final doc

ANDHRA LOYOLA INSTITUTE OF ENGINEERING AND

TECHNOLOGY

VIJAYAWADA, KRISHNA DISTRICT

MINI PROJECT REPORT

ON

“TRANSFORMER AND ITS PROTECTION”

Submitted in accordance with the curriculum requirements for third year

second semester of degree course in

BACHELOR OF TECHNOLOGY

In the branch of

ELECTRICAL AND ELECTRONIC ENGINEERING

Of

JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY, KAKINADA

YEAR-2011

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ANDHRA LOYOLA INSTITUTE OF ENGINEERING AND

TECHNOLOGY

VIJAYAWADA, KRISHNA DISTRICT

CERTIFICATE

This is to certify that this mini project entitled “TRANSFORMER AND ITS

PROTECTION” has been completed by R.RATNA SAGAR (08HP1A0241),

K.MANJEET (08HP1A0235) in partial fulfillment of the award the degree in

BACHELOR OF TECHNOLOGY in ELECRICAL AND ELECTRONIC

ENGINEERING of JAWAHARLAL NEHRU TECHNOLOGICAL

UNIVERSITY, KAKINADA during the academic year 2011-2012.

Project Guide Head of the Department

Principal External Examiner

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ACKNOWLEGEMENT

We would like to express our sincere thanks to LANCO, KONDAPALLI for providing us with an opportunity to undergo mini project at your esteemed organization. I thank few people in this regard

Mr.S.Sundaramoorthy- Executive director (project)Mr.K.Hari Krishna Rao - General Manager (O&M)Mr.K.Tirumala Rao-AGM (Maintenance), granting us permission.

We would like to express our sincere gratitude to our project guide Mr.K.Baskar Rao , Deputy Manager for his valuable guidance. We were privileged to experience a sustained enthusiastic and involved interest from his side. We would also like to thank Mr. Ravindra for his conscious effort simplifies the concepts and facilities better understanding of the subject.

We would like to extend our gratitude to our Associative Professor Mrs. Anantha Lakshmi H.O.D, Department of Electrical and Electronics for his consistent encouragement and effort.

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CONTENTS

1. ABSTRACT 5

2. ABOUT LANCO 6

3. OVER VIEW OF LANCO POWER PLANT 7-23

3.1 INTRODUCTION

3.2 COMBINE CYCLE POWER PLANT

3.3 HEAT RECOVERY STEAM GENERATOR

3.4 BLACK START SYSTEM

3.5 PROCESS LAYOUT AT LANCO

3.6 GENERAL PRICIPLES OF DESIGN CONCEPTS

4. INTRODUCTION TO PROTECTIVE SYSTEMS 24-25

5. TRANSFORMERS 26-36

6. POWER SYSTEM PROTECTION 37-39

6.1 NECESSITY OF PROTECTIVE SYSTEM

6.2 BASIC REQUIREMENTS OF PROTECTIVE SYSTEM

6.3 IMPORTANCE OF PROTECTIVE RELAYING

7. TRANSFORMERS USED IN

LANCO POWER 41-48

7.1 GENERATOR TRANSFORMER

7.2 AUXILARY TRANSFORMER

7.3 STATION TRANSFORMER

7.4 TRANSFORMER OIL PRESERVATION SYSTEM

8. POWER TRANSFORMER PROTECTION 49-85

8.1 CLASSIFICATION OF TRANSFORMERS

8.2 TRANSFORMER FAULTS

8.3 PROTECTION BY FUSE

8.4 PRIMARY BACK-UP PROTECTION

8.5 DIFFERENTIAL PROTECTION

8.6 OVER CURRENT PROTECTION

8.7 RESTRICTED OVER CURRENT AND EARTH

FAULT PROTECTION

8.8 COMBINED EARTH FAULT AND PHASE FAULT

PROTECTION

8.9 RESTRICTED EARTH FAULT PROTECTION

8.10 OVERLOAD PROTECTION

8.11 THERMAL OVER HEATING PROTECTION OF

LARGE TRANSFORMERS

8.12 HOT-SPOT THERMOMETER

8.13 LEAKAGE TO FRAME PROTECTION

8.14 OVERFLUXING PROTECTION

8.15 MECHANICAL PROTECTION

8.16 DEVICES USED FOR PROTECTION

9. MODERN TRENDS IN

TRANSFORMER PROTECTION 86-88

10. CONCLISION 89

11. BIBLIOGRAPHY 90

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1. ABSTRACT:

Protective systems have been undergoing improvements/modifications keep

in step with the requirements of larger & larger generating stations and complexity

of interactions.

Protective systems are the heart of any power system. They play a very

important role in controlling and protecting various equipment in power system.

Therefore for reliable operation of any plant, protective systems are very

important. Keeping in phase with the development of advanced electronics, the

shape and size of protective systems are also getting major changes. Static &

microprocessor based relays came into existence which precisely control & protect

the system from spurious faults.

Therefore in our project we studied various protective schemes that are

employed for Transformers in “LANCO KONDAPALLI POWER PLANT”.

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2. ABOUT LANCO:

The LANCO odyssey began more than two decades ago in civil engineering and

core sector. The challenges and opportunities in a resurgent India following

economic liberalization saw LANCO engineer and consolidate itself a single apex

entity, LANCO infratech ltd.

LANCO infratech ltd is one of India’s top business conglomerates and among the

fastest growing .LANCO infratech has subsidiaries and divisions across synergistic

span of verticals. These include construction, power, EPC, infrastructure, property

development and wind energy. LANCO projects, operational and underway, are

spread across India.

A member of the UN global compact, LANCO infratech is recognized for its good

corporate governance and corporate social responsibility initiatives led by the

LANCO infratech builds on a tradition and culture where trust comes first……and

the credo is inspiring growth. It has won many awards from different organizations

in different fields.

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3. OVERVIEW OF LANCO POWER PLANT:

3.1. Introduction:

The plant is located at IDA (industrial development area) kondapalli Vijayawada,

which works on combined cycle power plant (CCPP) with total capacity of 734

MW. The plant consists of two gas turbines and two heat recovery steam

generators (HRSG) and one steam turbine.

The gas turbine uses fuel as natural gas or naphtha and stating fuel as high speed

diesel (HSD) and steam turbine uses fuel as water .they have an long term

agreement with GAIL(gas authority of India limited) to supply natural gas. The gas

is supplied from tatipaka near amalapuram through 200km pipeline. Naphtha is

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alternate fuel; it is used in the shortage of natural gas. It is stored in the naphtha

storage tanks.

For starting the gas turbine air is drawn from the atmosphere into the compressor

(with the help of cranking motor) and the compressed air and fuel is brought to

combustion chamber where it is ignited which produces enough energy to rotate

the gas turbine, which generates the power of 15KV. The exhaust gas from the

turbine consists enough temperature, which is used in HRSG to produce steam,

which is used for the running the steam turbine producing power.

The power that is generated is to be transmitted to Andhra Pradesh state grid. The

generated voltage 15KV is stepped up to 220KV by generator transformer. The

plant consists of 3-generator transformers, 2-station transformers and 2-unit

auxiliary transformers with their productive equipment.

The combined cycle power plant has a better efficiency of 45% to 55%compared to

other power plants, because of higher heat rate. It has five feeders namely

kondapalli-1, kondapalli-2, chilakulu-1, chilakulu-2, gudivada. It has three system

busses namely main bus-1, main bus-2 and transfer bus. It maintains standards set

by APTRANSCO.

In the Power Sector, gas turbine drive generators are used.

Gas turbines range in size from less than 100 KW up to about 140.000 KW. The

gas turbine has found increasing application due to the following potential

advantages over competive equipment.

• Small size and weight per horsepower

• Rapid loading capability

• Self-contained packaged unit

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• Moderate first cost

• No cooling water required

• Easy maintenance

• High reliability

• Waste heat available for combined cycle application.

• Low Gestation Period

• Low Pollution Hazards

The function of a gas turbine in a combined cycle power plant is to drive a

generator which produces electricity and to provide input heat for the steam cycle.

Power for driving the compressor is also derived from gas turbine.

3.2. Combined Cycle

Combined Cycle power plant integrates two power conversion cycles namely.

Brayton Cycle (Gas Turbines) and Rankine Cycle (Conventional steam power

plant) with the principal objective of increasing overall plant efficiency.

3.2.1. Brayton Cycle

Gas Turbine plant-operate on Brayton Cycle in which air is compressed this

compressed air is heated in the combustor by burning fuel combustion produced is

allowed to expand In the Turbine and the turbine is coupled with the generator.

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FIG-1.GAS TURBINE

Without losses the theoretical cycle process is represented by 1’ 2’ 3’ 4’

In the actual process losses do occur. Deviation from the theoretical process, results

from the fact that compression and expansion are not performed

Isentropically but polytropically which is conditioned by heat dissipation

(expansion) and heat supply (Compression) caused by various flow and fraction by

losses.

In the combined cycle mode, the Brayton Cycle is chosen as the topping cycle due

to the high temperature of the exhaust of the gas turbine (point 4 in the P.V

diagram). In modern gas turbines the temperature of the exhaust gas is in the

Range of 500 to 550 0C.

Reference to the T.S. diagram may indicate the amount of heat that is produced,

converted into mechanical energy and extracted from this process. For the

evaluation of the cyclic process, two parameters are of greatest importance;

1) Thermal efficiency 2) Process working capacity

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Thermal efficiency is obtained from chemical binding energy of the fuel and

mechanical energy available at the shaft of the gas turbine.

Thermal efficiency ( th ) as follows:

th = Energy at GT shaft

Chemical Energy of fuel

= (Q Input. - Q output)/ Q Input

= 1 - (Q Output/ Q Input)

Working capacity is also obtained from the difference between the amounts of heat

supplied and removed. This is achieved by increasing P2 that is increasing gas

inlet temperature T3.

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FIG-2 BRAYTON CYCLE

3.2.2.Rankine Cycle

The conversion of heat energy to mechanical energy with the aid of steam is carried

out through this cycle. In its simplest form the cycle works as follows (fig.2).

The initial state of the working fluid is water (point-3) which, at a certain

temperature is compressed by a pump (process 3-4) and fed to the boiler. In the

boiler the compressed water is heated at

Constant pressure (process 4-5-6-1). Modern steam power plants have steam

temperature in the range of 500 0C to 550 0C at the inlet of the turbine.

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FIG-3 RAKINE CYCLE

3.2.3. Combining two Cycles to Improve Efficiency

We have seen in the above two cycles that gas turbine exhaust is at a

Temperature of 500–550 0C and in Rankine Cycle heat is required to generate steam

at the temperature of 500-550 0C. So, why not use the gas-turbine exhaust to

generate steam in the Rankine cycle and save the fuel required to heat the water?

Combined Cycle does just the same.

The efficiency of Gas Turbine cycle alone is 30% and the efficiency of Rankine

Cycle is 35%. The overall efficiency of combined cycle comes to 48%.

3.2.4. Types of Combined Cycles

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It is basically of two types, namely Unfired Combined cycle and Fully Fired

combined cycle.

1. Unfired combined Cycle

The basic system is shown in figure- 3. In this system the exhaust gas is used only

for raising steam to be fed to the steam turbine for power generation.

The conventional fossil fuel fired boiler of the steam power plant is replaced with a

‘Heat Recovery Steam Generator’ (HRSG). Exhaust gas from the gas turbine is led

to the HRSG where heat of exhaust gas is utilized to produce steam at desired

parameters as required by the steam turbine.

However, non-reheat steam turbine is the preferred choice for adopting this type of

system as usually the live steam temperature for HRSG will be solely controlled by

the gas turbine exhaust temperature which is usually around 500 0C.

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Fig-4 UNFIRED COMBINED CYCLE

In recent development, with the introduction of Dual Pressure Cycles more heat is

recovered in the HRSG and steam with higher pressure and temperature can be

generated. But higher capital investment and sometimes necessity of supplemental

firing system makes the system complex and costly.

2. Fully Fired Combined Cycle

Fig – 4 shows the basic schematic of this cycle. In this system the heat of

Exhaust gas from gas turbine is used for two purposes as described below:

Heat contained in exhaust gas is used to heat feed water to a desire

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Fig-5 BOILER REPOWERING SYSTEM EXHAUST HEAT EXCHANGER

Temperature at the inlet to the boiler. This leads to the reduction or elimination of

the extraction steam requirement from the steam turbine. In case, the steam turbine

has a larger steam swallowing capacity to generate more power the amount of

steam which is being extracted from steam turbine for regenerative feed heating

could be made to expand in the turbine to increase its base load capacity and

improve the overall efficiency. In case the steam turbine does not have the capacity

to swallow extra steam available due to cutting down of extraction, the fuel being

fired in the boiler can be cut down to generate less steam by an amount equivalent

to steam required for extractions and thus improving the overall efficiency due to

less consumption of fuel.

Gas turbine exhaust contains about 14 to 16 % oxygen (by weight) and can be used

as hot secondary air in the conventional fossil fired furnaces. So the heat required to

heat the secondary air will be saved and can be used for other purposes. FD fan

power consumption will also be reduced to a great extent.

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3.3. HEAT RECOVERY STEAM GENERATOR:

A heat recovery steam generator or HRSG is an energy recovery heat

exchanger that recovers heat from a hot gas stream. It produces steam that can

be used in a process or used to drive a steam turbine.

A common application for an HRSG is in a combined-cycle power station,

where hot exhaust from a gas turbine is fed to an HRSG to generate steam

which in turn drives a steam turbine. This combination produces electricity

more efficiently than either the gas turbine or steam turbine alone. Another

application for an HRSG is in diesel engine combined cycle power plants,

where hot exhaust from a diesel engine, as primary source of energy, is fed to

an HRSG to generate steam which in turn drives a steam turbine. The HRSG is

also an important component in cogeneration plants. Cogeneration plants

typically have a higher overall efficiency in comparison to a combined cycle

plant. This is due to the loss of energy associated with the steam turbine.

HRSGs consist of four major components: the Evaporator, Superheater,

Economizer and Water preheater. The different components are put together to

meet the operating requirements of the unit.

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FIG-6 HEAT RECOVERY STEAM GENERATOR

Modular HRSGs can be categorized by a number of ways such as direction of

exhaust gases flow or number of pressure levels. Based on the flow of exhaust

gases, HRSGs are categorized into vertical and horizontal types. In horizontal type

HRSGs, exhaust gas flows horizontally over vertical tubes whereas in vertical type

HRSGs exhaust gas flow vertically over horizontal tubes. Based on pressure levels,

HRSGs can be categorized into single pressure and multi pressure. Single pressure

HRSGs have only one steam drum and steam is generated at single pressure level

whereas multi pressure HRSGs employ two (double pressure) or three (triple

pressure) steam drums. As such triple pressure HRSGs consist of three sections: an

LP (low pressure) section, a reheat/IP (intermediate pressure) section, and an HP

(high pressure) section. Each section has a steam drum and an evaporator section

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where water is converted to steam. This steam then passes through super heaters to

raise the temperature and pressure past the saturation point.

3.4. PROCESS AT LANCO KONDAPALLI POWER PLANT:

This is a (stage-1) 368.144 MW combined cycle power plant situated at IDA

(Industrial development area) kondapalli. The plant includes 2-gas turbine

generators, 2-heat recovery steam generators each supplying steam to single

condensing steam turbine generator. The main fuel used is natural gas and the

starting fuel is HSD (high speed diesel). The output power of the generators is at

15KV.the two gas turbines are fed with gas as fuel, whose flue gases at the output

are used to produce steam in heat recovery steam generator (HRSG), which drives

the steam turbine. The power is produce by 3 units of 120MW each.

The gas turbines are capable of being operated on simple cycle by exhausting into

the atmosphere, without the utilization of its exhaust gas for steam generation in

HRSG and the consequent use of the same in steam turbine generator for power

generation. This uncertainty arises during the initial commissioning the plant when

the gas turbine alone is started up and stabilized attending to the heating problems

as well as the last stage. For any reason the HRSG is not available for service.

During availability the HRSG are capable of being positively isolated by means of

a “GUILLOTINE DAMPER”. a DIVERTER DAMPER is provided in the exhaust

of gas turbine for directing/controlling the exhaust either to the atmosphere or into

the HRSG direct to the surface condenser in envisaged for matching the required

parameter of steam for steam turbine with that or steam generated in the

HRSG ,during the start up of the steam turbine also by pass system maintaining the

HRSG in the service in case of a trip out of the steam turbine facilities the

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matching of the steam parameters, in case a steam turbine generator has to take

again into service.

3.5. START UP POWER:

The function of the starting system is to crank the gas turbine up to the required

speed until: it becomes self sustaining.

One method of starting large gas turbine is by using a motor driven hydraulic

starting system. Alternatively, the GTG can be started by using a frequency

converter to rotate the generator which drives the turbine for starting.

Typical hydraulic starting systems for each gas turbine consist of the following:

• Starting motor, electric AC induction motor

• Hydraulic torque converter

• Auxiliary Gear

• Couplings

The electric starting motor drives the hydraulic torque generator through a coupling.

The hydraulic torque converter consists of an impeller, which forces the fluid

against hydraulic starting motor. The hydraulic torque converter is coupled to the

accessory gear, which is connected to the gas turbine shaft. The torque converter

receives hydraulic fluid from hydraulic and lube oil reservoir during

Operation. When gas turbine reaches self-sustaining speed the starting device is

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disconnected and shut down. To break the inertia of the starting motor and reduce

the starting current a pony motor is provided. Gas turbines of GE and WH designs

are provided with starting motor system for cranking purpose.

3.5.1. Black Start System

To start a gas turbine in the event of AC-power failure an emergency black start

system is provided. It also helps in safe coasting down of the gas turbine and its

auxiliaries following a ‘trip’ in the event of grid collapse. The black start system

consists of a separate diesel engine or a gas turbine driven synchronous generator

connected to station switch gear bus. It can be operated manually from local or

remote and also it automatically comes into operation following a black out

condition. Capacity of the black start unit should be such that it can supply the total

auxiliary power required to start a gas turbine from standstill condition.

The LANCO KONDAPALLI POWER PLANT gas turbine is provided for

emergency black-start purpose and all other projects are provided with diesel

generator set for the same duty. In LANCO the start up power of gas turbine

generator is catered by 6.6KV station switchgear by starting gas turbine generator

cranking motor (HT) and also feeding gas turbine generator (LT) auxiliaries by

connecting the tie between 6.6KV unit and station switch gears. The tie breaker

can be closed only on dead bus closing that is with both incomer and bus couplers

open. Upon successful starting and synchronizing of generator to grid, the 6.6KV

unit switch gear incomer breakers can be closed under synchronization

immediately automatic tripping of station per unit tie breakers will take place

through trip selection switch.

3.6. GENERAL PRINCIPLES OF DESIGN CONCEPT:-

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The basic concept of the electrical system as a

whole is based on the requirements for the safe and reliable performance of the gas

turbine and steam turbine generator sets and the interconnected electrical system

with provision for easy maintenance and overhauling.

3.6.1. AUXILIARY POWER SYSTEM:-

Auxiliaries of the gas turbine generator and steam turbine

generator range from large capacity motors to small fractional horsepower motors.

Larger motors range from 160KV is fed from 6.6KV buses and smaller motors

from 415KV buses.

3.6.2. SYSTEM /NEUTRAL GROUNDING:-

Generator neutral grounding:-

Generator neutral is grounded through neutral grounding

transformer with resistor to limit earth fault current to 10Amps.

220KV system grounding:-

As per ANSI/IEEE standard 142-1982, system with

voltages above 15KV is to be effectively grounded. Hence, the 220kv systems

have solidly earthen.

6.6KV system grounding:-

Resistance grounding is used at medium voltages

primarily due to the following advantages.

1. Electric shocks hazards to personnel due to stray ground fault currents in

the ground return path is returned.

2. Transient over voltages can be limited.

3. Mechanical stresses in circuits and apparatus carrying fault current is

reduced.

4. Burning and melting effects in faulted electric equipment are reduced. In

view of the above and also advantage of immediate and selective tripping

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of grounded circuit, low resistance grounding is envisaged for limiting

the theoretical ground fault current to 500Amps.

415KV system grounding:-

415V system is widely distributed and 3-phase, 4-wire

system is required to meet the requirement of power to control, indication,

annunciation, etc.

According to Indian electricity rule no.61,the neutral

conductor of a 3-phase , 4- wire low voltage and medium voltage system shall be

earthed and there shall not be inserted in the connection with earth any impedance

cut-out or circuit breaker. IEEE standard no.142 also recommends low voltage

system (600V and below) to be operated solidly grounded.

220KV bus bar systems:-

For the 220KV outdoor switchyard, double main and transfer

bus is provided. Bus coupler breaker is used for connecting main bus-1 and main

bus-2. The bus transfer breaker is provided between main and transfer bus. 220kv

switchyard is having the provision for connecting three numbers generator

transformers, two number station transformers and five number APSEB grid

feeders.

6.6.6KV systems:-

The 6.6KV unit switch gear of unit bus as well as 6.6KV station

switch gear of station bus is sectionalized into two sections each section being fed

from individual 15/16.9 KV transformer (in case of station bus) , 2 * 100% rated .

The unit bus is feeding unit auxiliaries of one steam – generating unit. The station

bus is used to supply power for the start-up as well as station auxiliaries.

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415V system:-

The 415V units as well as station switchboards are

sectionalized into two buses each being fed from 6.6KV/433V transformer, 2 *

100% rated. The 415V switch boards are carrying the 415 V plant loads.

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4. INTRODUCTION TO PROTECTIVE SYSTEM:

Protective relaying is an integral part of any electrical power system. The

fundamental objective of system protection is to quickly isolate a problem so

that the unaffected portions of the system can continue to function. The flip side

of this objective is that the protection system should not interrupt power for

acceptable operating conditions, including tolerable transients.

The choice of protection depends upon several aspects such as type, rating of

the protected equipment, its important location, probable abnormal conditions,

costs etc.

A fault in electrical equipment is defined as a defect in its electrical circuit

due to which the flow of current is diverted the intended.

Faults can be minimized by improving system design, improving quality of

component, better and adequate protective relaying, better operation and

maintenance; however the fault can’t be entirely eliminated.

The protective relay senses the abnormal condition in a part of power system

and given an alarm or isolate that part from the healthy system.

When abnormal conditions occur three basic objectives must always be met:

All endangered equipment must be protected from damage

The faulted components must be isolated and if not damaged, reenergized as

rapidly as possible.

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Service interruption must be minimized.

A power transformer constitutes an important and expensive component in a

power system. It is, therefore essential to provide an efficient protective relay

scheme to protect the transformer from any severe damage which might likely

to be caused by short-circuited faults within the equipment itself or any

sustained overload or fault conditions in the power systems

.

Protective relaying is necessary for every power transformer. The choice of

protection depends upon several aspects such as type, rating of transformer, its

location, its importance, probable abnormal conditions, cost etc. There are

several transformers of various ratings. Each needs certain adequate protection.

The protective relaying senses the abnormal conditions give an alarm or

isolate that part from the healthy system. The relaying are compact, self

contained devices which respond to abnormal condition. The relay

distinguishes the normal and abnormal conditions. When an abnormal

condition occurs relay closes its contacts there by trip circuit breaker opens and

faulty part is disconnected from the supply. The entire process is automatic and

fast.

Circuit breakers are switching devices which can interrupt normal and

abnormal currents. Besides relays and circuit breaker there are several other

important components in the protective relaying scheme. These include

protective current transformer, voltage transformers, protective relays, time

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delay relays, Auxiliary relays, trip circuits, secondary circuits, auxiliary and

accessories etc.

5. TRANSFORMERS

5.1INTRODUCTION:

A transformer is a static piece of equipment used either for raising or lowering

The voltage of an a.c. supply with a corresponding decrease or increase in

current. It essentially consists of two windings, the primary and secondary,

wound on a common laminated magnetic core as shown in Fig. (7.1). The

winding connected to the a.c. source is called primary winding (or primary) and

The one connected to load is called secondary winding (or secondary). The

Alternating voltage V1 whose magnitude is to be changed is applied to the

primary. Depending upon the number of turns of the primary (N1) and secondary

(N2), an alternating e.m.f. E2 is induced in the secondary. This induced e.m.f. E2

in the secondary causes a secondary current I2. Consequently, terminal voltage

V2 will appear across the load. If V2 > V1, it is called a step up-transformer. On

The other hand, if V2 < V1, it is called a step-down transformer.

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FIG-7 TRANSFORMER

5.2. Working

When an alternating voltage V1 is applied to the primary, an alternating flux is

set up in the core. This alternating flux links both the windings and induces

e.m.f.s E1 and E2 in them according to Faraday’s laws of electromagnetic

Induction. The e.m.f. E1 is termed as primary e.m.f. and e.m.f. E2 is termed as

secondary e.m.f.

E1=-N1 * dø/dt

E2=-N2*dø/dt

Therefore, E2 = N2

E1 N1

Note that magnitudes of E2 and E1 depend upon the number of turns on the

Secondary and primary respectively. If N2 > N1, then E2 > E1 (or V2 > V1) and

We get a step-up transformer. On the other hand, if N2 < N1, then E2 < E1 (or V2

< V1) and we get a step-down transformer. If load is connected across the

secondary winding, the secondary e.m.f. E2 will cause a current I2 to flow

through the load. Thus, a transformer enables us to transfer a.c. power from one

circuit to another with a change in voltage level.

5.3. Construction of a Transformer

We usually design a power transformer so that it approaches the characteristics

Of an ideal transformer. To achieve this, following design features are

Incorporated:

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(i) The core is made of silicon steel which has low hysteresis loss and high

Permeability. Further, core is laminated in order to reduce eddy current

loss. These features considerably reduce the iron losses and the no-load

current.

(ii) Instead of placing primary on one limb and secondary on the other, it is a

usual practice to wind one-half of each winding on one limb. This

ensures tight coupling between the two windings. Consequently, leakage

flux is considerably reduced.

(iii) The winding resistances R1 and R2 are minimized to reduce I2R loss and

resulting rise in temperature and to ensure high efficiency.

5.4.Types of Transformers

Depending upon the manner in which the primary and secondary are wound on

The core, transformers are of two types’ viz., (i) core-type transformer and (ii)

Shell-type transformer.

(i) Core-type transformer. In a core-type transformer, half of the primary

winding and half of the secondary winding are placed round each limb as

shown in Fig. (8). This reduces the leakage flux. It is a usual practice to

place the low-voltage winding below the high-voltage winding for

mechanical considerations.

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FIG-8 CORE TYPE TRANSFORMERS

(ii) Shell-type transformer. This method of construction involves the use of a

double magnetic circuit. Both the windings are placed round the central

limb (See Fig. 9), the other two limbs acting simply as a low-reluctance

flux path.

FIG-9. SHELL-TYPE TRANSFORMERS

The choice of type (whether core or shell) will not greatly affect the efficiency

of the transformer. The core type is generally more suitable for high voltage and

small output while the shell-type is generally more suitable for low voltage and

high output.

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5.5.Cooling of Transformers

In all electrical machines, the losses produce heat and means must be provided

to keep the temperature low. In generators and motors, the rotating unit serves as

a fan causing air to circulate and carry away the heat. However, a transformer

has no rotating parts. Therefore, some other methods of cooling must be used.

Heat is produced in a transformer by the iron losses in the core and I2R loss in

the windings. To prevent undue temperature rise, this heat is removed by

cooling.

(i) In small transformers (below 50 kVA), natural air cooling is employed

i.e., the heat produced is carried away by the surrounding air.

(ii) Medium size power or distribution transformers are generally cooled by

housing them in tanks filled with oil. The oil serves a double purpose,

carrying the heat from the windings to the surface of the tank and

insulating the primary from the secondary.

(iii) For large transformers, external radiators are added to increase the

cooling surface of the oil filled tank. The oil circulates around the

transformer and moves through the radiators where the heat is released to

surrounding air. Sometimes cooling fans blow air over the radiators to

accelerate the cooling process.

5.6.Three-phase transformer

A three-phase transformer can be constructed by having three primary and three

secondary windings on a common magnetic circuit. The basic principle of a 3-

phase transformer is illustrated in Fig. (10). The three single-phase coretype

transformers, each with windings (primary and secondary) on only one leg

have their unwound legs combined to provide a path for the returning flux. The

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primaries as well as secondaries may be connected in star or delta. If the primary

is energized from a 3-phase supply, the central limb (i.e., unwound limb) carries

the fluxes produced by the 3-phase primary windings. Since the phasor sum of

three primary currents at any instant is zero, the sum of three fluxes passing

through the central limb must be zero. Hence no flux exists in the central limb

and it may, therefore, be eliminated. This modification gives a three leg coretype

3-phase transformer. In this case, any two legs will act as a return path for

the flux in the third leg. For example, if flux is Ø in one leg at some instant, then

Flux is Ø/2 in the opposite direction through the other two legs at the same

Instant. All the connections of a 3-phase transformer are made inside the case

and for delta-connected winding three leads are brought out while for star

connected Winding four leads are brought out.

FIG. (10).THREE PHASE TRANSFORMERS

For the same capacity, a 3-phase transformer weighs less, occupies less space

and costs about 20% less than a bank of three single-phase transformers.

Because of these advantages, 3-phase transformers are in common use,

Especially for large power transformations.

A disadvantage of the three-phase transformer lies in the fact that when one

Phase becomes defective, the entire three-phase unit must be removed from

service. When one transformer in a bank of three single-phase transformers

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becomes defective, it may be removed from service and the other two

Transformers may be reconnected to supply service on an emergency basis until

Repairs can be made.

5.6.Three-Phase Transformer Connections

A three-phase transformer can be built by suitably connecting a bank of three

Single-phase transformers or by one three-phase transformer. The primary or

secondary windings may be connected in either star (Y) or delta (Δ)

arrangement. The four most common connections are (i) Y-Y (ii) Δ-Δ (iii) Y-Δ

and (iv) Δ-Y. These four connections are shown in Fig. (). In this figure, the

windings at the left are the primaries and those at the right are the secondaries.

The primary and secondary voltages and currents are also shown. The primary

line voltage is V and the primary line current is I. The phase transformation ratio

K is given by;

K =Primary phase voltage = N1

Secondary phase voltage N2

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FIG-11.THREE PHASE CONNECTIONS

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(i) Y-Y Connection. In the Y-Y connection shown in Fig. (11), 57.7%

(Or 1/ 3) of the line voltage is impressed upon each winding but full line

175

Current flows in each winding. Power circuits supplied from a Y-Y bank

Often create serious disturbances in communication circuits in their

Immediate vicinity. Because of this and other disadvantages, the Y-Y

connection is seldom used.

(ii) Δ-Connection. The Δ-Δ connection shown in Fig. (11) is often used

for moderate voltages. An advantage of this connection is that if one

Transformer gets damaged or is removed from service, the remaining two

Can be operated in what is known as the open-delta or V-V connection. By

Being operated in this way, the bank still delivers three-phase currents and

Voltages in their correct phase relationships but the capacity of the bank is

Reduced to 57.7% of what it was with all three transformers in service.

(iii) Y-Δ Connection. The Y-Δ connection shown in Fig. (11) is suitable

For stepping down a high voltage. In this case, the primaries are designed

For 57.7% of the high-tension line voltages.

(iv) Δ-Y Connection. The Δ-Y connection shown in Fig. (11) is

Commonly used for stepping up to a high voltage.

5.7.Applications of Transformers

There are four principal applications of transformers viz.

(i) Power transformers (ii) distribution transformers

(iii) Autotransformers (iv) instrument transformers

(i) Power Transformers. They are designed to operate with an almost

Constant load which is equal to their rating. The maximum efficiency is

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Designed to be at full-load. This means that full-load winding copper losses

Must be equal to the core losses.

(ii) Distribution Transformers. These transformers have variable load which

Is usually considerably less than the full-load rating. Therefore, these are

Designed to have their maximum efficiency at between 1/2 and 3/4 of full load.

(iii) Autotransformers. An autotransformer has only one winding and is used

In cases where the ratio of transformation (K), either step-up or step down,

Differs little from 1. For the same output and voltage ratio, an

Autotransformer requires less copper than an ordinary 2-winding

Transformer. Autotransformers are used for starting induction motors

(Reducing applied voltage during starting) and in boosters for raising the

Voltage of feeders.

(iv) Instrument transformers. Current and voltage transformers are used to

Extend the range of a.c. instruments.

(a) Current transformer

A current transformer is a device that is used to measure high alternating current

In a conductor. Fig. (12) illustrates the principle of a current transformer. The

Conductor carrying large current passes through a circular laminated iron core.

The conductor constitutes a one-turn primary winding. The secondary winding

Consists of a large number of turns of much fine wire wrapped around the core

As shown. Due to transformer action, the secondary current is transformed to a

Low value which can be measured by ordinary meters.

Secondary current, Is= Ip * Np/Ns

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FIG-12.CURRENT TRANSFORMER

For example, suppose that IP = 100 A in Fig. (7.55) and the ammeter is capable

of measuring a maximum of 1 A. Then,

Ns=Np * Ip/Is = 1* 100/1 = 100

(b) Voltage transformer

It is a device that is used to measure high alternating voltage. It is essentially a

step-down transformer having small number of secondary turns as shown in Fig.

(7.56). The high alternating voltage to be measured is connected directly across

the primary. The low voltage winding (secondary winding) is connected to the

voltmeter. The power rating of a potential transformer is small (seldom exceeds

300 W) since voltmeter is the only load on the transformer.

Vs = Vp * Np/Ns

FIG-13.VOLTAGE TRANSFORMER

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6.POWER SYSTEM PROTECTION:

6.1. Necessity of protection system:

Modern power systems are growing fast with more generators,

transformers and large networks in the systems. For system operation a high degree

of reliability is required. In order to protect the system from damage due to undue

currents and abnormal voltages caused by the faults the need for reliable protective

devices, such as relays and circuit breakers arises. The most common electrical

hazard against which protection is required is the short circuit. however , there are

many other abnormal conditions- e.g. overloads , under voltage and over voltage,

open phase , unbalanced phase currents, reversal of power , under frequency ,over

frequency , over temperature , power swings, instability for which protection is

desired.

As a rule, on the occurrence of short circuits which may lead to

heavy disturbances in normal operation (damage to equipment, impermissible drop

in voltage etc...), the protective scheme is designed to disconnect or isolate the

faulty part from the system without any delay. The protective scheme is designed

to energies and alarm or signal whenever the overloads and short circuits do not

present a direct danger to the faulted circuit element and the entire installation.

The main function to protective relaying are to detect the presence of faults, their

locations and initiate the action for quick removal from service of any element of a

power system when it suffers a short circuit or when it starts to operate in any

element of a power system of any abnormal manner that might cause damage or

otherwise interfere with effective with effective operation of the rest of the system.

The relaying equipment is added in this task by the circuit breakers that are capable

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of disconnecting the faulty element when they are called upon to do so by the

relaying equipment.

Circuit breakers are generally located so that each generator, transformer, bus,

transmission line etc.can be completely disconnected from the rest of the system.

The circuit breakers must have sufficient capacity so that they can carry

momentarily the maximum short circuit current that can flow through them and

then interrupt it.

6.2. Basic requirements of protective system:

Protective system is an extremely important part of the power system as

it provided to operate under abnormal conditions to prevent failure or isolate

trouble and limit its effect. Every protection system which isolates a faulty element

is required to satisfy basic quantities. The quantities of protective relaying are

named as

1. Reliability

2. Selectivity

3. Sensitivity

4. Speed of operation

5. Discrimination

6. Power consumption

7. Adequateness

8. Time of operation

9. Stability

10.System security

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The qualities should be carefully considered while selecting protection schemes

power system protection. Cost is also equally important. After better protective

system costs more.

The operating current should not be kept too small for following reasons:

The protection should not operate on maximum loads.

The protection should not operate under fault conditions, or faults somewhere else

in the system.

6.3. Importance of protective relaying:

Protective relaying is necessary with almost every electrical

plant, and no part of the power system is left unprotected. The choice of protection

depends up on several aspects such as type and rating of the protective equipment,

its importance, location, probable abnormal conditions, cost etc... Between

generators and final load points, there are several electrical equipment and

machines of various ratings. Each needs adequate protection. The protective

relaying senses the abnormal conditions in a part of the power system and gives an

alarm or isolated that part from the healthy system. The relay should distinguish

between normal and abnormal condition.

Protective relaying technology has evolved from single

function electromechanical units to static units and now in to digital arena. The

development of low cost microprocessor technology has made possible the

multifunction digital relay where many relaying functions can be combined into a

single unit. In the past, the engineer applied many individual units. The failure of

the single protective unit was not a major concern since there were other discrete

relays in the protective system that would provide a back up to the unit that is out

of service. If a multifunctional relay was out of the service, the consequences can

be more servers since many protective functions may be incorporated in the unit.

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With proper planning, the multifunction digital relays can provide a better overall

protection system. When proper planning, the multifunction digital relays, the cost

of each protective function needs to be justified verses the added protection the

relays provide. In many cases, typically on less critical generators, only minimal

protection was applied in order to reduce costs. Today, with digital relays, these

comprises do not need to be made. This allows the protection engineer to design a

complete protection system with less concern about costs.

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7. TRANSFORMERS USED IN LANCO POWER PLANT

7.1. Generator transformer:-

The generators are connected to the 220KV

switchyard through 15KV/220KV step-up generator transformers. Generator

transformers are designed to deliver the total output of the generating unit into the

system covering the entire operating range of the generator capability and have the

following salient technical features.

Type : oil filled, outdoor type

Voltage ratio : 220KV/15KV

Frequency : 50Hz

Vector group : YNd1

Percentage impedance : 15%

Capacity : 98/128/160MVA

Type of cooling : ONAN/ONAF/OFAF

The generator transformers are provided with ± 10% ON-LOAD tap changer on

HV winding suitable for unidirectional power flow.

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7.2. Unit auxiliary transformers:-

Unit transformers are sized to cater to unit auxiliaries for full load

operation of the both gas turbine generating unit and unit auxiliaries of one steam

generating unit. The loads are calculated on the basis of running load with

sufficient margin. while arriving at the rating of transformer and impedance value ,

due considerations are given to short circuit level at the unit HT bus and also the

voltage drop at the unit buses when the largest motor (BFP-1500KW)connected to

the unit bus is started with other operating loads of unit bus in running condition.

The unit transformer is having the following salient features.

Type : oil filled, outdoor type

Voltage ratio : 15KV/6.9KV

Frequency : 50Hz

Vector group : Dyn11

Percentage impedance : 10%

Capacity : 12/15MVA

Type of cooling : ONAN/ONAF

The unit transformer is provided with ± 10% ON-LOAD tap changer on HV

winding suitable for unidirectional power flow.

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7.3. Station transformer:-

The transformers are used to supply power for the start-up as well

as station auxiliaries. The size of the station transformers is calculated on the basis

of running loads with good margin.

Type : oil filled, outdoor type

Voltage ratio : 220KV/6.9KV

Frequency : 50Hz

Vector group : Ynyn0

Percentage impedance : 10%

Capacity : 10/12MVA

Type of cooling : ONAN/ONAF

The station transformer is provided with ± 10% ON-LOAD tap changer on HV

winding.

A hollow insulating with ceramic support column through which passes the

insulating tie rod and which is attached to the one-minute power frequency.

1. Withstand voltage : 20KV (rms)

2. 1.2/50 µs , impulse withstand voltage: 60KV (peak)

3. Remote auto operation of fans:

Page | 44

For remote auto operations of fans (fan1 to fan8) fans (on RTCC panel) kept

on auto mode. Coil of contractor will get energies and it is normally open

contact closes.

Operations of pumps:

For checking the healthiness of individual pump test

mode can be adopted. Based on requirement if the internal temperature high

we can run the pumps.

1. Local manual operation of pumps: all the 4-pumps switch is kept on

local manual mode and pump switch are kept on in service mode. Bus

wired out in the circuit as pump contractor get energized and pump-1

to pump-4 starts running.

2. Local auto operation of pumps : for automatic operation of the pump

group ‘A’,’B’,’C’,’D’ pump switch is kept on local auto mode and

pump switches are kept on in service mode . When the temperature of

windings attains preset value, normally open contact of will close and

another contractor get energized.

3. Remote auto operation of pumps: - for remote auto operation of pumps

(pump-1 to pump-4) pump switch (on RTCC panel) kept on auto

mode. Coil of contractor will get energized and its number contact

closes. Pump group –‘A’,’B’,’C’ and ’D’ start running as described

above.

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7.4. Transformer oil preservation system:

Transformer oil deterioration takes place due to moisture. Moisture can appear in a

transformer from three sources, via. By leakage past gasket, by absorption from air

in contact with the surface,

Or by its formation with the transformer as a product of deterioration as insulation

ages at high temperature. The effect of moisture in oil is to reduce the electric

strength, especially if loose fibers or duct particles are present.

Page | 46

Method available to reduce oil contamination from moisture are silica gel

breather , thermosyphon filter, sealed conservator tanks using gas cushion , rubber

diaphragm or air-cell seals refrigerated dryers.

7.4.1. Thermo-syphon filters:

It is well recognized that molecular sieve has got an extreme

affinity to absorb moisture and acidity. Therefore in order to maintain the

transformer oil in moisture free condition in certain cases the transformer is

provided with thermo-syphon filter.

The constructional details of such a filter are shown in fig-15. Normally the filter is

mounted directly on the transformer tank. The filter, generally of cylindrical shape,

has a number of perforated steel trays filled with absorbent materials generally

employed are silica gel and active alumina.

FIG-15 THERMO-SYPHON FILTER

Page | 47

During the operation of a transformer, oil keeps circulating through this filter by

thermo-syphon action. The oil during circulation comes in contact with the

molecular sieve and the moisture from the oil is absorbed by molecular sieve.

The molecular sieve is inactive to hot transformer oil and hence it does not affect

the property of the transformer oil in any way.

Thermo-syphon filter is providing with isolating valve at either end to facilitate its

removal for maintenance. An oil sump is also provided for the collection of

sediments. To drain out the oil from the filter a drain plug is provided.

A periodical check of oil will reveal the acidity and water content level, if these

tend to show a rising trend the filter should be reactivated.

7.4.2. Silica gel breathers:

A silica gel breather is most commonly employed as a means of

preventing moisture ingress. It is connected to the conservator tank, which is fitted

to transformer to allow for the changes in volume due to temperature variations. As

the load reduces, air is drawn into the conservator through a cartridge packed with

silica gel desiccant, which effectively dries the air. Freshly regenerated gel is very

efficient, it will dry the air down to a dew point of below -40Zc, but quickly falls in

efficiency. A well maintained silica gel breather will generally operate with a dew

point of -35Zc, as long as a large enough quantity of gel has been used for the

cycling duty.fig shows such a breather.

Page | 48

FIG-16 SILICAGEL BREATHER

Silica gel is reactivated by heating in a shallow pan at a temperature of 150Z to 200Zc

for two to three hours when the crystal should have regained their blue tint.

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8. POWER TRANSFORMER PROTECTION

8.1 CLASSIFICATION OF TRANSFORMERS

Classification of power transformer for purpose of protective gear

application would be to take into account.

The voltage class

The M.V.A rating

Type of connections and number of windings.

Method of grounding the Y-connected neutrals.

The function it has to perform.

The schematic layout adopted

8.2 TRANSFORMER FAULTS

Any numbers of conditions have been the reason for an electrical transformer

failure. Statistics show that

Winding failures most frequently cause transformer faults (ANSI=IEEE, 1985).

Insulation deterioration,

Often the result of moisture, overheating, vibration, voltage surges, and mechanical

stress created during Transformer through faults is the major reason for winding

failure.Voltage regulating load tap changers, when supplied, rank as the second

most likely cause of a transformer Fault. Tap changer failures can be caused by a

Page | 50

malfunction of the mechanical switching mechanism, High resistance load

contacts, insulation tracking, overheating, or contamination of the insulating oil.

Transformer bushings are the third most likely cause of failure. General aging,

contamination, Cracking , internal moisture, and loss of oil can all cause a bushing

to fail. Two other possible reasons Are vandalism and animals that externally flash

over the bushing. Transformer core problems have been attributed to core

insulation failure, an open ground strap, or Shorted laminations. Other

miscellaneous failures have been caused by current transformers, oil leakage due to

inadequate tank welds, oil contamination from metal particles, overloads, and

overvoltage.

POWER TRANSFORMER PROTECTION

Differential Protection

HV Restricted Earth Fault Protection

HV Back Up Over Current Protection

HV Back Up Earth Fault Protection

Over Fluxing Protection

Mechanical Protections

Buchholz Protection

Oil/Winding Temperature HI-HI Protection

Pressure Relief Device Actuator

Page | 51

8.3PROTECTION BY FUSE:

Power transformer up to a limited capacity rating and voltage level can be

protected by means of high rupturing capacity fuses provided on the primary side.

While this method is simple and cheapest since no other costly switch gear

equipment is needed, it has many draw backs.

A fuse cannot detect the low current transformer earth faults. Besides, the

fuse is incapable of distinguishing faults currents from the transients magnetizing

in rush currents and normal load currents. A fuse would operate whether the fault

is in the transformer zone or outside the transformer zone. It is also not possible to

accomplish simultaneous interruption of all three phases in the event of a fault in

any one of the phases. In view of these limitations, the fuse protection of

transformers has a limited application and is generally employed where some

relaxation could be made in the degree of supply continuity and the amount of

unbalanced loading.

PROTECTIVE RELAYS:

Functions of Protective Relays:

To sound an alarm or close the trip circuit breaker so as to disconnect a

transformer during abnormal conditions such as over-load, under voltage,

temperature rise, unbalanced load, reverse power, under frequency, short-circuit,

etc.

To disconnect the abnormally operating transformer so as to prevent the

subsequent faults. E.g., over-load protection protects the transformer and prevents

insulation failure.

Page | 52

To localize the effect of fault by disconnecting the fault part from the

healthy part, causing least disturbance to the healthy system.

PROTECTIVE ZONE:

A part of the system protected by a certain protective schemes is called

protective zone or zone of protection. The entire power-system is covered by a

several protective zones and no part of the system is left unprotected.

The boundary of a protective zone is determined by the location of current

transformer. Hence the current transformer is located such that the circuit breaker

is covered in the protective zones. The zone can be precisely identified in unit

systems. Unit system is one in which the protection responds to faults in the

protected zone alone, and it does not responds to faults beyond the protected zone.

Each zone has certain protective scheme each protection do not have exact zone

boundary.

8.4 PRIMARY BACK-UP PROTECTION:

Primary protection (Main Protection) is the essential protection provided for

protecting an equivalent machine.

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ABNORMAL CONDITIONS AND STRESSES:

Power transformers are used in high voltage systems for transferring large

loads. They are subjected to voltage stresses, current stresses, thermal stresses and

electromagnetic stresses during their operation.

Voltage stresses are caused by normal voltage, power frequency over

voltage, impulse over voltages. They effect the internal and external insulation.

Current stresses are caused by normal current and short circuit currents

flowing through the transformer windings. The current stresses result in:

Temperature rise

Electromagnetic forces.

Environmental effects are caused by alternative variation in the ambient

temperature, atmospheric dust and pollution.

Mechanical stress: during short circuit winding and bushings are subjected to

dynamic forces. Hence transformers are to be protected from all the above

abnormalities

8.5 Transformer Differential Protection:-

The Purpose of this protection is to detect the phase to phase faults inside the

transformer. The differential protection responds to the vector difference between

two similar quantities .In protection of transformer C.T.’s are connected at each

end of the transformer.

Page | 54

The CT.’s secondary’s are connected in star or delta and the pilot wires are

connected between C.T’s of each end. The C.T connections and C.T ratios are such

that the current fed into the pilot wires from both the ends are equal during normal

conditions and it varies during fault conditions. During the internal faults such as

phase to phase or phase to ground the balance is disturbed. The out of balance

current I1-I2 flows through the relay operating coils. To avoid unwanted operation

restraining bias coils are provided in series with pilot wires. The ampere turns

provide by the bias coil or restraining coil is proportional to average of I1 and I2.

FIG 17. DIFFERENTIAL PROTECTION OF TRANSFORMER

DIFFICULTIES IN DIFFERENTIAL PROTECTION:

The differential protection may operate wrongly due to the following causes

even when there is no internal fault in the transformers.

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The difference in pilot wire lengths:

The current transformer and machine to be protected are located at

different sites and normally it is not possible to connect relay coil to equi potential

points. The difficulty is over come by connecting adjustable resistors in series with

the pilot wires .These are adjusted on size to obtain the equi potential points.

C.T. Ratio Errors During short circuits:

The current transformers may have almost equal ratio at normal currents.

But during short circuit conditions the primary current are unduly large .the ratio

errors of C.T.’.S on either side differs during these conditions.

Saturation of C.T. magnetic circuit during short-circuits conditions:

Due to these causes the relay may operate even for external faults. The

relay may lose its stability through these faults.

Magnetizing currents in rush in Transformer while switching in Tap Change:

When the transformer is connected to supply a large current in rush

takes place (6 to 10 times the full load current). This certainly causes operational of

differential relay though there is no fault in the transformer .To avoid this difficulty

harmonic restraint is provided for the differential relay.

TAP CHANGING:

The tap changing causes change in the transformation of the transformer.

There by the C.T.Ratio do not match with the new tap setting resulting current in

pilot wires even during healthy conditions. This aspect is taken care of by biased

differential relay.

Page | 56

C.T.CONNECTIONS:

The percentage differential relay for 3 phase transformer has 3 operating

coils and 3 restarting coils. These are connected to pilot wires on the secondary of

the current transformer. The connections are such that, for each phase, the

differential current (I1-12) flows through the operating coil.

In both the cases three current transformers are required at each side of the

protected transformer. The connections of the C.T.’s secondary are such that

during normal conditions and for external faults, no current should flow through

the relay operating coil. The differential protection provides the instantaneous

protection (less than 1.0 seconds and no internal time delay) within the protective

zone.

Phase to phase faults.

Phase to ground faults.

It does not detect the faults and a high speed over relay is required for this purpose.

8.6 OVER CURRENT PROTECTION:

Differential protection is uneconomical for power transformer below

5MVA.In such cases over current protection is employed as main protection

against phase faults. For transformers above 5MVA over current protection is used

in addition to differential protection because the latest cannot respond to through

faults and if this through faults persists for longer duration it creates stresses in the

transformer. For small distribution transformers over current protection is provided

by means of fuse on HV side.

Page | 57

HARMONIC RESTRAINT:

In this method the predominant harmonic currents present in the inrush

current are filtered out from the operating coil circuit by means of tuning and

utilize for applying a blocking feature to the differential current relays at the time

of transformer energization.Its limitations are the danger of relay failure during

internal faults when harmonic components and dc of sets could also be generated

due to CT saturation arcing at the point of fault.

8.7RESTRICTED OVER CURRENT AND EARTH FAULT PROTECTION:

Over current and earth fault protection is provided as main protection for

medium transformers where differential protection is not provided.

Differential protection is generally uneconomical for the power a

transformer below 1 M.V.A. in such case over current protection is employed as

main protection against phase faults. For the transformer above 1 M.V.A., if

differential protection is used as main protection over current protection is used in

addition as backup for sustained through faults. Earth fault protection is provided

in addition to phase fault protection.

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FIG 18. COMBINED RESTRICTED & EARTH FAULT PROTECTION

8.8 COMBINED EARTH FAULT AND PHASE FAULT PROTECTION:

It is convenient to incorporate phase fault and earth fault relay in a combined

phase fault and earth fault protection. The increase in current of phases causes

corresponding increase in respective secondary currents. The secondary current

flows through respective relay unit. Very often only two phase relays are provided

instead of three because in case phase faults current in any at least 2 phases must

increase. Hence, two relay units are enough. The earth fault relay is residually

connected.

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8.9 RESTRICTED EARTH FAULT PROTECTION:

Earth fault relays connected in residual circuit of line C.T’s give protection

against earth faults on the delta or unearthed star connected winding of

transformer. Earth faults on secondary side are not reflected on the primary side,

when primary winding is delta connected or has unearthed star point. In such cases

an earth fault relay connected in residual circuit of 3 C.T’.s on primary side

operates on internal earth faults on secondary side do not produce zero sequence

currents on primary side.

Restricted earth fault protection may be use high speed tripping for faults on

star connected earthed secondary winding of power transformer protection.

When fault occurs very near to the neutral point of the transformer the

voltage available for driving earth fault current is small. Hence faults current

would be low. If the relay is to sense such faults, it has too sensitive and would

therefore, operative spurious signals, external faults and switching surges. Hence

the practice is to set the relay such that it operates for earth fault current of the

order of the 15% of the rated winding current. Such setting protects restricted

portion of the winding.

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FIG 19: RESTRICTED EARTH FAULT PROTECTION SCHEME

8.10 OVER LOAD PROTECTION:

The permissible over load and their duration depends upon the type of

cooling and insulation class of transformer. Higher over loads are permissible for a

shorter duration.

Permissible duration of over Load

Over load % : 125 150 175 200 300

Duration : 125 45 15 10 1

(In minutes)

Hence for substation transformers over load protection is generally arranged

to initiate alarm in unattended stations, over load protection is arranged to trip the

breaker after request time delay.

The transformer with utility equipment is prone to sudden over loads.

(Furnace transformer, Motor transformer). The over load protection for such

transformer is also given the requisite time delay. While selecting the over current

protections of transformer, the following aspects need consideration.

Magnetizing current inrush: inverse relays are not affected by the current

inrush as they have enough time lags. Instantaneous over current relays should

high set to avoid mal operation. The fault current on primary side and secondary

side of power transformer are different for phase-phase faults. Lower value should

be selected for setting over current relays. Primary full load current should be

considered while setting the over current relay.

Page | 61

The setting of inverse over current relay is generally 125% of transformer

rating to take care of normal over loads. Enough time delay should be provided as

for the application.

The setting of instantaneous over current relay on primary side should be

more than asymmetrical value of fault current for 3 phase fault on secondary side

of transformer. This setting is generally adequate to take care of magnetizing

current inrush.

Same set of current transformers should not be used for differential

protection and over current protection.

8.11 THERMAL-OVER HEATING PROTECTION OF LARGE

TRANSORMERS:

Thermo couples or resistor temperature detectors are kept near each

winding. These are connected to a bridge circuit. When temperature increases

above safe value, an alarm is sounded. If measures are not taken, the circuit

breaker is tripped after a certain temperature. Some typical settings for oil

temperatures are as follows

Switch of fans : 60oc

Alarm: 95oc

Trip : 120oc

Oil temperature indicated by a thermometer. An oil thermometer, which is

similar for all oil filled transformer, can be consider as a partially effective

protective device when equipped with alarm contacts connected to give remote

Page | 62

warning of abnormally high temperature. Its location is such that it naturally

monitors the hottest fluid that exists in the transformers. The same thermometer is

often used to start fan motors on transformers equipped with automatic air blast to

increase the name plate KVA rating.

Alarm contacts used in conjunction with an oil thermometer are adjustable

but are typically set in a sequence that brings fans at liquid temperature of 600c and

actuate a switch contact should the temperature reach 90oc. For a typical design at

300c ambient, the fans are brought in to operation at about 90 percent rated load

where as the alarm is given at about 130% rated load. These percentages will vary

for each design and are dependent upon the actual ambient above 30oc and higher

at ambient under 30oc.Switches are usually capable of readjustment through a

range of 10oc.

8.12 HOT SPOT THERMOMETER (Winding Temperature Device):

The thermometer bulb is located in a pocket near the winding. The bulb is

also heated by a small heater connected across CT secondary. There by the heat

given to the bulb is a function of load current as well as the temperature of oil near

winding. The device is matched with heating curve of the transformer winding.

The reading of hot spot thermometer is related to actual thermal condition of

transformer than that of oil temperature indicator

8.13 LEAKAGGE –TO-FRAME PROTECTION:

Leakage-to –frame protection is a very simple system suitable for

small power transformers .It consists of a current transformer slipped over the

earthen-connection of the power transformer casing with single-pole over current

relay connected directly across the secondary through a setting –resistor. The

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power –transformer is mounted on concrete are similar base so that it is lightly

insulated from earth .an earth fault in either winding of the power transformer

causes current to flow through the main earthling connection ,thus energizing the

current transformer and operating the relay

FIG 20 FRAME LEAKAGE PROTECTION SCHEME

8.14 OVER-FLUXING PROTECTION (High magnetic flux protection):

Increase in power frequency voltage causes increase in working magnetic

flux; thereby increase the iron loss and magnetizing current. The core and core

bolts get heated and the lamination insulation is effected. Over fluxing protection is

provided for generator-transformers and feeder transformers where it is a

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possibility of over-fluxing due to sustained over voltage. Over-fluxing causes over

heating of core and insulation failure.

The resistance and capacitance are connected to secondary of VT. The

voltage drop across the resistance is a function of V/F, where V is the line to earth

voltage and F is the frequency. This voltage is fed to the volts ‘per hertz’ relay.

The magnetic flux in the transformer core is a function of V/F; hence the

relay senses magnetic flux condition. Over fluxing relay is provided with enough

time lags.

The flux density (B) in the transformer core is proportional to the volts/HZ

of supply voltage, i.e. B is proportional to V/F.

8.15 MECHANICALPROTECTION:-

The mechanical protection of a transformer is to say that which protects the

transformer from moisture contact, to protect against incipient faults such as hot

spot generation and cooling system failure etc….,

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8.16 DEVICES USED FOR PROTECTION:-

8.16.1 Buchholz Relay:-

INTRODUCTION:-

Buchholz relay is a gas actuated relay used for the protection of the transformer

against almost all types of internal faults. It is employed for protection against

incipient faults such as hot spot generation, cooling system failure etc. Minor faults

are indicated through alarm signal provided. Major faults are detected by Buchholz

a highly reliable type of equipment, yet, in order to ensure the continuity of service

that modern conditions demanded, protective devices are required. This device will

disconnect the supply when large scale damage is caused by a fault to the

apparatus or to other connected apparatus. In this the alarm elements will be

operated after a specified volume of gas has collected to give an alarm indication.

FIG-21 .BUCHHOLZ RELAY

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EXAMPLES:-

1. Core bolt insulation failure

2. Short circuited laminations due to burns

3. Bad contact

4. Over heating of windings

The alarm will also operate in oil leakage. Gas operated relay,

commonly known as buchholz relay, is generally used for protection of oil

immersed transformers, reactors etc.

As it is known, all types of faults occurring with oil filled transformers are

accompanied by more or less violent generation of gases which the heat liberates

from the oil. The phenomenon has been fully utilized to provide complete internal

protection of transformers. The falling oil which may eventually lead to a

dangerous situation is also detected by this relay in time.

CONSTRUCTION:-

The Relay is hydraulic device, arranged in the pipe line between the

main transformer tank and the conservator tank. It comprises a cast iron housing

which contains essentially two floats

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FIG-22.BUCHHOLZ RELAY CONNECTION TO THE TRANSFORMER

Upper and lower. Each float carries a mercury switch, from which are taken to a

box.

The necessary pet cocks for gas release, site testing, and a drain plug on

the body of the housing are provided. Inspection windows are fitted on both sides

of the relay housing to see the oil level and to see the volume of gas collected on a

calibrated scale in the centimeters.

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MERCURY SWITCHSES RATING:-

Since the role of the mercury switches is most critical in the

performance of the buchholz relay, the rating of the switches is carefully chosen.

Their current rating is 2 Amps. At 240 Volts A.C or D.C.

PRINCIPLE OF OPERATION:-

When the transformer is healthy, the entire relay housing remains filled

with oil and the buoyancy of the respective floats tilts the mercury switch to the

“open” position.

When a severe or incipient fault occurs in the transformer, small

bubbles of as will be generated and these,

Attempting to pass from the tank to the oil conservator, will be trapped in the

upper portion of the relay housing. As this gas

Accumulates, the oil in the relay depresses, causing the upper float to tilt, there by

closing the mercury switch. The alarm circuit which is usually connected to this

switch gets energized to ring an alarm bell.

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SAFETY DEVICES AND FITMENTS WITH POWER TRANSFORMERS:

The electrical protection systems can sense the abnormal condition by

measuring current/voltage. Besides electrical relays, a power transformer can be

provided with the following safety and monitoring devices.

1) Fluid level gauge

2) Vacuum gauge

3) Pressure / Vacuum switch

4) Sudden Pressure Relay

5) Pressure Relief Value.

6) Fluid Temperature Indicator.

7) Hot Spot Temperature Indicator

8) Gas Temperature Indicator

9) Combustible limit relays

10) Conservator

11) Breather.

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PROTECTION OF TRANSFORMER IN PARALLEL:

The following protections are necessary in case of transformers operating in

parallel.

1) Over current protection

2) Earth-fault protection

3) Direction over current and directional earth fault relays on secondary side to

prevent the healthy section feeding in to faulty section.

The feedback is prevented by operation of directional over current relay of faster

setting. By operation of this directional over current relay, the corresponding C.B

is quickly tripped and the feedback from the healthy section is prevented. The

current coils of O.C relay and o.c. relay on secondary side may be connected in

series.

PROTECTION OF GROUNDING TRANSFORMER:

The C.T secondary is data connected. An over current relay with time lag is

inserted in the delta. The zero sequence currents circulate in this delta. The time

settings of this relay are selected to coordinate with thermal rating of the earthing

resistor (if used) or with time setting of other fault relays. The earthing transformer

is disconnected by opening the circuit breaker, on persistent earth fault.

The other three relays provide protection against faults in the grounding

transformer. The job instantaneous relays set between 25-50% of continuous

current rating of grounding transformer. Buchholz relay is also is used. Earth fault

protection is provided by residually connected relay.

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8.16.2LOW OIL LEVEL-FLUID LEVEL GUAGE:

Low oil level is a harmful condition because internal insulation clearance,

creep ages etc. between loads, bushing and tanks are left in air when the oil drops

below the specified level. Low oil could result from 1) initial mistake to full

sufficient oil up to the 2) leakage of the oil through the tank.

If the cooling tubes are partially cooled or nearly at ambient temperature, it

is an indication that the oil is not circulating in the cooling tubes and oil level has

dropped below the desired level. The cooling tubes are warned and level indicator

gives an alarm, it may be a false alarm and level indicator needs checking. Its

position may be improper.

The level indicator has a float ended arm. The float is suspended in the oil.

When the oil level drops down, the float tilts the arm there by closing the alarm

contacts. Both low and high level alarm contacts are provided.

8.16.3THE DELAY RELAYS:

Here an intentional time delay 5 to 8 on 50 Hz basis is introduced in the

relay operating time to over side the short time inrush current. This scheme while

being simple can’t be generally recommended for the large transformers as the

time delay can result in severe damage to the transformer during internal faults.

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UNDER VOLTAGE RELAY ACROSS THE RELAY OPERATING COIL:

The under voltage relay contacts which are closed when the transformer is

de energized, open out after the transformer is reconnected to the supply. A

tripping suppressor device is connected in the circuit for the relay to operate when

there is a fault while energizing. For this the under voltage contacts are connected

in series with differential relay operating contacts.

8.16.4 TRIPPING SUPPRESSOR DEVICE:

Here the under-voltage relay contacts are connected in series with the

differential relay operating contacts and they open out if the transformer is healthy

at the time of energeization. The main limitation with the scheme is the possible

delay by the timer for the 10 current internal faults which affects the voltage, but

slightly.

8.16.5 ON LOAD TAP CHANGER (OLTC):-

By use of this type of switches taps can be changed

without de-energizing the transformer. The switches are operated by manually or

electrically. Electrical operation is possible locally or remote.

Arrangement for operating two of more OLTC in Parallel is also done through

their remote control cubicle.

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Write up on OLTC control scheme

SELECTION-A

1) OLTC & Supply details:-

1. Make of OLTC : BHEL

2. Type of OLTC : M 111 600Y

3. Motor supply : 415V, 3phase,

4wire, 50Hz, AC

4. Control supply : 55-0-55V, AC

5. Auxiliary supply for: 240V, 1phase, AC Heater and illumination

Circuit.

Operation Available:-

1. Local electrical independent

2. Remote electrical independent/automatic

3. Remote Group Simulations

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Paralleling Feature:-

The paralleling of two or more OLTC’s is by means of multi contact radial switch

(MBB concepts) is provided in drive mechanism. The ensure that transformer will

operate in parallel only when they are on same tap position number.

SELECTOR SWITCHES:-

The following selector switches are provided in drive mechanism:

1. For local-remote selection of control switch is provided in drive mechanism.

For control of OLTC from drive mechanism, put this switch on local whereas for

operation from RTCC this should be put on remote position.

2.Sequence selector switch is for master follower operation mounted. When only

one transformer independently is required to operate from remote switch to be put

on independent mode. For simultaneous parallel operation of two or more

transformer this should be put on master for unit being used as control.

3. OLTC control selector auto-manual “csl” for manual selection of OLTC,

provided in RTCC panel. For operation put this switch on auto mode, for manual

operation put this switch on auto mode.

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DESCRIPTION OF LOCAL CONTROL SCHEME

SECTION-B

1. Motor circuit:-

The motor terminals U, V, W are connected to power source R, Y, B (415volt,

3phase, 50Hz AC) via brake contactor and motor contactor limit switch, hand

interlock safety switch and motor protection switch.

2. Heater circuit:-

The heater circuit is connected to a 2410volts, 50Hz supply

through terminals. The heater is continuously switched ON, where as another

heater is switched ON via the Thermostat.

General notes:

Store the valve on its base duty packed in polythene bag to ensure that dirt or other

solid particles. Do not remove the protection cover when it is stored. Enter the

valid value port from tank side. Keep the flat gasket to duty tied to the valve.

Remove it only when it is to be installed. The gasket should be kept dry.

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8.16.6 TEMPERATURE INDICATORS:-

At every yearly maintainence inspection the level of oil in the pockets holding the

thermometer bulbs should be checked and the oil replenished, if required. The

capillary tubing should be fastened down again if it has become loose. Dial glasses

should be kept clean and if broken, replaced as soon as possible to prevent damage

to the instrument. Temperature indicators found reading incorrectly should be

filled with oil. Also check the pockets for the presence of water and if found, clean

the pocket and refill with fresh oil and seal the opening properly. If armoring of

capillary is exposed, then retaping must be done by PVC tape.

FIG-23 TEMPERATURE INDICATORS

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8.16.7 PRESSURE RELIEF DEVICE:-

Model: s6 150mm(6”) diameter Model: t6150mm(6”) diameter port

opening switch, indicating flag and rain guard, suitable for oil filled transformer

port opening switch, indicating flag and rain guard, suitable for synthetic liquid

filled transformer.

FIG-24. PRESSURE RELIEF DEVICE

Application:-

This PRV is used on power transformer and similar applications.

When pressure in the tank rises above predetermined safe limit, this valve operates

and performs functions

1. Allows to pressure drop by instantaneously

opening a port of about 150mm diameter.

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2. Gives visual indication of valve operation by raising a flag.

3. Operates a micro switch.

Specifications:-

1. Liquid in tank : Synthetic liquid Transformer

Oil as per order

2. Operating pressure: 0.42,0.49,0.5,0.7kg/cm2

Any one value as per use

3. Operating tolerance :+0.07kg/cm2 (LBSQ.INCH)

4. Operating time: Instantaneous

5. Valve resetting: Automatic

6. Switch resetting: Manual

7. Visual indicator resetting: Manual

8. Operating temperature: 0 to 10o centigrade

9. Environment : Indoor or Outdoor

10. Switch :Micro switchD.P.D.T

11. Port opening : 150, Nominal

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Tests:-

Routine tests are carried working:-

This PRV has an internal flange with six holes for counting. The valve

cannot on PRV.A test certificate is prepared and sent with consignment.

Construction of it will be mounted vertically or horizontally on the

tank. The PRV has got a port of about 150mmwhen press diameter. This port is

scaled by a stainless steel diaphragm. This ‘O’ ring and is kept pressed by two

heavy duty springs, there by keeping the port closed. The other side of the

diaphragm exposed to tank pressure. Whenever the pressure in the diaphragm tank

rises due to any reason, the same pressure acts on the diaphragm from inside.

When pressure rises above the predetermined safe limit, the diaphragm gets lifted

from its seat. The lifting is instantaneous and allows vapours, gases and liquids to

come out of the tank. The diaphragm regains its position as soon as pressure in the

tank drops below set limit. The lift of diaphragm is utilized to operate flag and

micro switch with the help of the rod, the flag and the switch remain operated until

they are manually.

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Check for valve:-

The PRV is generally packed in a corrugated box. The method of packing may

vary. The PRV should be checked as follows, before it is installed.

If PRV is found operating at pressure out of tolerance limit, it will have to be

returned to us for resetting, changing of bursting pressure cannot be done outside

our factory.

While conducting such tests at your care should be taken that small

particles do not have passage through the port opening. Such particles are likely to

get trapped between diaphragm and gaskets which will affect functioning of PRV.

It should also be noted that inaccurate pressure gauge may lead to wrong

conclusions.

Check for installations:-

The following checks should be observed before installation of PRV on the

transformer.

1) Check the orientation of pad is properly done before

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installation.

2) Check that the proper bolts, M 12*30 with thick

Washers are taken for the installation. Bolts other

than this size should not be used.

3) We have supplied a gasket with PRV. Any other

Gasket supplied with the valve can be used with due

Precaution.

4) Provide cable gland of one bus conduct with suitable

bus for cable to be use

5) After all above checks, the PRV is ready for the Installation.

Installation:-

Each valve should be cleaned from inside with compressed air jet. All particles

should be removed from inside tank. While cleaning, care is taken that the jet does

not attack the switch and the flag mechanism. The PRV should be installed

considering the following aspects.

1) The indicating flag is easily seen from distance. While installing remove the

dirt and clean the mounting surface.

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2) The rubber gasket supplied with the valve is accessible essentially for

manual resetting and for routine manual checking.

3) The rubber gasket supplied with the valve should be properly cleaned &

should be located in the recess carefully. Tighten M 12 bolts evenly with

care.

4) Remove rain guard. Remove the cover of switch box (15). Connect control

circuits to terminal plate provided in the switch box through cable gland.

5) Check the operation of the flag and by switch by lifting operation rod as

done during checking. After checking of flag and control circuit, replace

cover of switch and rain guard.

6) The PRV can be put to service.

Faults and Remedies:-

The valve has rugged construction and it is not likely to get damaged easily.

However, the indicating flag is a delicate item. Hence it is likely to get damaged.

The indicating flag must be replaced.

Major parts have the damage for the diaphragm, base, and valve cover.

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Abnormal condition Protection Remarks

Incipient fault below oil level resulting

in decomposition of oil, faults between

phases & earth.

Buchholtz relay sounds alarm (gas

actuated relay)

Buchholtz relay used for transformers of

rating 500 KVA and above.

Large internal faults:

Phase to phase

Phase to ground,

Below oil level

Buchholtz relay

Trips the circuit

Breaker

Buchholtz relay slow

And less sensitive.

Buchholtz relay for tap change also

Fault in tap changer

1)percentage differential

protection

2) High speed high set over

current relay.

Percentage differential protection used

for transformers of rating above 5 MVA

Saturation of Magnetic circuit 1)over fluxing protection

2)over voltage protection

For important generator transformer

with bus bar protection

Earth faults 1) differential protection

2) Earth fault relay.

For transformers of and above 5 MVA

a)instantaneous restricted E.F.Relay

b) Time lag E.F.relay

Through faults (external faults)

1) grade time lag over

current relay

2) 2) HRC fuses

Protection of distribution transformers

Small distribution transformers up to

500 KVA

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Abnormal condition Protection Remarks

Over loads 1) thermal over load relays

2) Temperature relay

Generally temperature indicators are

provided on the transformers

Temperature increase is indicated on

control board also. Fans started at

certain temperature

High voltage surges due to lighting,

switching

1) Horn Gaps

Lightning

arresters

Not favored for important transformers.

In addition to LA for incoming lines.

Protection of Generator

Transformer Together

Generator- Transformer over all differential Protection

Generator protection Generator differential protection

Stator earth fault protection

Negative phase sequence Protection Against unbalanced loading

Interturn fault

Reverse power protection

Field Failure Protection.

Rotor earth fault protection

Temperature sensors in slots

Over current relays in stator and rotor circuits

Lightening arresters generator over voltage protection

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Protection of Generator

Transformer Together

Generator- Transformer over all differential Protection

Protection of Unit Auxiliary

Transformer

Differential protection

Restricted earth Fault protection

Buchooltz Relay

Over current protection

Winding and oil temperature sensors

Over flux protection

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9. MODERN TRENDS IN TRANSFORMER PROTECTION

Micro-Processor Based Relays:

The increased growth of power systems both in size and complexity has

brought about the need for the fast and reliable relays to protect major equipment

and to maintain system stabilility. The electromagnetic relays has several

drawbacks such as high burden on the instrument transformers, high operating

time, contact problems etc. though successfully used the static relays suffer from a

no. of disadvantages such as inflexibility, inadaptability to changing the system

conditions and complexity.

The concept of digital protection employing computers which show much

promise in providing improved performance has evolved during past two decades.

Digital computers can easily fulfill the protection requirements of modern power

system without difficulties. But their cost is 15 to 20 times more than that of

conventional protective relaying schemes. The cost of protective scheme should be

about of 1% of the cost of the equipment to be protected.

The main features which have encouraged the design and development of

micro processor based relays there are economic compactness, reliability,

flexibility and improved performance over conventional relays. A no. of desired

relaying characteristics such as over current, directional, impedance, mho

quadrilateral, elliptical etc can be obtained using the same interface. Digital relays

are user-friendly.

The primary protection for the A.C generator shall be an integrated digital

protection system including protection functions such control monitoring,

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diagnostic and communication capabilities. A high degree of dependability and

security shall be provided by extensive self diagnostic routines and an optical

redundant D.C supply. The protection function shall operate over the range of 31-

79 Hz with same accuracy at normal system frequency.

Digital system Electro Mechanical System Comments

100 % Stator Protection

Complete over Excitation

Protection

Unbalanced Armature

currents Protection

Loss of excitation (two zones)

Reverse Power

IAV

Two set point over

excitation protection

INC77

Generally one zone of

protection

Lack of sensitivity for some

applications

IAV protects 90-95% choice of 27TN

and 64G in digital system for 100%

stator ground protection.

Digital system better coordinates with

transformer and generator capability

curves.

More sensitive protection for negative

sequence current condition.

Possibility of false trip with one zone

protection during power swing.

Digital system offers better sensitivity.

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Advantages of Micro Processor Relay:

Ability to combine a large no of protective and monitoring functions in

single relay unit.

Measured values of variables are processed digitally by micro processor.

High level of flexibility.

Increased reliability due to self checking.

User friendly yet higher capable.

High speed.

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10. CONCLUSION

Electrical system of the LANCO kondapalli power plant is well equipped for

minimizing the risks of the plant equipments, personnel & also the reliability of

the system for the maximizing the plant reliability.

Electrical system of the LANCO kondapalli power plant has well

generation, transmission, distribution, and utilization are the four main stages

related to the electrical power system high voltage generation which is limited by

size and insulation requirements. The electrical power generated at low levels is

stepped up before transmission. Thus connection of incoming generations and

switch up operations are done in switch yard.

Excellent proven protection is given to the generator, transformer

equipments. So as to limit the damage to faulted equipment, minimum possibility

of fire or explosion, minimum hazard to personnel. 6.6Kv and 415Vmedium and

low voltage distribution networks are providing power to plant auxiliaries like

lighting fire fighting system, compressors, and emergency fuel pumps.

The above study gives the brief idea of the protective schemes of the transformers.

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BIBLOGRAPHY:

1. Transformers by bhel sanka sen. Bhel

2. Transformers manual

3. www.googlesearch.com

4. www.wikipedia.com

5. Lanco project manual

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