bus-bar voltages.  Second in coming

voltage when the synchronoscope stop zero we close the C.B. and the supply is taken on bus

bar.

8.4.2 Synchronoscope:-

A synchronoscope is used to determine the current instance of closing the switch, which

connects new supply to bus bar.

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8.4.3 Basic Equipment Or Requirement Of Protective Relays:-

Basic requirements of protective relays are as follows

1. Speed

Protective relaying should do's connect a faulty element as quickly as possible.

2. Selectivity

The ability of the protective relay to determine the point of which have the fault occurs and

select the nearest circuit breaker tripping of which will lead the clearing of fault with min-or

so damage to the system.

3. Sensitivity

It is the capacity of the relaying to operate relay under the actual condition that produces the

last operating condition tendency.

Depending upon the method of element connected primary relay (series element connect

directly on the circuit of protective element) and secondary relay (sensing element connected

through a current and voltage transformer).

The figure of control panel board is shown as below:-

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Figure-8.6- Control Panel Board

8.5 Relay

There are many type of relays-

1. Over current relays

2. I.D.M.T. fault relay

3. Impedance relay

4. Earth fault relay

5. Buchhloz relay

6. Differential relay

7. Auxiliary relays

8.5.1 Over Current Relay:-

It is used in over current protection scheme. Over current protection is the name given to

protected relay scheme devised to rise in current in a protected circuit of to a safe value

inherent simplicity of operation and reliability in operation has resulted in over current

protection having obtained the widest application in short circuit protection scheme and a

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mean of protection against abnormal condition's of operation etc in power x-mission circuit

as here is grid station when the short circuit occurs the fault current which is very much

higher than the normal current flow through the relay i.e. from proportional due to C.T. and

the over current relay because operations (because flow is more than the present value) i.e. is

more than Ix where Ip is relay picking up or operating currents now due to close of C.B. the

signal is go to trip coil of C.B. trip.

a. Electromagnetic relay

b. Induction over current relays

8.5.2 Inverse Time Characteristics Relay:-

The relay using here having the inverse time characteristics having the time delays dependent

upon current value.  This characteristic is being available in relay of special design. 

There are

1. Electromagnetic Induction type

2. Permanent magnetic moving coil type

3. Static type

8.5.3 Earth Fault Relay:-

The earth fault relay and over current relay resembles because when the conductor break by

any reason it is earthed meant, it is short circuited and fault current which flow in many times

to normal current, so there is always over current fault so now we have the over current relay

and both are same.  These relays can also be Electro-magnetic induction and static relay.

8.5.4 Directional Relay:-

The non-directional relay discussed above can operate for fault in either direction in order to

achieve operation for the fault current flowing in a specific direction.  It is necessary to add

an additional element; such a relay which corresponds to fault current flow in a particular

direction is closes called a directional relay.  These relays are added in the panel.

When a fault takes place, the fault current flows through the current coil of relay which

produces a flux in the lower magnet of the directional while the current in the voltage coil

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produces another flux in the upper magnets.  The flux produces torque tending to close it's

contact (directional element contacts).  The relay also flows through the windings over the

magnet of the non-directional elements.  Since this winding provides a closed path the

induced emf circulates a current, which therefore produce another flux.

8.6 Battery Room

8.6.1 Introduction:-

Storage battery is the most dependable source of supply of D.C. power required for closing

and tripping of circuit breakers, operation of automatic protective devices; signaling

equipment, remote control apparatus, telephone service and emergency lighting in case of

power plants and sub-stations.  Correctly selected and properly maintained battery will

withstand heavy stresses and strains during service without causing much headaches to the

maintenance Engineer. D.C. Auxiliary power supply is provided from storage batteries

maintained continuously charged by some type of supply set or a charger.  The voltage of the

auxiliary supply is maintained at 110/220 .The battery room should be ready in all respects by

fulfilling the following minimum requirements-

1. The walls and the ceiling of the battery room should be well black washed and should

remain clean and dry.

2. The flooring of the battery room shall be acid resistant tiles and material.

3. The battery room should be well lit.  there should be no direct sun light on the cells.

4. Suitable exhaust fans shall be fixed to provide a minimum of six air charges per hour.

5. The exhaust fans shall be suitably distributed and placed on the wall, which open to

atmosphere, equally sufficient air inlet should be provided to prevent any negative pressure

developing in the room.

6. Necessary blowers are to be provided to maintain sufficient air inlet in to room

7.Never the entrance door should be kept closed which will lead to a negative pressure

developing in the batter room due to the continuous operation of exhaust fans.

8. Inlet air should be free from effluents (such as chlorine, acetic acid).

8.6.2 Advantages of Storage Batteries:-

1. High Reliability

VIT/DOEE/2011-2012/PTS/052

2. Independences of A.C. power circuit conditions of existence of the faults.

Figure:-8.7 view of Battery Room

VIT/DOEE/2011-2012/PTS/053