A mini project report on

Study of TRANSFORMER AND THEIR PROTECTION,

STORAGE BATTERIES AND EARTHING A.P.TRANSCO 220/132/33kv substation, Kakinada

Submitted in partial fulfillment of the requirement for the award of the degree of

BACHELOR OF TECHNOLOGY

In

ELECTRICAL AND ELECTRONICS ENGINEERING

By

1. M.V.V.S.Gangadhar 09P31A0229

2. K. Rajesh Kumar 09P31A0221

3. M. Phanindra V Varma 09P31A0231

4. M.D.Madhavi 09P31A0230

5. D. MangaTayaru 09P31A0215

Under the esteemed guidance of

Internal Guide External Guide

Mr.D.Surya Prakash, M.Tech Mr. K.V.Ramana, B.Tech

Asst. Professor Asst.Divisional Engineer

220/132/33KV Substation, Kakinada.

Department of Electrical and Electronics Engineering

SRI SAI ADITYA INSTITUTE OF SCIENCE AND TECHNOLOGY

(Affiliated to JNTUK, Kakinada and Approved by AICTE, New Delhi)

Surampalem, East Godavari District

Year 2009-2013

CERTIFICATE

This is certify that the mini project report entitled Study of Transformer and Their Protection, Storage Batteries and Earthingbeing submitted by

1. M.V.V.S.Gangadhar 09P31A0229

2. K. Rajesh Kumar 09P31A0221

3. M. Phanindra V Varma 09P31A0231

4. M.D.Madhavi 09P31A0230

5. D. MangaTayaru 09P31A0215

in partial fulfillment of the requirement for the award of the degree ofBachelor of Technology in Electrical and Electronics Engineering. It is record of bonified work, carried out by them under esteemed guidance and supervision of Mr.D.Surya Prakash, M.Tech.

Project guide Head of the Department

Mr.D.Surya Prakash, M.Tech Mr.J.Pavan, M.Tech, (Ph.D.)

Asst. Professor Electrical and Electronics Engineering

Electrical and Electronics Engineering Sri Sai Aditya Inst. of Sci. &Tech.

Department of Electrical and Electronics Engineering

SRI SAI ADITYA INSTITUTE OF SCIENCE AND TECHNOLOGY

(Affiliated to JNTUK, Kakinada and Approved by AICTE, New Delhi)

Surampalem, East Godavari District

Year 2009-2013.

ABSTRACT

This project deals with the Transformers such as power transformers and instrument

transformers used in 220/132/33kv substation, Kakinada. The protection schemes used in this

substation are differential protection, buchholtz relay protection and over flux relay.

Transformer is an important device in electrical transmission and distribution.

Transformer is used to step down and step up the voltages. In distribution, the transformer is

used to step down high voltages to low voltages (i.e. from 33KV to 400/230V).This step

down of voltage is necessary for the appliances we use based on this voltage.

The different types of storage batteries which are used for the purpose of delivering

power to the control unit and other auxiliary equipments in the substation by the process of

charging and discharging.

Storage batteries are used for supplying power for operating automatic control

circuits, the protective relay systems, as well as the emergency lighting circuits in electric

power stations and substations.

Generally earthing practices are used for the safety purpose. In this we covered the

topics like classification of earthing, earthing methods, earthing materials and requirements

for good earthing.

Earthing practices adopted at general stations, substations, distribution structures and

lines are of great importance. Earthing means connecting frame of electrical equipment or

some electric part of the system to earth. Earthing is provided for safety of personnel;

improve reliability of power supply etc

INDEX

CHAPTER TOPIC PAGE NO

1. INTRODUCTION 01

2. TRANSFORMERS 02

2.1.Basic Theory of Transformer 02

2.2. Current Transformer 05

2.3. Potential Transformer 06

2.4. Applications of Transformers 08

3. CAUSES OF TRANSFORMER FAILURE AND PREVENTION 09

3.1. Differential protection 10

3.2. Buchholz Relay 12

3.3. Over fluxing relay 14

4. STORAGE BATTERIES 16

4.1. Principle of Operation 17

4.2. Maintenance schedule of Batteries 19

5. SUBSTATION EARTHING 20

5.1. Classification of earthing 20

5.2. Earthing Materials 22

5.3. Components of a Substation 23

6. CONCLUSION 28

7. BIBLIOGRAPHY 29

LIST OF FIGURES

S.NO. PAGE NO.

1. Figure no.1 POWER TRANSFORMER 02

2. Figure no.2 TRANSFORMER INTERNAL CORE AND WINDINGS 04

3. Figure no.3 CURRENT TRANSFORMER. 06

4. Figure no.4 POTENTIAL TRANSFORMER 07

5. Figure no.5 DIFFERENTIAL PROTECTION 12

6. Figure no.6 VECTOR DIAGRAM 12

7. Figure no.7 CONSERVATOR TANK 14

8. Figure no.8 BUCHHOLZ RELAY 14

9. Figure no.9 OVERFLUXING RELAY 16

10. Figure no.10 COMPONENTS OF EARTH RESISTANCE IN AN ELECTRODE 22

LIST OF TABLES

S.NO. PAGE NO.

1. Table no.1 POWER TRANSFORMER SPECIFICATIONS 03

2. Table no.2 33KV CURRENT TRANSFORMER RATINGS 06

3. Table no.3 33KV POTENTIAL TRANSFORMER RATINGS 07

4. Table no.4 DIFFERENTIAL PROTECTION CONNECTION WAYS 11

5. Table no.5 MAINTENANCE SCHEDULE OF BATTERIES 19

6. Table no.6 OVERHEAD LINE TERMINATIONS 26

CHAPTER-1

INTRODUCTION

A 132/33KV EHT substation at Kakinada was commissioned in the year 2-7-1986

and upgraded as 220KV substation in the year 25-9-1997 as a part of primary transmission

system into large network of APSEB now the AP TRANSCO. The 220/132-33KV EHT

substation Kakinada, situated near the coastal area of East Godavari District which receives

power from M/s Spectrum Power Generation limited UPPADA (Godavari Combined Cycle

Power Plant) near sea UPPADA.

A 220MW of generation power at M/S SPG LTD. Uppada is being received through

2no, 220KV circuits (220KV SPECTRUM CIRCUITS 1&2).The power of 140MW app.has

been utilized to fed the loads of Kakinada town and rural areas of Kakinada,132/33KV

Ramachandrapuram substation, 132/11KV yanam substation, and to NFCL,GFCL EHT

industrial consumers. Sometimes the power is also transmitted to peddapuram, Amalapuram

and Rajole depends up on the grid conditions. Out of 220MW, 60MW app.of power again

transmitted to AP grid through 220KV Kakinada-bomuru feeders.

There are 2no. 100MVA, 220/132 KV Crompton Greaves Limited make power

transformers, 2no.132/33KV power transformers available and in service in this substation to

step down the power from higher voltages to lower voltage as a part of the secondary

transmission/substation of large network of AP TRANSCO. There are 2no.station

transformers (250KVA, 100KVA capacities) connected to 33 KV bus to feed the auxiliaries

of substation to feed the station lighting. The operation of entire equipment of the substation

is being controlled in remote by means of C&R panels equipment installed in control room

safety.

A station battery system of 200AH capacity, 220V DC has been used to feed the loads

of control circuits like closing/tripping circuits of breakers, indication circuits, and auxiliary

supply to relay and so on.

1CHAPTER-2

TRANSFORMERS

Electrical Power Transformer is a static device which transforms electrical energy

from one circuit to another without any direct electrical connection and with the help of mutual

induction between to windings. It transforms power from one circuit to another without

changing its frequency but it differs the voltage level and current level.

2.1. Basic Theory of Transformer:

The working principle of transformer depends upon Faraday's laws of

Electromagnetic Induction. Actually mutual induction between two or more winding is

responsible for transformation action in an electrical transformer. According to these Faraday's

laws, Rate of change of flux linkage with respect to time is directly proportional to the

induced EMF in a conductor or coil".

Say you have one winding which is supplied by an alternating electrical source. The

alternating current through the winding produces a continually changing flux or alternating flux

surrounds the winding. If any other winding is brought nearer to the previous one, obviously

some portion of this flux will link with the second. As this flux is continually changing in its

amplitude and direction, there must be a change in flux linkage in the second winding or coil.

According to Faraday's laws of Electromagnetic Induction, there must be an EMF induced in the

second. If the circuit of the latter winding is closed, there must be current flows through it. This

is the simplest form of electrical power transformer and this is most basic of working principle

of transformer.

POWER TRANSFORMER

Figure no. 1

2

The winding which takes electrical power from the source, is generally known as

Primary Winding of transformer. Here it is first winding. The winding which gives the desired

output voltage due to mutual induction in the transformer, is commonly known as Secondary

Winding of Transformer. Here it is second winding.

Main constructional parts of transformer:

So three main parts of a transformer are,

1. Primary winding of transformer - which produces magnetic, flux when it is connected to

electrical source.

2. Magnetic Core of transformer - the magnetic flux produced by the primary winding, will pass

through this low reluctance path linked with secondary winding and creates a closed magnetic

circuit.

3. Secondary Winding of transformer - the flux, produced by primary winding, passes through

the core, will link with the secondary winding. This winding is also wound on the same core and

gives the desired output of the transformer.

Transformer is one of the most efficient electrical device due to the absence of rotating parts.

And it has a maximum efficiency around 99%.

The power transformer used is 200KVA POWER TRANSFORMER. Its

specifications are as shown.

Table no.1

FULL LOAD 200KVA

RATED NO LOAD VOLTAGE HV-33KV,LV-433V

RATED NO LOAD CURRENT HV-35A,LV-2666.67A

TYPR OF COOLING NATURAL AIR,OIL FILLED ONAN

IMPEDANCE VOLTAGE 8.432%

AMBIENT TEMPERATURE 40 C

OIL TANK CAPACITY 22000LTS

TOTAL WEIGHT WITH OIL 7360KG

VECTOR GROUP REF DYN1

QUANTITY 1 NO

TAP CHANGER OFF LOAD TAP CHANGER

3

TYPES OF TRANSFORMER:

Transformers can be categorized in different ways, depending upon their purpose, use,

construction etc. The types of transformer are as follows,

Step Up Transformer & Step Down Transformer - Generally used for stepping up and down the voltage level of power in transmission and distribution power network.

Three Phase Transformer& Single Phase Transformer is generally used in three phase power system as it is cost effective than later but when size matters it is preferable to use bank of three

Single Phase Transformeras it is easier to transport three single phase unit separately than one

single three phase unit.

Transformer generally used in transmission network is normally known as Power Transformer,distribution transformer is used in distribution network and this is lower rating transformer

and current transformer&potential transformer, we use for relay and protection purpose in

electrical power system and in different instruments in industries are called Instrument

Transformer.

Two Winding Transformer & Auto Transformer is generally used where ratio between High Voltage and Low Voltage is greater than 2. It is cost effective to use later where the ratio

between High Voltage and Low Voltage is less than 2.

Outdoor Transformer & Indoor Transformer - Transformers designed for installing at outdoor is Outdoor Transformer and Transformers designed for installing at indoor is Indoor Transformer.

TRANSFORMER INTERNSL CORE AND WINDINGS

Figure no.2

4

The above mentioned form of transformer is theoretically possible but not practically, because in open air very tiny portion of the flux of the first winding will link with second so the current

flows through the closed circuit of latter, will be so small that it may be difficult to measure.

The rate of change of flux linkage depends upon the amount of linked flux, with the second winding. So it desired to be linked almost all flux of primary winding, to the secondary winding.

This is effectively and efficiently done by placing one low reluctance path common to both the

winding. This low reluctance path is core of transformer, through which maximum number of

flux produced by the primary is passed through and linked with the secondary winding. This is

most basic theory of transformer.

Instrument Transformers:1. Current transformer

2. Potentialtransformer

Instrument transformers means current transformer&voltage transformer are used in

electrical power system for stepping down currents and voltages of the system for metering and

protection purpose. Actually relays and meters used for protection and metering, are not

designed for high currents and voltages.

2.2Current Transformer:

A current transformer (CT) is an instrument transformer in which the secondary

current is substantially proportional to primary current and differs in phase from it by ideally

zero degree.

A CT is similar to an electrical power transformer to some extent, but there is some

difference in construction and operation principle. For metering and indication purpose,

accuracy of ratio, between primary and secondary currents is essential within normal working

range. Normally accuracy of current transformer required up to 125% of rated current, as

because allowable system current must be below 125% of rated current. Rather it is desirable the

CT core to be saturated after this limit since the unnecessary electrical stresses due to system

over current can be prevented from the metering instrument connected to the secondary of the

current transformer as secondary current does not go above a desired limit even primary current

of the CT rises to a very high value than its ratings. So accuracy within working range is main

criteria of a current transformer used for metering purpose. The degree of accuracy of a

Metering CT is expressed by CT Accuracy Class or simply Current TransformerClass or CT

Class.

But in the case of protection, the CT may not have the accuracy level as good as

metering CT although it is desired not to be saturated during high fault current passes through

primary. So core of protection CT is so designed that it would not be saturated for long range of

currents. 5

If saturation of the core comes at lower level of primary current the proper reflection

of primary current will not come to secondary, hence relays connected to the secondary may not

function properly and protection system losses its reliability.

CURRENT TRANSFORMER

Figure no.3

RATINGS OF CURRENT TRANSFORMER IN 220/132/33KV SUBSTATION

Table no.2

CT RATIO 50-25/1-1A

SPECIFICATIONS OUTDOOR

FREQUENCY 50HZ

QUANTITY 20LIT

TOTAL WEIGHT 80KG

BURDEN 15VA

2.3. Potential Transformer:

Potential Transformer or Voltage Transformer is used in electrical power system for

stepping down the system voltage to a safe value which can be fed to low ratings meters and

relays. Commercially available relays and meters used for protection and metering, are designed

for low voltage. This is a simplest form of Potential Transformer Definition.

A Voltage Transformer theory or Potential Transformer theory is just like theory of general

purpose step down transformer. Primary of this transformer is connected across the phases or

and ground depending upon the requirement. Just like the transformer, used for stepping down

purpose, potential transformer i.e. PT has lowers turns winding at its secondary.

6

The system voltage is applied across the terminals of primary winding of that

transformer, and then proportionate secondary voltage appears across the secondary terminals of

the PT.

POTENTIAL TRANSFORMER

Figure no.4

The secondary voltage of the PT is generally 110V. In an ideal Potential Transformer

or Voltage Transformer when rated burden connected across the secondary the ratio of primary

and secondary voltages of transformer is equal to the turns ratio and furthermore the two

terminal voltages are in precise phase opposite to each other. But in actual transformer there

must be an error in the voltage ratio as well as in the phase angle between primary and

secondary voltages.

RATINGS OF POTENTIAL TRANSFORMER IN 220/132/33KV

SUBSTATION

Table no.3

TYPE 33PY/D,SINGLE

PHASE

RATIO 33KV/53/110/53

FREQUENCY 50HZ

RATING 100VA

INSULATION

LEVEL

70/170KV

7

2.4. Applications of Transformers:

A major application of transformers is to increase voltage before transmitting

electrical energy over long distances through wires. Wires have resistance and so dissipate

electrical energy at a rate proportional to the square of the current through the wire. By

transforming electrical power to a high-voltage (and therefore low-current) from for

transmission and back again afterward, transformers enable economical transmission of power

over long distances.

Consequently, transformers have shaped the electricity supply industry, permitting

generation to be located remotely from points of demand. All but a tiny fraction of the worlds

electrical power has passed through series of transformers by the time it reaches the

consumer.Transformers are also used extensively in electronic products to step down the supply

voltage to a level suitable for low voltage circuits they contain. The transformer also electrically

isolates the end user from contact with the supply voltage.

Signals and audio transformers are used to couple stages of amplifiers and to match

devices such as microphones and record players to the input of amplifiers. Audio transformers

allowed telephone circuits to carry on a two-way conversation over a single pair of wires. A

balloon transformer converts a signal that is referenced to ground to a signal that has balanced

voltages to ground, such as between external cables and internal circuits.

8CHAPTER-3

CAUSES OF TRANSFORMER FAILURE AND PREVENTION

The various probable causes of failures and the preventive maintenance necessary are

given below:

As explained earlier, the transformer mainly consists:

1. Magnetic Circuits

2. Electrical Circuits

3. Insulation(Dielectric) terminals

4. Tank, Oil, etc.

Transformer failure and safety hazards can be avoided or minimized by ensuring that

the conductors and equipment are properly sized, protected and adequately grounded. Incorrect

installation of transformers can results in fires from improper protection, as well as electric

shock from inadequate grounding.

Once the transformer is placed, the tank must be permanently grounded with a correctly sized and properly installed permanent ground.

Access should be restricted to the transformer liquid- filled compartment in conditions of excessive humidity or rain.

Dry air should be continuously pumped into the gas space if humidity exceeds 70%. Transformer should be given protection against rain such that no water gets inside. All equipment used in the handling of the fluid (hoses, pumps, etc.) should be clean and dry.

If the insulating liquid for inspection is drawn out, its level should not go below the top of

windings.

Sufficient gas pressure must be maintained to allow a positive pressure of 1 psi to 2 psi at all times (even at low ambient temperature) when liquid-filled transformers are stored outside.

Final inspection of the transformer is essential before it is energizes. All electrical connections, bushings, draw lead connections should be checked.

Upon loading the transformer should be kept under observation during the first few hours of operation. All temperatures and pressures should be checked in the transformer tank during the

first week of operation.

Surge arresters must be installed and connected to the transformer bushing/terminals with the shortest possible leads to protect the equipment from line switching surges and lighting.

9

The various protection used for various transformer faults are:Fault-Type Protection-used

Primary winding ph-ph fault ----------------------------- Differential, over current

Primary winding ph-earth fault ----------------------------- Differential, over current

Secondary winding ph-ph fault ----------------------------- Differential

Secondary winding ph-earth fault------------------------------ Differential, restricted earth fault

Internal fault ------------------------------ Differential, Buchholz

Core fault ------------------------------ Differential, Buchholz

Tank fault ------------------------------- Differential, Buchholz

3.1. DIFFERENTIAL PROTECTION

The differential protection used for transformers is based on the principle of current

circulation. This type of protection is mostly used for transformers as this responds not only to

inter turn fault but also provides protection against phase-to-phase faults. Following are the

complicated features in transformers and there remedial measures:

1. In a power transformer, the currents in primary and secondary are to be compared.

As these two currents are usually different, therefore the use of identical transformers will give

differential current and operate the relay even under no load conditions. The difference in

magnitude of currents in primary and secondary of power transformers is compensated by

different turns ratios of C.T.s. If T is the turns ratio of power transformer, then the turns ratio

of C.T.s on lv side is made T times the turns ration of the C.T.s on hv side. When this condition

is fulfilled the secondaries of the two C.T.s will carry same current under normal conditions.

And thus no current will flow through the relay and it remains in operative.

2. There is usually a phase difference between the primary and secondary currents of a 3-phase

power transformer. Even if C.T.s of proper transformation ratios are used, a differential current

will flow through the relay under normal condition and cause relay operation. The correction for

phase difference is effected by appropriate connections of C.T.s. The C.T.s on one side of the

power transformer are connected in such a way that the resultant current fed into the pilot wires

are displaced in phase from the individual phase currents in the same direction as, and by an

angle equal to, the phase shift between the power transformers primary and secondary currents.

The table below shows the type of connections to be employed for C.T.s in order to compensate

for the phase difference in the primary and secondary currents of power transformer.

10

CONNECTION WAYS:

Table no.4

S.NO POWER

TRANSFORMER CONNNECTIONS

CURRENT

TRANSFORMER

CONNECTIONSPRIMARY SECONDARY PRIMARY SECONDARY

1. Star with neutral earthed

Delta Delta Star

2. Delta Delta Star Star

3. Star Star with neutral earthed

Delta Delta

4. Delta Star with neutral earthed

Star Delta

Another factor, which has to be considered, is the in rush of magnetizing current.

When the transformer is switched to supply the magnetizing current may assume very high

values momentarily and may cause operation of the relay even though they are transient. This

can be avoided by using relays with time delay characteristics. Figure 5 shows the differential

protection for transformer. In this the power transformer is delta- star connected. On delta side

the C.T.s are connected in star and on the star side the C.T.s are connected in delta as in fig.5.

Under normal working conditions the circulating currents caused by the primary and

secondary load current in the relay circuit will balance; but under fault conditions the balance

will no longer be there and the relay will be energized to trip the circuit breakers on the primary

and secondary side. In order to understand the phase difference in the two sides consider fig 5.

The primary is connected in delta and the set of current transformers CT1 is connected in star,

while the secondary is connected in star and the set of current transformers CT2 is connected in

delta. Fig 6 illustrates the vector diagram in reference to primary and secondary sides of current

transformer.

11

In fig 6.a IRP , IYP and IBP are the phase currents in the primary side, while IR is the line

current on the same side in line R as shown in fig 6.a., the corresponding secondary current of

current transformers CT1 on the primary side is in phase with IR and is represented as IRS in fig

6.b. The current in the secondary side of the power transformer is represented as IR, IY and IB in

fig 6, the phase current in the secondary winding of the current transformers CT2 is represented

as IR, IY and IB in fig 6.c. The current in pilot wire of CT2 is represented as IRS. Now when we

consider fig 6.b and 6.d its clear that the currents in the pilot wires are in phase.

DIFFERENTIAL PROTECTION

Figure 5

Figure 6

12

3.2. Buchholz Relay:

Buchholz Relay in transformer is an oil container housed the connecting pipe from

main tank to conservator tank. It has mainly two elements. The upper element consists of a float.

The float is attached to a hinge in such a way that it can move up and down depending upon the

oil level in the Buchholz Relay Container. One mercury switch is fixed on the float. The

alignment of mercury switch hence depends upon the position of the float. The lower element

consists of a baffle plate and mercury switch.

This plate is fitted on a hinge just in front of the inlet (main tank side) of Buchholz

Relay in transformer in such a way that when oil enters in the relay from that inlet in high

pressure the alignment of the baffle plate along with the mercury switch attached to it, will

change. In addition to these main elements a Buchholz Relay has gas release pockets on top. The

electrical leads from both mercury switches are taken out through a molded terminal block.

Buchholz Relay principle:

Buchholz Relay function is based on very simple mechanical phenomenon. It is

mechanically actuated. Whenever there will be a minor internal fault in the transformer such as

an insulation faults between turns, break down of core of transformer, core heating, the

transformer insulating oil will be decomposed in different hydrocarbon gases, CO2 and CO. The

gases produced due to decomposition of transformer insulating oil will accumulate in the upper

part the Buchholz Container which causes fall of oil level in it. Fall of oil level means lowering

the position of float and there by tilting the mercury switch. The contacts of this mercury switch

are closed and an alarm circuit energized. Sometime due to oil leakage on the main tank air

bubbles may be accumulated in the upper part the Buchholz Container which may also cause fall

of oil level in it and alarm circuit will be energized. By collecting the accumulated gases from

the gas release pockets on the top of the relay and by analyzing them one can predict the type of

fault in the transformer.

More severe types of faults, such as short circuit between phases or to earth and faults

in the tap changing equipment, are accompanied by a surge of oil which strikes the baffle plate

and causes the mercury switch of the lower element to close. This switch energized the trip

circuit of the Circuit Breakers associated with the transformer and immediately isolate the faulty

transformer from the rest of the electrical power system by inter tripping the Circuit Breakers

associated with both LV and HV sides of the transformer. This is how Buchholz Relay

functions.

13

CONSERVATOR TANK

Figure 7

BUCHHOLZ RELAY

Figure 8

3.3. Over fluxing relay:

A transformer is designed to operate at or below a maximum magnetic flux density in

the transformer core. Above this design limit the eddy currents in the core and near by

conductive components cause overheating which within a very short time may cause severe

damage. The magnetic flux in the core is proportional to the voltage applied to the winding

divided by the impedance of the winding. The flux in the core increases with either increasing

voltage or decreasing frequency. During startup or shutdown of generator-connected

transformers, or following a load rejection, the transformer may experience an excessive ratio of

volts to hertz, that is, become overexcited.

14

.

When a transformer core is overexcited, the core is operating in a non-linear magnetic

region, and creates harmonic components in the exciting current. A significant amount of current

at the 5th harmonic is characteristic of over excitation.

Over fluxing in Transformers:

The transformer works on the principle of mutual induction between the primary and

secondary windings. The induction is caused by the constantly varying magnetic flux that links

the two windings. The flux density in the windings is directly proportional to the induced

voltage and inversely proportional to the frequency and the number of turns in the winding.

Magnetic Flux Voltage/Frequency

Over fluxing is a dangerous situation in which the magnetic flux density increases to

extremely high levels. The high flux density can induce excessive eddy currents in the windings

and in other conductive parts inside the transformers.

The heat generated by these eddy currents can damage the windings and the insulation. The high

flux density also causes magnetostriction inside the transformer core and produces noise. The

powerful Magnetostrictive forces can also cause damage. The winding temperatures may also

increase due to the heat produced.

The magnetic flux density is dependent on the current flowing through the primary

windings in a transformer. This current is dependent on the voltage applied across the windings

and the winding impedance. The impedance is dependent on the frequency of the applied

voltage. If the nominal voltage is applied at a reduced frequency, the low inductive reactance

will cause a higher current to flow through the windings.

Over fluxing is usually encountered in Transformers which are directly connected to

the generator. It usually occurs when the generator is being started or stopped. As the rpm of the

generator and consequently the frequency of the power falls, the same system voltage induces a

higher magnetic flux. Modern Automatic Voltage regulators are equipped with V/Hz limiters

which limit the voltage in accordance with the frequency.

15

.

OVERFLUXING RELAY

Figure 9

Over fluxing can be prevented by the use of an Over fluxing relay. An over fluxing is

an adaptation of an overvoltage relay. The PT voltage is connected across a resistor and a

capacitor in series. The voltage sensing relay is connected across the capacitor. The relay

operates in the event of an over fluxing and isolates the transformer.

16

STORAGE BATTERIES

Station batteries are used for supplying power for operating automatic control circuits,

the protective relay system, as well as the emergency circuits in lighting circuits in electrical

power Station and substations. These constitutes independent source of operative D.C.power and

guarantee operation of the above mentioned circuits irrespective of any fault occurring in the

power Stations or sub-Stations, even in the event of complete disappearance of A.C. service

voltage in the installation. Storage batteries are assembled with certain number of accumulator

cells depending on the working voltage of the respective D.C. circuits.

Fuse protection of equipment or a circuit serves the dual purpose of detecting excessive

current and breaking the supply by self-melting. These two functions are discharged collectively

by relays and circuit breakers with the assistance of storage batteries. The relay detect the over

current conditions and close the trip circuit of the breaker. The storage batteries give the

necessary operative D.C.power for the tripping mechanism of the circuit breaker to open the

circuit and isolate the fault. At the time of failure of A.C.supply, Station will be left in the

darkness even without D.C.lighting if the battery is not maintained in proper condition. The

much needed communication either by power line carrier or by magneto during emergencies

may not work if the Station battery is not maintained in good condition.

Electric Battery is a device for the direct transformation of chemical energy to electrical energy.

Storage Battery is cells which are reversible to a high degree, i.e., those in which the

chemical condition and as well as physical state of the electrodes after discharging are brought

back to the original condition simply by causing current to flow in opposite direction i.e.

charging. In a primary battery the parts which react chemically require renewal where as in

storage battery the reactions are completely reversible and initial chemical condition may be

restored after partial or complete discharge. The passing of electric current through the cell so as

to bring it to a chemical condition in which it is capable of supplying electricity to an external

circuit called charging.

The quantity of electricity used is known as charge and is measured in ampere-hours.

The connection of cell to an external circuit in such a way that a current flow through the cell in

reverse of charge is known as discharging.

17

The quantity of electricity usually expressed in ampere-hours, which may be taken from a

cell at a given rate of discharge under specified conditions of voltage and temperature is known

as capacity.

4.1. Principle of Operation:

There are only two types of batteries. They are

1. Acid type

2. Alkaline type

The acid type is prominently used.

Lead Acid Battery:

The essential parts of accumulator cell are

1. The plates (electrodes)

2. The electrolyte

3. The container

In lead acid accumulator the active material is lead peroxide on the positive plate and

spongy lead on the negative plate. The electrolyte used is dilute sulphuric acid. The principle of

operation is chemical transformation of the positive lead peroxide and the negative spongy lead

into lead sulphate when discharging and the converse process when charging. When discharging

lead sulphate is deposited on the positive plate and part of sulphuric acid is used up, water being

formed. The chemical reactions are represented by the following equations:

PbO2+Pb+2H2SO4-------2PbSO4+2H2OThe specific gravity of the acid therefore decreases. In the course of discharging the

voltage of the cell at the first falls slowly and then more quickly. After a certain drop in voltage

has taken place the discharging must be stopped if the cell is to remain in good condition. The

mean cell voltage of lead acid battery is 2.1 to 2.2 volts and the specific gravity of the electrolyte

is 1.23 to 1.26 in a fully charged condition.

Nickel-Iron (NI-Fe) Battery :( Alkaline Type)

The nickel- iron accumulator has as its active material. In the positive plates a nickel

alloy, in the negative plates an ironalloy.The electrolyte used is a 21% solution of caustic potash

(KOH) with the addition of a small quantity lithium hydrate. On the discharge the OH ions of

KOH travel to the negative, the iron therefore becoming oxidized. The K ions travel to the

positive and reduce Ni (OH) 4 to Ni (OH) 2.

18

During charge the converse action takes place.The chemical reactions are represented by

the following equations:

Ni(OH)4+ KOH+Fe---------Ni(OH)2+ KOH+Fe(OH)2

It be noticed that the electrolyte acts merely as a vehicle for the transfer of OH from the

one plate to the other and does not take part in any chemical change. As a result the specific

gravity does not change to the extent as in the ordinary lead acid cell. The average voltage of

nickel- iron cell is 1.5v and the specific gravity of the electrolyte is 1.2.

Small batteries will have glass, container and incase of large batteries, containers are made

of wood and lined with sheet lead.Battery container are also made of plastics, hard rubber and

vitreous clay.

Selection of Batteries:

The most widely employed type is the lead acid storage battery because of its high cell

voltage and reasonable cost. The alkaline accumulator such as nickel- iron or nickel-cadmium are

less frequently used because of their lower voltage per cell than lead- acid type. Alkaline cells

are remarkably in sensitive to electrical or mechanical strains.

Voltage: For power station and substation where remote control operation of switchgear is

involved a 220V battery is normally selected taking into account other purpose for which the

battery is used such as emergency lighting. 110 or 120 batteries are also use in a few stations. In

other cases where remote control operations are not involved such as small 33/11KV substations

one or two 32V batteries may serve the purpose.

Number of Cells:

Normally 110 cells for 220V station battery and 15 cells for 32V station battery of lead-

acid are used.

Capacity:

The ampere-hour capacity of a battery on discharge is determined by a number of

factors of which the following are the most important.

a) Final limiting voltage

b) Discharging rate

19

c) Number, design and dimensions of plates

d) Design of separators

e) Quantity of electrolyte

f) Specific gravity of electrolyte

g) Temperature etc.

Charger:

The general practice is to install rectifier equipment. Two separate chargers one with

small capacity for idle charging and other of larger capacity for boost charging are

recommended for important stations. For small stations both idle and boost charging functions

can be combined in one charger by suitably cutting in and with resistors.

4.2. Maintenance schedule of Batteries:

Table no.5

S.NO. ITEMS OF MAINTENANCE PERIODICITY

1. Taking specific gravity and voltage of pilot cells daily

2. Checking gravity and voltage of each cell

A) lead-acid

B) NI-Fe cells

before and after charging(Weekly)

monthly

3. Cleaning of terminals applying Vaseline and

topping with distilled water

weekly for lead-acid

4. Overhaul of NI-Fe batteries Yearly

5. Leakage test by lamp or voltmeter method Each shift

6. Checking all connections of charger and battery

for tightness

Quarterly

20

CHAPTER-5

SUBSTATION EARTHING

5.1. CLASSIFICATION OF EARTHING:

The function of an earthing system is to provide an earthing system connection to

which transformer neutrals or earthing impedances may be connected in order to pass the

maximum fault current. The earthing system also ensures that no thermal or mechanical damage

occurs on the equipment within the substation, thereby resulting in safety to operation and

maintenance personnel. The earthing system also guarantees equipotential bonding such that

there are no dangerous potential gradients developed in the substation.

Earthing can be classified into the following categories based on the purpose for

which the part of the equipment connected to the general mass of the earth.

a) System Earthing

b) Equipment Earthing

System Earthing:

Earthing associated with current carrying parts of the equipment is caked System

Earthing. The system security, reliability, performance, voltage stabilization, all relied only on

the system Earthing. E.g. Earthing Neutral of Transformer, Surge arrester Earthing.

System Earthing Methods:

a) Solid Earthing

b) Resistance Earthing

c) Reactance Earthing

d) ThroGrounding Transformer

Equipment Earthing:

Earthing associated with non-current carrying parts of Electrical equipment is caked

Equipment Earthing. Safety of operator, consumer, and safety of their property are mainly

based on Equipment Earthing. E.g. Body of the Transformer, Body of Motor.

21

Requirement of Good Earthing:

a) Good earth should have low resistance.

b) It should stabilize circuit potential with respect to ground and limit overall potential rise.

c) It should protect men material from injury or damage due to over voltage.

d) It should provide low impedance path to fault currents to ensure prompt and consistent operation of protective

relays, surgearrester etc

e) It should keep maximum potential gradient along the surface of the substation within safe limits during ground

fault.

22

COMPONENTS OF EARTH RESISTANCE IN AN ELECTRODE

Figure 10

In designing the substation, three voltages have to be considered.

1. Touch Voltage : This is the difference in potential between the surface potential and the

potential at earthed equipment while a man is standing and touching the earthed structure.

2. Step Voltage : This is the potential difference developed when a man bridges a distance of

1m with his feetwhile not touching any other earthed equipment.

3. Mesh Voltage: This is the maximum touch voltage that is developed in the mesh of the

earthing grid.

5.2. EarthingMaterials:

1. Conductors: Bare copper conductor is usually used for the substation earthing grid. The

copper bars themselves usually have a cross-sectional area of 95 square millimeters, and they are

laid at a shallow depth of 0.25-0.5m, in 3-7m squares. In addition to the buried potential earth

grid, a separate above ground earthing ring is usually provided, to which all metallic substation

plant is bonded.

23

2. Connections : Connections to the grid and other earthing joints should not be soldered

because the heat generated during fault conditions could cause a soldered joint to fail. Joints are

usually bolted, and in this case, the face of the joints should be tinned.

3. Earthing Rods: The earthing grid must be supplemented by earthing rods to assist in the

dissipation of earth fault currents and further reduce the overall substation earthing resistance.

These rods are usually made of solid copper, or copper clad steel.

4. Switchyard Fence Earthing: The switchyard fence earthing practices are possible and are

used by different utilities. These are:

(i) Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter. The fence is then

bonded to the grid at regular intervals.

(ii) Place the fence beyond the perimeter of the switchyard earthing grid and bond the fence to its

own earthing rod system. This earthing rod system is not coupled to the main substation earthing

grid.

5.3. Components of a Substation:

The substation components will only be considered to the extent where they influence substation layout.

Circuit Breakers:

There are two forms of open circuit breakers:

1. Dead Tank - circuit breaker compartment is at earth potential.

2. Live Tank - circuit breaker compartment is at line potential.

The form of circuit breaker influences the way in which the circuit breaker is

accommodated. This may be one of four ways.

Ground Mounting and Plinth Mounting: The main advantages of this type of mounting

are its simplicity, ease of erection, ease of maintenance and elimination of support structures. An

added advantage is that in indoor substations, there is the reduction in the height of the building.

A disadvantage however is that to prevent danger to personnel, the circuit breaker has to be

surrounded by an earthed barrier, which increases the area required.

Retractable Circuit Breakers: These have the advantage of being space saving due to the

fact that isolators can be accommodated in the same area of clearance that has to be allowed

between the retractable circuit breaker and the live fixed contacts.

24

Another advantage is that there is the ease and safety of maintenance. Additionally

such a mounting is economical since at least two insulators per phase are still needed to support

the fixed circuit breaker plug contacts.

Suspended Circuit Breakers: At higher voltages tension insulators are cheaper than post or

pedestal insulators. With this type of mounting the live tank circuit breaker is suspended by

tension insulators from overhead structures, and held in a stable position by similar insulators

tensioned to the ground. There is the claimed advantage of reduced costs and simplified

foundations, and the structures used to suspend the circuit breakers may be used for other

purposes.

Current Transformers:

CT's may be accommodated in one of six manners:

Over Circuit Breaker bushings or in pedestals.

In separate post type housings.

Over moving bushings of some types of insulators.

Over power transformers of reactor bushings.

Over wall or roof bushings.

Over cables. In all except the second of the list, the CT's occupy incidental space and do not affect the size

of the layout. The CT's become more remote from the circuit breaker in the order listed above.

Accommodation of CT's over isolator bushings, or bushings through walls or roofs, is usually

confined to indoor substations.

Isolators:

These are essentially off load devices although they are capable of dealing with small

charging currents of bus bars and connections. The design of isolators is closely related to the

design of substations. Isolator design is considered in the following aspects:

Space Factor

Insulation Security

Standardization

Ease of Maintenance

Cost 25

Some types of isolators include:

Horizontal Isolation types

Vertical Isolation types

Moving Bushing types

Conductor Systems:

An ideal conductor should fulfill the following requirements:

Should be capable of carrying the specified load currents and short time currents.

Should be able to withstand forces on it due to its situation. These forces comprise self-weight,

and weight of other conductors and equipment, short circuit forces and atmospheric forces such

as wind and ice loading.

Should be corona free at rated voltage.

Should have the minimum number of joints.

Should need the minimum number of supporting insulators.

Should be economical.

The most suitable material for the conductor system is copper or aluminum. Steel may be used

but has limitations of poor conductivity and high susceptibility to corrosion.

In an effort to make the conductor ideal, three different types have been utilized, and these

include:

Flat surfaced Conductors Stranded Conductors Tubular Conductors

Insulation:

Insulation security has been rated very highly among the aims of good substation design.

Extensive research is done on improving flashover characteristics as well as combating

pollution. Increased creepage length, resistance glazing, insulation greasing and line washing

have been used with varying degrees of success.

Power Transformers:

EHV power transformers are usually oil immersed with all three phases in one tank.

Auto transformers can offer advantage of smaller physical size and reduced losses.

The different classes of power transformers are:

o.n.: Oil immersed, natural cooling

o.b.: Oil immersed, air blast cooling

o.f.n.: Oil immersed, oil circulation forced

o.f.b.: Oil immersed, oil circulation forced, air blast cooling.

Power transformers are usually the largest single item in a substation. For economy of

service roads, transformers are located on one side of a substation, and the connection to

switchgear is by bare conductors. Because of the large quantity of oil, it is essential to take

precaution against the spread of fire. Hence, the transformer is usually located around a sump

used to collect the excess oil.

Transformers that are located and a cell should be enclosed in a blast proof room.

Overhead Line Terminations:

Two methods are used to terminate overhead lines at a substation.

Tensioning conductors to substation structures or buildings

Tensioning conductors to ground winches.

The choice is influenced by the height of towers and the proximity to the substation.

The following clearances should be observed:

Table no.6

VOLTAGE LEVEL MINIMUM GROUND CLEARANCE

less than 66kV 6.1m

66kV - 110kV 6.4m

110kV - 165kV 6.7m

greater than 165kV 7.0m

27

CHAPTER-6

CONCLUSION

This project gives detail study on the operation of different equipment present in

220/132/33KV Achampeta substation at Kakinada. The potential transformer is used for

stepping down the voltage from 220 to 132KV or 220 to 33KV and protection.

The differential protection used for transformers is based on the principle of current

circulation. This type of protection is mostly used for transformers as this responds not only to

inter turn fault but also provides protection against phase-to-phase faults.

Buchholz Relay function is based on very simple mechanical phenomenon. It is

mechanically actuated. More severe types of faults, such as short circuit between phases or to

earth and faults in the tap changing equipment, are accompanied by a surge of oil which strikes

the baffle plate and causes the mercury switch of the lower element to close.

Over fluxing can be prevented by the use of an Over fluxing relay. An over fluxing is

an adaptation of an overvoltage relay. The PT voltage is connected across a resistor and a

capacitor in series. The voltage sensing relay is connected across the capacitor. The relay

operates in the event of an over fluxing and isolates the transformer.

Various types storage batteries used for emergency lightening and to supply power for

protecting circuits during fault conditions. Also about various earth measures to provide low

resistance path for heavy fault currents to flow through the ground.

The function of an earthing system is to provide an earthing system connection to

which transformer neutrals or earthing impedances may be connected in order to pass the

maximum fault current. The earthing system also ensures that no thermal or mechanical damage

occurs on the equipment within the substation, thereby resulting in safety to operation and

maintenance personnel.

28

CHAPTER-7

BIBLIOGRAPHY

1. POWER SYSTEMS 10TH edition by J.B.GUPTHA,S.K.KATARIA & SONS

PUBLICATIONS.

2. ELECTRICAL MACHINES 7TH edition BY P.S.BHIMBRA, KHANNA PUBLICATIONS.

3. ELECTRICAL TECHNILOGY 22ND edition by B.L.THERAJA & A.K.THERJA,

S.CHAND PUBLICATIONS.

4. ELECTRICAL POWER SYSTEMS 2ND edition by P.S.R.MURTHY, BS PUBLICATIONS

5. POWER SYSTEM ANALYSIS AND DESIGN6TH edition by

DR.B.R.GUPTHA, S.CHAND PUBLICATIONS.

29

ACKNOWLEDGEMENT

We would like to express our unbounded gratefulness to Mr.K.V.Ramana,

Asst.Divisional Engineer, A.P.TRANSCO 220/132/33KV SUBSTATION for his valuable

guidance throughout the project.

We are very thankful to our project guide Mr.D.Surya Prakash, M.Tech for his valuable

guidance throughout the project.

We express deep sense of guidance and heartfelt thanks to our head of the department

Mr.J.Pavan, M.Tech, HOD, Electrical & Electronics Engg, for his encouragement, cheerful

motivation and impartial suggestions at each stage of endeavor.

We would like to express our sincere thanks to the principal Dr.CH.Srinivasa Rao Ph.D

for extending college facilities for successful completion of mini project.

We thank and express our gratitude to all the faculty members and other non teaching

staff for their support directly and indirectly that we could make this project in to a successful

one.

With sincere regards

M.V.V.S.Gangadhar 09P31A0229

K.Rajesh Kumar 09P31A0221

M.Phanindra V Varma 09P31A0231

M.D.Madhavi 09P31A0230

. D.MangaTayaru 09P31A0215