HARCOURT BUTLER TECHNOLOGICAL INSTITUTE

KANPUR

TO WHOM IT MAY CONCERN

I hereby certify that Shailesh Khandelwal Roll No 0904520022 and Raj Singh Chauhan Roll

no 0904520018 of Harcourt Butler Technological Institute, Kanpur undergone six weeks

industrial training from 12th June, 2012 to 21st July 2012 at our organization to fulfill the

requirements for the award of degree of B.Tech Electrical Engineering. They worked on

Power Plant Overview project during the training under the supervision of Mr. G. D. Sharma.

During their tenure with us we found them sincere and hard working.

We wish them a great success in the future.

Signature of the Student

Shailesh Khandelwal

Raj Singh Chauhan

ii

CONTENTS

1. Acknowledgement

2. Introduction to the Company

a. About the Company

b. Vision

c. Strategies

d. Evolution

3. Introduction to the Project

4. Project Report

a. Operation

i. Introduction

ii. Steam Boiler

iii. Steam Turbine

iv. Turbine Generator

b. EMD – I

i. Motors

ii. Switchgear

iii. High Tension Switchgear

iv. Direct On Line Starter

c. EMD – II

i. Generator

ii. Transformer

5. References

iii

ACKNOWLEDGEMENT

With profound respect and gratitude, we take the opportunity to convey our thanks to

complete the training here. We express gratitude to the Program Manager and other faculty

members of Electrical Engineering Department of Harcourt Butler Technological Institute,

Kanpur for providing this opportunity to undergo industrial training at National Thermal

Power Corporation, Badarpur, New Delhi. We do extend my heart felt thanks to Ms. Rachna

Singh Bhal for providing us this opportunity to be a part of this esteemed organization. We are

extremely grateful to Mr. G.D.Sharma, Superintendent of In-Plant Training at BTPS-NTPC,

Badarpur for his guidance during whole training. We are extremely grateful to all the

technical staff of BTPS-NTPC for their co-operation and guidance that helped me a lot during the

course of training. We have learnt a lot working under them and I will always be indebted of them for this

value addition in us.

Finally, we are indebted to all whosoever have contributed in this report work and friendly

stay at Badarpur Thermal Power Station, Badarpur, New Delhi.

1

INTRODUCTION TO THE COMPANY

• About the Company

• Vision

• Strategies

• Evolution

National Thermal Power Corporation Limited

Badarpur Thermal Power Station

Badarpur, New Delhi

ABOUT THE COMPANY

NTPC, the largest power Company in India, was setup in 1975 to accelerate power

development in the country. It is among the world’s largest and most efficient power

generation companies. In Forbes list of World’s 2000 Largest Companies for the year 2007,

NTPC occupies 411th place.

A View of Badarpur Thermal Power Station, New Delhi

NTPC has installed capacity of 29,394 MW. It has 15 coal based power stations (23,395

MW), 7gas based power stations (3,955 MW) and 4 power stations in Joint Ventures (1,794

MW). The company has power generating facilities in all major regions of the country. It

plans to be a 75,000 MW Company by 2017.

NTPC has gone beyond the thermal power generation. It has diversified into hydro power,

coalmining, power equipment manufacturing, oil &gas exploration, power trading &

distribution. NTPC is now in the entire power value chain and is poised to become an

Integrated Power Major. NTPC's share on 31 Mar 2008 in the total installed capacity of the

country was 19.1% and it contributed 28.50% of the total power generation of the country

during 2007-08. NTPC has set new benchmarks for the power industry both in the area of

power plant construction and operations. With its experience and expertise in the power

sector, NTPC is extending consultancy services to various organizations in the power

business. It provides consultancy in the area of power plant constructions and power

generation to companies in India and abroad. In November 2004, NTPC came out with its

Initial Public Offering (IPO) consisting of 5.25% as fresh issue and 5.25% as offer for sale by

Government of India. NTPC thus became a listed company with Government holding 89.5%

of the equity share capital and rest held by Institutional Investors and Public. The issue was a

resounding success. NTPC is among the largest five companies in India in terms of market

capitalization.

Recognizing its excellent performance and vast potential, Government of the India has

identified NTPC as one of the jewels of Public Sector 'Navratnas'- a potential global giant.

Inspired by its glorious past and vibrant present, NTPC is well on its way to realize its vision

of being "A worldclass integrated power major, powering India's growth, with increasing

global presence"

VISION

A world class integrated power major, powering India's growth with increasing global

presence.

Mission

Develop and provide reliable power related products and services at competitive prices,

integrating multiple energy resources with innovative & Eco-friendly technologies and

contribution to the society

View of a well flourished power plant

Core Values ---- BCOMIT

• Business ethics

• Customer Focus

• Organizational & Professional Pride

• Mutual Respect & Trust

• Innovation & Speed

• Total Quality for Excellence

STRATEGIES

Technological Initiatives

Introduction of steam generators (boilers) of the size of 800 MW

Integrated Gasification Combined Cycle (IGCC) Technology

Launch of Energy Technology Center -A new initiative for development of

technologies with focus on fundamental R&D

The company sets aside up to 0.5% of the profits for R&D

Roadmap developed for adopting ‘Clean Development

Mechanism’ to help get / earn ‘Certified Emission Reduction

Corporate Social Responsibility

As a responsible corporate citizen NTPC has taken up number of CSR initiatives

NTPC Foundation formed to address Social issues at national level

NTPC has framed Corporate Social Responsibility Guidelines committing up to 0.5%

of net profit annually for Community Welfare Measures on perennial basis

The welfare of project affected persons and the local population around NTPC

projects are taken care of through well drawn Rehabilitation and Resettlement policies

The company has also taken up distributed generation for remote rural areas

Environment Management

All stations of NTPC are ISO 14001 certified

Various groups to care of environmental issues

The Environment Management Group

Ash Utilization Division

A forestation Group

Centre for Power Efficiency & Environment Protection

Group on Clean Development Mechanism

NTPC is the second largest owner of trees in the country after the Forest department.

Partnering government in various initiatives

Consultant role to modernize and improvise several plants across the country

Disseminate technologies to other players in the sector

Consultant role “Partnership in Excellence” Programme for improvement of PLF of

15Power Stations of SEBs.

Rural Electrification work under Rajiv Gandhi Grameen Vidyutikaran Yojana.

EVOLUTION

1975 NTPC was set up in 1975 with 100% ownership by the Government of India.

In the last 30 years, NTPC has grown into the largest power utility in India.

1997 In 1997, Government of India granted NTPC status of “Navratna’ being one of

the nine jewels of India, enhancing the powers to the Board of Directors.

NTPC became a listed company with majority Government ownership of

2004 89.5%. NTPC becomes third largest by Market Capitalization of listed

companies

The Company rechristened as NTPC Limited in line with its

2005 changing business portfolio and transforms itself from a thermal power utility

to an integrated power utility.

2008 National Thermal Power Corporation is the largest power generation company

in India. Forbes Global 2000 for 2008 ranked it 411th in the world.

TPC is the largest power utility in India, accounting for about 20% of India’s installed

capacity

INTRODUCTION TO THEMAL POWER PLANT

• Introduction

• Classification

• Functioning

INTRODUCTION

Power Station (also referred to as generating station or power plant) is an industrial facility

for the generation of electric power. Power plant is also used to refer to the engine in ships,

aircraft and other large vehicles. Some prefer to use the term energy center because it more

accurately describes what the plants do, which is the conversion of other forms of energy,

like chemical energy, gravitational potential energy or heat energy into electrical energy.

However, power plant is the most common term in the U.S., while elsewhere power station

and power plant are both widely used, power station prevailing in many Commonwealth

countries and especially in the United Kingdom.

A coal-fired Thermal Power Plant

At the center of nearly all power stations is a generator, a rotating machine that converts

mechanical energy into electrical energy by creating relative motion between a magnetic field

and a conductor. The energy source harnessed to turn the generator varies widely. It depends

chiefly on what fuels are easily available and the types of technology that the power company

has access to.

In thermal power stations, mechanical power is produced by a heat engine, which transforms

thermal energy, often from combustion of a fuel, into rotational energy. Most thermal

power stations produce steam, and these are sometimes called steam power stations. About

80% of all electric power is generated by use of steam turbines. Not all thermal energy can be

transformed to mechanical power, according to the second law of thermodynamics.

Therefore, there is always heat lost to the environment. If this loss is employed as useful heat,

for industrial processes or district heating, the power plant is referred to as a cogeneration

power plant or CHP (combined heat-and-power) plant. In countries where district heating is

common, there are dedicated heat plants called heat-only boiler stations. An important class

of power stations in the Middle East uses byproduct heat for desalination of water.

CLASSIFICATION

BY FUEL

• Nuclear power plants use a nuclear reactor's heat to operate a steam turbine

generator.

• Fossil fuelled power plants may also use a steam turbine generator or in the case

of natural gas fired plants may use a combustion turbine.

• Geothermal power plants use steam extracted from hot underground rocks.

• Renewable energy plants may be fuelled by waste from sugar cane, municipal

solid waste, landfill methane, or other forms of biomass.

• In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-

density, fuel.

• Waste heat from industrial processes is occasionally concentrated enough to use

for power generation, usually in a steam boiler and turbine.

BY PRIME MOVER

• Steam turbine plants use the dynamic pressure generated by expanding steam to turn

the blades of a turbine. Almost all large non-hydro plants use this system.

• Gas turbine plants use the dynamic pressure from flowing gases to directly operate

the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to

supply “peak" energy during periods of high demand, though at higher cost than base-

loaded plants. These may be comparatively small units, and sometimes completely

unmanned, being remotely operated. This type was pioneered by the UK, Prince town

being the world’s first, commissioned in 1959.

• Combined cycle plants have both a gas turbine fired by natural gas, and a steam

boiler and steam turbine which use the exhaust gas from the gas turbine to produce

electricity. This greatly increases the overall efficiency of the plant, and many new

base load power plants are combined cycle plants fired by natural gas.

• Internal combustion Reciprocating engines are used to provide power for isolated

communities and are frequently used for small cogeneration plants. Hospitals,

office buildings, industrial plants, and other critical facilities also use them to provide

backup power in case of a power outage. These are usually fuelled by diesel oil,

heavy oil, natural gas and landfill gas.

• Micro turbines, Sterling engine and internal combustion reciprocating engines are low

cost solutions for using opportunity fuels, such as landfill gas, digester gas from

water treatment plants and waste gas from oil production.

FUNCTIONING

Functioning of thermal power plant:

In a thermal power plant, one of coal, oil or natural gas is used to heat the boiler to convert

the water into steam. The steam is used to turn a turbine, which is connected to a generator.

When the turbine turns, electricity is generated and given as output by the generator, which is

then supplied to the consumers through high-voltage power lines.

Process of a Thermal Power Plant

Detailed process of power generation in a thermal power plant

1) Water intake: Firstly, water is taken into the boiler through a water source. If water

is available in a plenty in the region, then the source is an open pond or river. If water

is scarce, then it is recycled and the same water is used over and over again.

2) Boiler heating: The boiler is heated with the help of oil, coal or natural gas. A

furnace is used to heat the fuel and supply the heat produced to the boiler. The

increase in temperature helps in the transformation of water into steam.

3) Steam Turbine: The steam generated in the boiler is sent through a steam turbine.

The turbine has blades that rotate when high velocity steam flows across them. This

rotation of turbine blades is used to generate electricity.

4) Generator: A generator is connected to the steam turbine. When the turbine

rotates, the generator produces electricity which is then passed on to the power

distribution systems.

5) Special mountings: There is some other equipment like the economizer and air pre-

heater. An economizer uses the heat from the exhaust gases to heat the feed water. An

air pre-heater heats the air sent into the combustion chamber to improve the efficiency

of the combustion process.

6) Ash collection system: There is a separate residue and ash collection system in place

to collect all the waste materials from the combustion process and to prevent them

from escaping into the atmosphere. Apart from this, there are various other

monitoring systems and instruments in place to keep track of the functioning of all the

devices. This prevents any hazards from taking place in the plant.

PROJECT REPORT

• OPERATION

• EMD – I

• EMD – II

Module - I

OPERATION

• Introduction

• Steam Generator or Boiler

• Steam Turbine

• Electric Generator

INTRODUCTION

The operating performance of NTPC has been considerably above the national average. The

availability factor for coal stations has increased from 85.03 % in 1997-98 to 90.09 % in

2006-07, which compares favorably with international standards. The PLF has increased

from 75.2%in 1997-98 to 89.4% during the year 2006-07 which is the highest since the

inception of NTPC.

Operation Room of Power Plant

In a Badarpur Thermal Power Station, steam is produced and used to spin a turbine that

operates generator. Water is heated, turns into steam and spins a steam turbine which drives

an electrical generator. After it passes through the turbine, the steam is condensed in a

condenser; this is known as a Rankine cycle. Shown here is a diagram of a conventional

thermal power plant, which uses coal, oil, or natural gas as fuel to boil water to produce the

steam. The electricity generated at the plant is sent to consumers through high-voltage power

lines.

The Badarpur Thermal Power Plant has Steam Turbine-Driven Generators which has a

collective capacity of 705MW. The fuel being used is Coal which is supplied from the Jharia

Coal Field in Jharkhand. Water supply is given from the Agra Canal.

Table: Capacity of Badarpur Thermal Power Station, New Delhi

Sr. No. Capacity No. of Generators Total Capacity

1. 210MW 2 420MW

2. 95MW 3 285MW

Total 705MW

There are basically three main units of a thermal power plant:

1. Steam Generator or Boiler

2. Steam Turbine

3. Electric Generator

We have discussed about the processes of electrical generation further. A complete detailed

description of the three units is given further.

Typical Diagram of a Coal based Thermal Power Plant

1. Cooling Tower 10. Steam governor valve 19. Super heater

2. Cooling Water Pump 11. High pressure turbine 20. Forced draught fan

3. Transmission line (3-phase) 12. Deaerator 21. Reheater

4. Unit transformer (3-phase) 13. Feed heater 22. Air intake

5. Electric generator (3-phase) 14. Coal conveyor 23. Economiser

6. Low pressure Turbine 15. Coal hopper 24. Air preheater

7.Condensate extraction pump 16. Pulverised fuel mill 25. Precipitator

8. Condensor 17. Boiler drum 26. Induced draught fan

9. Intermediate pressure turbine 18. Ash hopper 27. Chimney Stack

Coal is conveyed (14) from an external stack and ground to a very fine powder by large metal

spheres in the pulverized fuel mill (16). There it is mixed with preheated air (24) driven by

the forced draught fan (20). The hot air-fuel mixture is forced at high pressure into the boiler

where it rapidly ignites. Water of a high purity flows vertically up the tube-lined walls of the

boiler, where it turns into steam, and is passed to the boiler drum, where steam is separated

from any remaining water. The steam passes through a manifold in the roof of the drum into

the pendant superheater (19) where its temperature and pressure increase rapidly to around

200 bar and 540°C, sufficient to make the tube walls glow a dull red. The steam is piped to

the high pressure turbine (11), the first of a three-stage turbine process. A steam governor

valve (10) allows for both manual control of the turbine and automatic set-point following.

The steam is exhausted from the high pressure turbine, and reduced in both pressure and

temperature, is returned to the boiler reheated (21). The reheated steam is then passed to the

intermediate pressure turbine (9), and from there passed directly to the low pressure turbine

set (6). The exiting steam, now a little above its boiling point, is brought into thermal contact

with cold water (pumped in from the cooling tower) in the condenser (8), where it condenses

rapidly back into water, creating near vacuum-like conditions inside the condensor chest. The

condensed water is then passed by a feed pump (7) through a deaerator (12), and pre-warmed,

first in a feed heater (13) powered by steam drawn from the high pressure set, and then in the

economiser (23), before being returned to the boiler drum. The cooling water from the

condensor is sprayed inside a cooling tower (1),creating a highly visible plume of water

vapor, before being pumped back to the condensor (8)in cooling water cycle. The three

turbine sets are sometimes coupled on the same shaft as the three-phase electrical generator

(5) which generates an intermediate level voltage (typically 20-25 kV). This is stepped up by

the unit transformer (4) to a voltage more suitable for transmission (typically 250-500 kV)and

is sent out onto the three-phase transmission system (3).Exhaust gas from the boiler is drawn

by the induced draft fan (26) through an electrostatic precipitator (25) and is then vented

through the chimney stack (27).

STEAM GENERATOR OR BOILER

The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its

walls are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter.

Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it

rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats

the water that circulates through the boiler tubes near the boiler perimeter. The water

circulation rate in the boiler is three to four times the throughput and is typically driven by

pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700 °F

(370 °C) and 3,200 psi (22.1MPa). It is separated from the water inside a drum at the top of

the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the

hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to

1,000 °F (540 °C) to prepare it for the turbine.

The steam generating boiler has to produce steam at the high purity, pressure and temperature

required for the steam turbine that drives the electrical generator. The generator includes the

economizer, the steam drum, the chemical dosing equipment, and the furnace with its steam

generating tubes and the super heater coils. Necessary safety valves are located at suitable

points to avoid excessive boiler pressure. The air and flue gas path equipment include: forced

draft (FD) fan, air preheated (APH), boiler furnace, induced draft (ID) fan, fly ash collectors

(electrostatic precipitator or baghouse) and the flue gas stack.

Schematic diagram of a coal-fired power plant steam generator

For units over about 210 MW capacity, redundancy of key components is provided by

installing duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating dampers.

On some units of about 60 MW, two boilers per unit may instead be provided.

Boiler Furnace and Steam Drum

Once water inside the boiler or steam generator, the process of adding the latent heat

of vaporization or enthalpy is underway. The boiler transfers energy to the water by the

chemical reaction of burning some type of fuel. The water enters the boiler through a section

in the convection pass called the economizer. From the economizer it passes to the steam

drum. Once the water enters the steam drum it goes down the down comers to the lower inlet

water wall headers. From the inlet headers the water rises through the water walls and is

eventually turned into steam due to the heat being generated by the burners located on the

front and rear water walls (typically). As the water is turned into steam/vapor in the water

walls, the steam/vapor once again enters the steam drum.

External View of an Industrial Boiler at Badarpur Thermal Power Station, New Delhi

The steam/vapor is passed through a series of steam and water separators and then dryers

inside the steam drum. The steam separators and dryers remove the water droplets from the

steam and the cycle through the water walls is repeated. This process is known as natural

circulation. The boiler furnace auxiliary equipment includes coal feed nozzles and igniter

guns, soot blowers, water lancing and observation ports (in the furnace walls) for observation

of the furnace interior. Furnace explosions due to any accumulation of combustible gases

after a trip-out are avoided by flushing out such gases from the combustion zone before

igniting the coal. The steam drum (as well as the super heater coils and headers) have air

vents and drains needed for initial startup. The steam drum has an internal device that

removes moisture from the wet steam entering the drum from the steam generating tubes. The

dry steam then flows into thesuperheater coils.

Geothermal plants need no boiler since they use naturally occurring steam sources. Heat

exchangers may be used where the geothermal steam is very corrosive or contains excessive

suspended solids. Nuclear plants also boil water to raise steam, either directly passing the

working steam through the reactor or else using an intermediate heat exchanger.

Fuel Preparation System

In coal-fired power stations, the raw feed coal from the coal storage area is first crushed into

small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is

next pulverized into a very fine powder. The pulverizes may be ball mills, rotating drum

grinders, or other types of grinders. Some power stations burn fuel oil rather than coal. The

oil must kept warm (above its pour point) in the fuel oil storage tanks to prevent the oil from

congealing and becoming unpumpable. The oil is usually heated to about 100°C before being

pumped through the furnace fuel oil spray nozzles.

Boiler Side of the Badarpur Thermal Power Station, New Delhi

Boilers in some power stations use processed natural gas as their main fuel. Other power

stations may use processed natural gas as auxiliary fuel in the event that their main fuel

supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the

boiler furnaces.

Fuel Firing System and Igniter System

From the pulverized coal bin, coal is blown by hot air through the furnace coal burners at an

angle which imparts a swirling motion to the powdered coal to enhance mixing of the

coal powder with the incoming preheated combustion air and thus to enhance the combustion.

To provide sufficient combustion temperature in the furnace before igniting the powdered

coal, the furnace temperature is raised by first burning some light fuel oil or processed natural

gas (by using auxiliary burners and igniters provide for that purpose).

Air Path

External fans are provided to give sufficient air for combustion. The forced draft fan takes

air from the atmosphere and, first warming it in the air pre-heater for better combustion,

injects it via the air nozzles on the furnace wall.

The induced draft fan assists the FD fan by drawing out combustible gases from the furnace,

maintaining a slightly negative pressure in the furnace to avoid backfiring through any

opening. At the furnace outlet, and before the furnace gases are handled by the ID fan, fine

dust carried by the outlet gases is removed to avoid atmospheric pollution. This is an

environmental limitation prescribed by law, and additionally minimizes erosion of the ID fan.

Auxiliary Systems

Fly Ash Collection

Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag

filters (or sometimes both) located at the outlet of the furnace and before the induced draft

fan. The fly ash is periodically removed from the collection hoppers below the precipitators

or bag filters. Generally, the fly ash is pneumatically transported to storage silos for

subsequent transport by trucks or railroad cars.

Bottom Ash Collection and Disposal

At the bottom of every boiler, a hopper has been provided for collection of the bottom ash

from the bottom of the furnace. This hopper is always filled with water to quench the ash and

clinkers falling down from the furnace. Some arrangement is included to crush the clinkers

and for conveying the crushed clinkers and bottom ash to a storage site.

STEAM TURBINE

Steam turbines are used in all of our major coal fired power stations to drive the generators

or alternators, which produce electricity. The turbines themselves are driven by steam

generated in’ Boilers' or 'Steam Generators' as they are sometimes called. Energy in the steam

after it leaves the boiler is converted into rotational energy as it passes through the turbine.

The turbine normally consists of several stages with each stage consisting of a stationary

blade (or nozzle) and a rotating blade. Stationary blades convert the potential energy of the

steam (temperature and pressure) into kinetic energy (velocity) and direct the flow onto the

rotating blades. The rotating blades convert the kinetic energy into forces, caused by pressure

drop, which results in the rotation of the turbine shaft. The turbine shaft is connected to a

generator, which produces the electrical energy. The rotational speed is 3000 rpm for Indian

System (50 Hz) systems and 3600 for American (60 Hz) systems.

In a typical larger power stations, the steam turbines are split into three separate stages, the

first being the High Pressure (HP), the second the Intermediate Pressure (IP) and the third the

Low-pressure (LP) stage, where high, intermediate and low describe the pressure of the

steam. After the steam has passed through the HP stage, it is returned to the boiler to be re-

heated to its original temperature although the pressure remains greatly reduced. The reheated

steam then passes through the IP stage and finally to the LP stage of the turbine.

A distinction is made between "impulse" and "reaction" turbine designs based on the

relative pressure drop across the stage. There are two measures for pressure drop, the pressure

ratio and the percent reaction. Pressure ratio is the pressure at the stage exit divided by the

pressure at the stage entrance. Reaction is the percentage isentropic enthalpy drop across the

rotating blade or bucket compared to the total stage enthalpy drop. Some manufacturers

utilize percent pressure drop across stage to define reaction

Steam turbines can be configured in many different ways. Several IP or LP stages can be

incorporated into the one steam turbine. A single shaft or several shafts coupled together may

be used. Either way, the principles are the same for all steam turbines. The configuration is

decided by the use to which the steam turbine is put, co-generation or pure electricity

production. For co-generation, the steam pressure is highest when used as process steam and

at a lower pressure when used for the secondary function of electricity production.

Nozzles and Blades

Steam enthalpy is converted into rotational energy as it passes through a turbine stage. A

turbine stage consists of a stationary blade (or nozzle) and a rotating blade (or bucket).

Stationary blades convert the potential energy of the steam (temperature and pressure) into

kinetic energy (velocity) and direct the flow onto the rotating blades. The rotating blades

convert the kinetic energy into impulse and reaction forces caused by pressure drop, which

results in the rotation of the turbine shaft or rotor. Steam turbines are machines which must

be designed, manufactured and maintained to high tolerances so that the design power output

and availability is obtained. They are subject to an umber of damage mechanisms, with two

of the most important being:

Erosion due to Moisture: -The presence of water droplets in the last stages of a turbine

causes erosion to the blades. This has led to the imposition of an allowable limit of about

12%wetness in the exhaust steam.

ELECTRIC GENERATOR

The steam turbine-driven generators have auxiliary systems enabling them to work

satisfactorily and safely. The steam turbine generator being rotating equipment generally has

a heavy, large diameter shaft. The shaft therefore requires not only supports but also has to be

kept in position while running. To minimize the frictional resistance to the rotation, the shaft

has a number of bearings. The bearing shells, in which the shaft rotates, are lined with a low

friction material like Babbitt metal. Oil lubrication is provided to further reduce the friction

between shaft and bearing surface and to limit the heat generated

A 95 MW Generator at Badarpur Thermal Power Station, New Delhi

Barring Gear (or Turning Gear)

Barring gear is the term used for the mechanism provided for rotation of the turbine

generator shaft at a very low speed (about one revolution per minute) after unit stoppages for

any reason. Once the unit is "tripped" (i.e., the turbine steam inlet valve is closed), the turbine

starts slowing or "coasting down". When it stops completely, there is a tendency for the

turbine shaft to deflector bend if allowed to remain in one position too long. This deflection is

because the heat inside the turbine casing tends to concentrate in the top half of the casing,

thus making the top half portion of the shaft hotter than the bottom half. The shaft therefore

warps or bends by millionths of inches, only detectable by monitoring eccentricity meters

But this small amount of shaft deflection would be enough to cause vibrations and damage

the entire steam turbine generator unit when it is restarted. Therefore, the shaft is not

permitted to come to a complete stop by a mechanism known as "turning gear" or "barring

gear" that automatically takes over to rotate the unit at a preset low speed. If the unit is shut

down for major maintenance, then the barring gear must be kept in service until the

temperatures of the casings and bearings are sufficiently low.

Condenser

The surface condenser is a shell and tube heat exchanger in which cooling water is circulated

through the tubes. The exhaust steam from the low pressure turbine enters the shell where it

is cooled and converted to condensate (water) by flowing over the tubes as shown in the

adjacent diagram. Such condensers use steam ejectors or rotary motor-driven exhausters for

continuous removal of air and gases from the steam side to maintain vacuum.

A Typical Water Cooled Condenser

For best efficiency, the temperature in the condenser must be kept as low as practical in order

to achieve the lowest possible pressure in the condensing steam. Since the condenser

temperature can almost always be kept significantly below 100oC where the vapor pressure of

water is much less than atmospheric pressure, the condenser generally works under vacuum.

Thus leaks of non-condensable air into the closed loop must be prevented. Plants operating in

hot climates may have to reduce output if their source of condenser cooling water becomes

warmer; unfortunately this usually coincides with periods of high electrical demand for air

conditioning. The condenser generally uses either circulating cooling water from a cooling

tower to reject waste heat to the atmosphere, or once-through water from a river, lake or

ocean.

Feed water Heater

A Rankine cycle with a two-stage steam turbine and a single feed water heater. In the case of

a conventional steam-electric power plant utilizing a drum boiler, the surface condenser

removes the latent heat of vaporization from the steam as it changes states from vapour to

liquid. The heat content (Btu) in the steam is referred to as Enthalpy. The condensate pump

then pumps the condensate water through a feed water heater. The feed water heating

equipment then raises the temperature of the water by utilizing extraction steam from various

stages of the turbine.

A Rankine cycle with a two-stage steam turbine and a single feed water heater

Preheating the feedwater reduces the irreversibility’s involved in steam generation and

therefore improves the thermodynamic efficiency of the system.

This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal

when the feedwater is introduced back into the steam cycle.

Superheater

As the steam is conditioned by the drying equipment inside the drum, it is piped from the

upper drum area into an elaborate set up of tubing in different areas of the boiler. The areas

known assuperheater and reheater. The steam vapor picks up energy and its temperature is

now superheated above the saturation temperature. The superheated steam is then piped

through the main steam lines to the valves of the high pressure turbine.

Deaerator

A steam generating boiler requires that the boiler feed water should be devoid of air and

other dissolved gases, particularly corrosive ones, in order to avoid corrosion of the metal.

Generally, power stations use a deaerator to provide for the removal of air and other

dissolved gases from the boiler feedwater. A deaerator typically includes a vertical, domed

deaerationsection mounted on top of a horizontal cylindrical vessel which serves as the

deaerated boiler feedwater storage tank.

Boiler Feed Water Deaerator (with vertical, domed aeration section and horizontal water

storage section)

There are many different designs for a deaerator and the designs will vary from one

manufacturer to another. The adjacent diagram depicts a typical conventional trayed

deaerator. If operated properly, most deaerator manufacturers will guarantee that oxygen in

the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).

Auxiliary Systems

Oil System

An auxiliary oil system pump is used to supply oil at the start-up of the steam turbine

generator. It supplies the hydraulic oil system required for steam turbine's main inlet steam

stop valve, the governing control valves, the bearing and seal oil systems, the relevant

hydraulic relays and other mechanisms. At a preset speed of the turbine during start-ups, a

pump driven by the turbine main shaft takes over the functions of the auxiliary system.

Generator High Voltage System

The generator voltage ranges from 10.5 kV in smaller units to 15.75 kV in larger units. The

generator high voltage leads are normally large aluminum channels because of their high

currents compared to the cables used in smaller machines. They are enclosed in well-

grounded aluminum bus ducts and are supported on suitable insulators. The generator high

voltage channels are connected to step-up transformers for connecting to a high voltage

electrical substation (of the order of 220 kV) for further transmission by the local power grid.

The necessary protection and metering devices are included for the high voltage leads. Thus,

the steam turbine generator and the transformer form one unit. In smaller units, generating at

10.5kV, a breaker is provided to connect it to a common 10.5 kV bus system

Other Systems

Monitoring and Alarm system

Most of the power plant’s operational controls are automatic. However, at times, manual

intervention may be required. Thus, the plant is provided with monitors and alarm systems

that alert the plant operators when certain operating parameters are seriously deviating from

their normal range

An Engineer monitoring the various parameters at NTPC, New Delhi

Battery Supplied Emergency Lightening & Communication

A central battery system consisting of lead acid cell units is provided to supply emergency

electric power, when needed, to essential items such as the power plant's control systems,

communication systems, turbine lube oil pumps, and emergency lighting. This is essential for

a safe, damage-free shutdown of the units in an emergency situation.

Module – II

EMD - I

• Motors

• Switchgear HT/LT

• Direct On Line Starter

ELECTRIC MOTORS

An electric motor uses electrical energy to produce mechanical energy. The reverse process

that of using mechanical energy to produce electrical energy is accomplished by a generator

or dynamo. Traction motors used on locomotives and some electric and hybrid automobiles

often performs both tasks if the vehicle is equipped with dynamic brakes

A high power electric motor

Categorization of Electric Motors

The classic division of electric motors has been that of Direct Current (DC) types vs

Alternating Current (AC) types. The ongoing trend toward electronic control further muddles

the distinction, as modern drivers have moved the commutator out of the motor shell. For this

new breed of motor, driver circuits are relied upon to generate sinusoidal AC drive currents,

or some approximation of. The two best examples are: the brushless DC motor and the

stepping motor, both being polyphase AC motors requiring external electronic control. There

is a clearer distinction between a synchronous motor and asynchronous types. In the

synchronous types, the rotor rotates in synchrony with the oscillating field or current

(eg. permanent magnet motors). In contrast, an asynchronous motor is designed to slip; the

most ubiquitous example being the common AC induction motor which must slip in order to

generate torque.

Comparison of Motor Types

At Badarpur Thermal Power Station, New Delhi, mostly AC motors are employed for

various purposes. We had to study the two types of AC Motors viz. Synchronous Motors and

Induction Motor. The motors have been explained further.

AC Motor

Internal View of AC Motors

An AC motor is an electric motor that is driven by an alternating current. It consists of two

basic parts, an outside stationary stator having coils supplied with AC current to produce a

rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque

by the rotating field.

There are two types of AC motors, depending on the type of rotor used. The first is the

synchronous motor, which rotates exactly at the supply frequency or a sub multiple of the

supply frequency. The magnetic field on the rotor is either generated by current delivered

through slip rings or a by a permanent magnet.

The second type is the induction motor, which turns slightly slower than the supply

frequency. The magnetic field on the rotor of this motor is created by an induced current.

Synchronous Motor

A synchronous electric motor is an AC motor distinguished by a rotor spinning with

coils passing magnets at the same rate as the alternating current and resulting magnetic field

which drives it. Another way of saying this is that it has zero slip under usual operating

conditions. Contrast this with an induction motor, which must slip in order to produce torque.

Sometimes a synchronous motor is used, not to drive a load, but to improve the power factor

on the local grid it's connected to. It does this by providing reactive power to or consuming

reactive power from the grid. In this case the synchronous motor is called a Synchronous

condenser. Electrical power plants almost always use synchronous generators because it's

very important to keep the frequency constant at which the generator is connected.

Advantages

Synchronous motors have the following advantages over non-synchronous motors:

• Speed is independent of the load, provided an adequate field current is applied.

• Accurate control in speed and position using open loop controls, eg. Stepper

motors.

• They will hold their position when a DC current is applied to both the stator

and the rotor windings.

• Their power factor can be adjusted to unity by using a proper field

current relative to the load. Also, a "capacitive" power factor, (current

phase leads voltage phase), can be obtained by increasing this current

slightly, which can help achieve a better power factor correction for the

whole installation.

• Their construction allows for increased electrical efficiency when a low speed

is required (as in ball mills and similar apparatus).

Examples

• Brushless permanent magnet DC motor.

• Stepper motor.

• Slow speed AC synchronous motor.

• Switched reluctance motor.

Induction Motor

An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the

rotating device by means of electromagnetic induction.

Three Phase Induction Motors

An electric motor converts electrical power to mechanical power in its rotor (rotating

part).There are several ways to supply power to the rotor. In a DC motor this power is

supplied to the armature directly from a DC source, while in an AC motor this power is

induced in the rotating device. An induction motor is sometimes called a rotating transformer

because the stator (stationary part) is essentially the primary side of the transformer and the

rotor (rotating part) is the secondary side. Induction motors are widely used, especially

polyphase induction motors, which are frequently used in industrial drives.

Induction motors are now the preferred choice for industrial motors due to their rugged

construction, lack of brushes (which are needed in most DC Motors) and — thanks to

modern power electronics — the ability to control the speed of the motor.

Construction

The stator consists of wound 'poles' that carry the supply current that induces a magnetic field

in the conductor. The number of 'poles' can vary between motor types but the poles are

always in pairs (i.e. 2, 4, 6 etc). There are two types of rotor:

1. Squirrel-cage rotor

2. Slip ring rotor

The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid

copper (most common) or aluminum that span the length of the rotor, and are connected

through a ring at each end. The rotor bars in squirrel-cage induction motors are not straight,

but have some skew to reduce noise and harmonics. The motor's phase type is one of two

types:

1. Single-phase induction motor

2. 3-phase induction motor

Principle of Operation

The basic difference between an induction motor and a synchronous AC motor is that in the

latter a current is supplied onto the rotor. This then creates a magnetic field which, through

magnetic interaction, links to the rotating magnetic field in the stator which in turn causes the

rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same

as the speed of the rotating magnetic field in the stator. By way of contrast, the induction

motor does not have any direct supply onto the rotor; instead, a secondary current is induced

in the rotor. To achieve this, stator windings are arranged around the rotor so that when

energized with a polyphone supply they create a rotating magnetic field pattern which sweeps

past the rotor. This changing magnetic field pattern can induce currents in the rotor

conductors. These currents interact with the rotating magnetic field created by the stator and

the rotor will turn. However, for these currents to be induced, the speed of the physical rotor

and the speed of the rotating magnetic field in the stator must be different, or else the

magnetic field will not be moving relative to the rotor conductors and no currents will be

induced. If by some chance this happens, the rotor typically slows slightly until a current is

re-induced and then the rotor continues as before. This difference between the speed of the

rotor and speed of the rotating magnetic field in the stator is called slip. It has no unit and the

ratio between the relative speed of the magnetic field as seen by the rotor to the speed of the

rotating field. Due to this an induction motor is sometimes referred to as an asynchronous

machine. Types:

• Based on type of phase supply

1. three phase induction motor (self starting in nature)

2. single phase induction motor (not self starting)

• Other

1. Squirrel cage induction motor

2. Slip ring induction motor

SWITCHGEAR HT/LT

The term switchgear, used in association with the electric power system, or grid, refers to the

combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical

equipment. Switchgear is used both to de-energize equipment to allow work to be done and to

clear faults downstream. The very earliest central power stations used simple open knife

switches, mounted on insulating panels of marble or asbestos. Power levels and voltages

rapidly escalated, making open manually-operated switches too dangerous to use for anything

other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be

contained and safely controlled. By the early 20th century, a switchgear line-up would be a

metal-enclosed structure with electrically-operated switching elements, using oil circuit

breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6

equipment, allowing large currents and power levels to be safely controlled by automatic

equipment incorporating digital controls, protection, metering and communications.

A View Of Switchgear at a Power Plant

Types

A piece of switchgear may be a simple open air isolator switch or it may be insulated by

some other substance. An effective although more costly form of switchgear is "gas insulated

switchgear" (GIS), where the conductors and contacts are insulated by pressurized (SF6)

sulfur hexafluoride gas. Other common types are oil [or vacuum] insulated switchgear.

Circuit breakers are a special type of switchgear that are able to interrupt fault currents.

Their construction allows them to interrupt fault currents of many hundreds or thousands of

amps. The quenching of the arc when the contacts open requires careful design, and falls into

four types: Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil

through the arc. Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field,

and then rely upon the dielectric strength of the SF6 to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the

contact material), so the arc quenches when it is stretched a very small amount (<2-3 mm).

Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000

volts.

Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts

are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing

out the arc.

Circuit breakers are usually able to terminate all current flow very quickly: typically between

30ms and 150 ms depending upon the age and construction of the device. Several different

classifications of switchgear can be made:

By the current rating:

• By interrupting rating (maximum short circuit current that the device can

safely interrupt)

• Circuit breakers can open and close on fault currents

• Load-break/Load-make switches can switch normal system load currents

• Isolators may only be operated while the circuit is dead, or the load current is

very small.

By voltage class:

• Low Tension (less than 440 volts AC)

• High Tension (more than 6.6 kV AC)

By insulating medium:

• Air

• Gas (SF6 or mixtures)

• Oil

• Vacuum

By construction type:

• Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure

type)

• Outdoor

• Industrial

• Utility

• Marine

• Draw-out elements (removable without many tools)

• Fixed elements (bolted fasteners)

• Live-front

• Dead-front

• Open

• Metal-enclosed

• Metal-clad

• Metal enclose & Metal clad

• Arc-resistant

High Tension Switchgear at Thermal Power Plant

By IEC degree of internal separation:

• No Separation

• Bus bars separated from functional units

• Terminals for external conductors separated from bus bars

• Terminals for external conductors separated from functional units but not from

each other

• Functional units separated from each other

• Terminals for external conductors separated from each other

• Terminals for external conductors separate from their associated functional

unit

By interrupting device:

• Fuses

• Air Blast Circuit Breaker

• Minimum Oil Circuit Breaker

• Oil Circuit Breaker

• Vacuum Circuit Breaker

• Gas (SF6) Circuit breaker

By operating method:

• Manually-operated

• Motor-operated

• Solenoid/stored energy operated

By type of current:

• Alternating current

• Direct current

By application:

• Transmission system

• Distribution.

A single line-up may incorporate several different types of devices, for example, air-

insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the

same row of cubicles.

Ratings, design, specifications and details of switchgear are set by a multitude of standards.

In North America mostly IEEE and ANSI standards are used, much of the rest of the world

uses IEC standards, sometimes with local national derivatives or variations.

Functions

One of the basic functions of switchgear is protection, which is interruption of short-circuit

and overload fault currents while maintaining service to unaffected circuits. Switchgear

also provides isolation of circuits from power supplies. Switchgear also is used to enhance

system availability by allowing more than one source to feed a load.

Safety

To help ensure safe operation sequences of switchgear, trapped key interlocking

provides predefined scenarios of operation. James Harry Castell invented this technique in

1922. For example, if only one of two sources of supply is permitted to be connected at a

given time, the interlock scheme may require that the first switch must be opened to release a

key that will allow closing the second switch. Complex schemes are possible

HT Swithgear

High voltage switchgear is any switchgear and switchgear assembly of rated voltage higher

than1000 volts. High voltage switchgear is any switchgear used to connect or to disconnect a

part of a high voltage power system. These switchgears are essential elements for the

protection and for a safety operating mode without interruption of a high voltage power

system. This type of equipment is really important because it is directly linked to the quality

of the electricity supply. The high voltage is a voltage above 1000 V for alternating current

and above 1500 V for direct current.

High Tension Switchgear of a Thermal Power Plant

The high voltage switchgear was invented at the end of the 19th century for operating the

motors and others electric machines. It has been improved and it can be used in the whole

range of high voltage until 1100 kV.

Functional Classification

Disconnections and Earthing Switches

They are above all safety devices used to open or to close a circuit when there is no current

through them. They are used to isolate a part of a circuit, a machine, a part of an overhead-

line or an underground line for the operating staff to access it without any danger.

The opening of the line isolator or bus bar section isolator is necessary for the safety but it is

not enough. Grounding must be done at the upstream sector and the downstream sector on the

device which they want to intervene thanks to the earthing switches. In principle,

disconnecting switches do not have to interrupt currents, but some of them can interrupt

currents (up to 1600 A under 10 to 300V) and some earthing switches must interrupt induced

currents which are generated in a non-current-carrying line by inductive and capacitive

coupling with nearby lines (up to 160 A under 20 kV) ).

A Vacuum Circuit Breaker High Tension Switchgear

High-Current Switching Mechanism

They can open or close a circuit in normal load. Some of them can be used as a disconnecting

switch. But if they can create a short-circuit current, they can not interrupt it.

Contactor

Their functions are similar to the high-current switching mechanism, but they can be used at

higher rates. They have a high electrical endurance and a high mechanical endurance.

Contactors are used to frequently operate device like electric furnaces, high voltage motors.

They cannot be used as a disconnecting switch. They are used only in the band 30 kV to 100

kV.

Fuses

The fuses can interrupt automatically a circuit with an over current flowing in it for a fixed

time. The current interrupting is got by the fusion of an electrical conductor which is graded.

They are mainly used to protect against the short-circuits. They limit the peak value of the

fault current. In three-phase electric power, they only eliminate the phases where the fault

current is flowing, which is a risk for the devices and the people. Against this trouble, the

fuses can be associated with high-current switches or contactors. They are used only in the

band 30 kV to 100 kV.

Circuit Breaker

A high voltage circuit breaker is capable of making, carrying and breaking currents under the

rated voltage (the maximal voltage of the power system which it is protecting) : Under

normal circuit conditions, for example to connect or disconnect a line in a power system;

Under specified abnormal circuit conditions especially to eliminate a short circuit. From its

characteristics, a circuit breaker is the protection device essential for a high voltage

power system, because it is the only one able to interrupt a short circuit current and so to

avoid the others devices to be damaged by this short circuit. The international standard IEC

62271-100defines the demands linked to the characteristics of a high voltage circuit breaker.

The circuit breaker can be equipped with electronic devices in order to know at any moment

their states (wear, gas pressure…) and possibly to detect faults from characteristics

derivatives and it can permit to plan maintenance operations and to avoid failures. To operate

on long lines, the circuit breakers are equipped with a closing resistor to limit the over

voltages. They can be equipped with devices to synchronize the closing and/or the opening to

limit the over voltages and the inrush currents from the lines, the unloaded transformers, the

shunt reactances and the capacitor banks. Some devices are designed to have the

characteristics of the circuit breaker and the disconnect or. But their use is limited

DIRECT ON LINE STARTER

A direct on line starter, often abbreviated DOL starter, is a widely-used starting method

of electric motors. The term is used in electrical engineering and associated with electric

motors. There are many types of motor starters, the simplest of which is the DOL starter. A

motor starter is an electrical/electronic circuit composed of electro-mechanical and electronic

devices which are employed to start and stop an electric motor. Regardless of the motor type

(AC or DC), the types of starters differ depending on the method of starting the motor. A

DOL starter connects the motor terminals directly to the power supply. Hence, the motor is

subjected to the full voltage of the power supply. Consequently, high starting current flows

through the motor. This type of starting is suitable for small motors below 5 hp (3.75 kW).

Reduced-voltage starters are employed with motors above 5 hp. Although DOL motor

starters are available for motors less than 150 kW on 400 V and for motors less than 1 MW

on 6.6 kV. Supply reliability and reserve power generation dictates the use of reduced voltage

or not.

Internal View of a Direct On Line Starter

Major Components

There are four major components of a Direct On Line Starter. They are given as follows:

1. Switch

2. Fuse

3. Conductor (Electromagnetic)

4. Thermal Overload Relay (Heat & Temperature)

Auxiliary Components

According to our desire and use of work, we use auxiliary components in a DOL Starter.

There are basically two types of Auxiliary Components given as follows:

1. Auxiliary Conductor

2. Timer (Range – 0.5s to 60s)

DOL Reversing Starter

Most motors are reversible or, in other words, they can be run clockwise and anti-clockwise.

A reversing starter is an electrical or electronic circuit that reverses the direction of a

motor automatically. Logically, the circuit is composed of two DOL circuits; one for

clockwise operation and the other for anti-clockwise operation.

External View of a Direct On Line Starter

Example of Motor Starters

A very well-known motor starter is the DOL Starter of a 3-Phase Squirrel-Cage Motor. This

starter is sometimes used to start water pumps, compressors, fans and conveyor belts. With a

400V, 50 Hz, 3-phase supply, the power circuit connects the motor to 400V. Consequently,

the starting current may reach 3-8 times the normal current. The control circuit is typically

run at 24V with the aid of a 400V/24V transformer.

Motor Direction Reversal

Changing the direction of a 3-Phase Squirrel-Cage Motor requires swapping any two phases.

This could be achieved by a contactor KM1 swapping phase L2 and L3 between the supply

and the motor.

MODULE - II

EMD - II

• Generator

• Protection

• Transformer

GENERATORS

The basic function of the generator is to convert mechanical power, delivered from the shaft

of the turbine, into electrical power. Therefore a generator is actually a rotating mechanical

energy converter. The mechanical energy from the turbine is converted by means of a

rotating magnetic field produced by direct current in the copper winding of the rotor or field,

which generates three-phase alternating currents and voltages in the copper winding of the

stator (armature). The stator winding is connected to terminals, which are in turn connected to

the power system for delivery of the output power to the system.

A 210 MW Turbine Generator at Badarpur Thermal Power Station, New Delhi

The class of generator under consideration is steam turbine-driven generators, commonly

called turbo generators. These machines are generally used in nuclear and fossil fueled power

plants, co-generation plants, and combustion turbine units. They range from relatively small

machines of a few Megawatts (MW) to very large generators with ratings up to 1900 MW.

The generators particular to this category are of the two- and four-pole design employing

round-rotors, with rotational operating speeds of 3600 and 1800 rpm in North America, parts

of Japan, and Asia(3000 and 1500 rpm in Europe, Africa, Australia, Asia, and South

America). At Badarpur Thermal Power Station 3000 rpm, 50 Hz generators are used of

capacities 210 MW and 95 MW.

As the system load demands more active power from the generator, more steam (or fuel in a

combustion turbine) needs to be admitted to the turbine to increase power output. Hence

more energy is transmitted to the generator from the turbine, in the form of a torque. This

torque is mechanical in nature, but electromagnetically coupled to the power system

through the generator. The higher the power output, the higher the torque between turbine

and generator.

The power output of the generator generally follows the load demand from the system.

Therefore the voltages and currents in the generator are continually changing based on the

load demand. The generator design must be able to cope with large and fast load changes,

which show up inside the machine as changes in mechanical forces and temperatures. The

design must therefore incorporate electrical current-carrying materials (i.e., copper), magnetic

flux-carrying materials (i.e., highly permeable steels), insulating materials (i.e., organic),

structural members (i.e., steel and organic), and cooling media (i.e., gases and liquids), all

working together under the operating conditions of a turbo generator.

An open Electric Generator at Power Plant

Since the turbo generator is a synchronous machine, it operates at one very specific speed

to produce a constant system frequency of 50 Hz, depending on the frequency of the grid to

which it is connected. As a synchronous machine, a turbine generator employs a steady

magnetic flux passing radially across an air gap that exists between the rotor and the stator.

(The term “air gap “is commonly used for air- and gas-cooled machines). For the machines in

this discussion, this means a magnetic flux distribution of two or four poles on the rotor. This

flux pattern rotates with the rotor, as it spins at its synchronous speed. The rotating magnetic

field moves past a three-phase symmetrically distributed winding installed in the stator core,

generating an alternating voltage in the stator winding. The voltage waveform created in each

of the three phases of the stator winding is very nearly sinusoidal. The output of the stator

winding is the three-phase power, delivered to the power system at the voltage generated in

the stator winding. In addition to the normal flux distribution in the main body of the

generator, there are stray fluxes at the extreme ends of the generator that create fringing flux

patterns and induce stray losses in the generator. The stray fluxes must be accounted for in

the overall design.

Generators are made up of two basic members, the stator and the rotor, but the stator and

rotor are each constructed from numerous parts themselves. Rotors are the high-speed

rotating member of the two, and they undergo severe dynamic mechanical loading as well as

the electromagnetic and thermal loads. The most critical component in the generator are the

retaining rings, mounted on the rotor. These components are very carefully designed for high-

stress operation. The stator is stationary, as the term suggests, but it also sees significant

dynamic forces in terms of vibration and torsional loads, as well as the electromagnetic,

thermal, and high-voltage loading. The most critical component of the stator is arguably the

stator winding because it is a very high cost item and it must be designed to handle all of the

harsh effects described above. Most stator problems occur with the winding.

Stator

The stator winding is made up of insulated copper conductor bars that are distributed around

the inside diameter of the stator core, commonly called the stator bore, in equally spaced slots

in the core to ensure symmetrical flux linkage with the field produced by the rotor. Each slot

contains two conductor bars, one on top of the other. These are generally referred to as top

and bottom bars. Top bars are the ones nearest the slot opening (just under the wedge) and the

bottom bars are the ones at the slot bottom. The core area between slots is generally called a

core tooth.

Stator of a Turbo Generator

The stator winding is then divided into three phases, which are almost always wye connected.

We connection is done to allow a neural grounding point and for relay protection of the

winding. The three phases are connected to create symmetry between them in the 360 degree

arc of the stator bore. The distribution of the winding is done in such a way as to produce a

120degree difference in voltage peaks from one phase to the other, hence the term “three-

phase voltage.” Each of the three phases may have one or more parallel circuits within the

phase. The parallels can be connected in series or parallel, or a combination of both if it is a

four-pole generator. This will be discussed in the next section. The parallels in all of the

phases are essentially equal on average, in their performance in the machine. Therefore, they

each “see “equal voltage and current, magnitudes and phase angles, when averaged over one

alternating cycle. The stator bars in any particular phase group are arranged such that there

are parallel paths, which overlap between top and bottom bars. The overlap is staggered

between top and bottom bars. The top bars on one side of the stator bore are connected to the

bottom bars on the other side of the bore in one direction while the bottom bars are connected

in the other direction on the opposite side of the stator. This connection with the bars on the

other side of the stator creates a “reach” or “pitch” of a certain number of slots. The pitch is

therefore the number slots that the stator bars have to reach in the stator bore arc, separating

the two bars to be connected. This is always less than 180 degrees. Once connected, the stator

bars form a single coil or turn. The total width of the overlapping parallels is called the

“breadth.” The combination of the pitch and breadth create a “winding or distribution factor.”

The distribution factor is used to minimize the harmonic content of the generated voltage. In

the case of a two parallel path winding, these may be connected in series or parallel outside

the stator bore, at the termination end of the generator. The connection type will depend on a

number of other design issues regarding current-carrying ability of the copper in the winding.

In a two-parallel path, three-phase winding, alternating voltage is created by the action of the

rotor field as it moves past these windings. Since there is a plus and minus, or north and

south, to the rotating magnetic field, opposite polarity currents flow on each side of the stator

bore in the distributed winding. The currents normally flowing in large turbo generators can

be in the order of thousands of amperes. Due to the very high currents, the conductor bars in a

turbo generator have a large cross-sectional area. In addition they are usually one single turn

per bar, as opposed to motors or small generators that have multiple turn bars or coils. These

stator or conductor bars are also very rigid and do not bend unless significant force is exerted

on them.

Rotor

The rotor winding is installed in the slots machined in the forging main body and is

distributed symmetrically around the rotor between the poles. The winding itself is made up

of many turns of copper to form the entire series connected winding. All of the turns

associated with a single slot are generally called a coil. The coils are wound into the winding

slots in the forging, concentrically in corresponding positions on opposite sides of a pole. The

series connection essentially creates a single multi-turn coil overall, that develops the total

ampere-turns of the rotor (which is the total current flowing in the rotor winding times the

total number of turns).

There are numerous copper-winding designs employed in generator rotors, but all rotor

windings function basically in the same way. They are configured differently for different

methods of heat removal during operation. In addition almost all large turbo generators have

directly cooled copper windings by air or hydrogen cooling gas.

Rotor of a Turbo Generator

Cooling passages are provided within the conductors themselves to eliminate the temperature

drop across the ground insulation and preserve the life of the insulation material.

In an “axially” cooled winding, the gas passes through axial passages in the conductors, being

fed from both ends, and exhausted to the air gap at the axial center of the rotor. In other

designs, “radial” passages in the stack of conductors are fed from sub slots machined along

the length of the rotor at the bottom of each slot. In the “air gap pickup” method, the cooling

gas is picked up from the air gap, and cooling is accomplished over a relatively short length

of the rotor, and then discharged back to the air gap. The cooling of the end-regions of the

winding varies from design to design, as much as that of the slot section. In smaller turbine

generators the indirect cooling method is used (similar to indirectly cooled stator windings),

where the heat is removed by conduction through the ground insulation to the rotor body.

The winding is held in place in the slots by wedges, in a similar manner as the stator

windings. The difference is that the rotor winding loading on the wedges is far greater due to

centrifugal forces at speed. The wedges therefore are subjected to a tremendous static load

from these forces and bending stresses because of the rotation effects. The wedges in the

rotor are not generally a tight fit in order to accommodate the axial thermal expansion of the

rotor winding during operation. There are also many available designs and configurations for

the end-winding construction and ventilation methods. As in the rotor slots, the copper turns

in the end-winding must be isolated from one another so that they do not touch and create

shorts between turns. Therefore packing and blocking are used to keep the coils separated,

and in their relative position as the rotor winding expands from thermal effects during

operation. To restrain the end winding portion of the rotor winding during high-speed

operation, retaining-rings are employed to keep the copper coils in place.

Hydrogen Cooling System

As the hydrogen cooling gas picks up heat from the various generator components within the

machine, its temperature rises significantly. This can be as much as 46oC, and therefore the

hydrogen must be cooled down prior to being re-circulated through the machine for

continuous cooling. Hydrogen coolers or heat exchangers are employed for this purpose.

Hydrogen coolers are basically heat exchangers mounted inside the generator in the enclosed

atmosphere. Cooling tubes with “fins” are used to enlarge the surface area for cooling, as the

hydrogen gas passes over the outside of the finned tubes. “Raw water” (filtered and treated)

from the local river or lake is pumped through the tubes to take the heat away from the

hydrogen gas and outside the generator. The tubes must be extremely leak-tight to ensure that

hydrogen gas does not enter into the tubes, since the gas is at a higher pressure than the raw

water.

Stator Cooling Water System

The stator cooling water system (SCW) is used to provide a source of de-mineralized water to

the generator stator winding for direct cooling of the stator winding and associated

components. SCW is generally used in machines rated at or above 300 MVA. Most SCW

systems are provided as package units, mounted on a singular platform, which includes all of

the SCW system components. All components of the system are generally made from

stainless steel or copper materials.

Stator Water Cooling System

System Component

Pumps: Generally, ac motor driven pumps are used to deliver the cooling water to the

windings. In some instances a dc motor driven pump is used for emergency shutdown.

Heat Exchangers: Heat exchangers are provided for heat removal from the SCW. Raw

water from the local lake or river is circulated on one side of the cooler to remove the heat

from the de-mineralized SCW circulating on the other side of the heat exchanger.

Filters and/or Strainers: Full-flow filters and/or strainers, or a combination of both, are

employed for removal of debris from the SCW. Strainers are generally sized to remove debris

in the 20 to 50 µ range and larger and filters for debris in the range of 3 µ and larger. They

can be mechanical or organic type filters and strainers. Debris removal is important to reduce

the possibility of plugging in the stator conductor bar strands.

Stator Cooling Water System Storage or Makeup Tank: In the event the SCW is lost, or

the SCW system must be refilled after shutdown and draining, the system requires

replenishing. Therefore a storage tank to hold sufficient makeup water is required. Some

systems are open to atmosphere while others maintain a hydrogen blanket on top of the water

to keep the level of oxygen at a minimum.

Gas Collection and Venting Arrangement:

Since no SCW system is leak proof, there is some ingress of hydrogen and natural collection

of other gases such as oxygen in the SCW system. A means for venting off these gases is

required. Generally, the excess gases are vented to atmosphere. In some systems the venting

process is monitored and/or quantified and in other systems there is none. This is

manufacturer-specific.

TRANSFORMER

A transformer is a static device consisting of a winding, or two or more coupled windings,

with or without a magnetic core, for inducing mutual coupling between circuits. When an

alternating current flows in a conductor, a magnetic field exists around the conductor. If

another conductor is placed in the field created by the first conductor such that the flux lines

link the second conductor, then a voltage is induced into the second conductor. The use of a

magnetic field from one coil to induce a voltage into a second coil is the principle on which

transformer theory and application is based.

A 220 kV Transformer at Power Plant

ANSI/IEEE defines a transformer as a static electrical device, involving no continuously

moving parts, used in electric power systems to transfer power between circuits through the

use of electromagnetic induction. The transformer is one of the most reliable pieces of

electrical distribution equipment. It has no moving parts, requires minimal maintenance, and

is capable of withstanding overloads, surges, faults, and physical abuse that may damage or

destroy other items in the circuit. Often, the electrical event that burns up a motor, opens a

circuit breaker, or blows a fuse has a subtle effect on the transformer. Although the

transformer may continue to operate as before, repeat occurrences of such damaging

electrical events, or lack of even minimal maintenance can greatly accelerate the eventual

failure of the transformer.

The fact that a transformer continues to operate satisfactorily in spite of neglect and abuse is a

testament to its durability. However, this durability is no excuse for not providing the

proper care. Most of the effects of aging, faults, or abuse can be detected and corrected by a

comprehensive maintenance, inspection, and testing program

Transformers are exclusively used in electric power systems to transfer power by

electromagnetic induction between circuits at the same frequency, usually with changed

values of voltage and current. There are numerous types of transformers used in various

applications including audio, radio, instrument, and power. In Badarpur Thermal Power

Station, we deal exclusively with power transformer applications involving the transmission

and distribution of electrical power. Power transformers are used extensively by traditional

electric utility companies, power plants, and industrial plants.

The term power transformer is used to refer to those transformers used between the

generator and the distribution circuits, and these are usually rated at 220 kVA and above.

Power systems typically consist of a large number of generation locations, distribution points,

and interconnections within the system or with nearby systems, such as a neighboring utility.

The complexity of the system leads to a variety of transmission and distribution voltages.

Power transformers must be used at each of these points where there is a transition between

voltage levels. Power transformers are selected based on the application, with the emphasis

toward custom design being more apparent the larger the unit. Power transformers are

available for step-up operation, primarily used at the generator and referred to as generator

step-up (GSU)transformers, and for step-down operation, mainly used to feed distribution

circuits. Power transformers are available as single-phase or three-phase apparatus.

A Power Transformer at a Thermal Power Plant

Construction

A power transformer is a device that changes (transforms) an alternating voltage and current

from one level to another. Power transformers are used to “step up” (transform) the voltages

that are produced at generation to levels that are suitable for transmission (higher voltage,

lower current). Conversely, a transformer is used to “step down” (transform) the higher

transmission voltages to levels that are suitable for use at various facilities (lower voltage,

higher current).Electric power can undergo numerous transformations between the source and

the final end use point.

• Voltages must be stepped-up for transmission. Every conductor, no matter how large,

will lose an appreciable amount of power (watts) to its resistance (R) when a current

(T) passes through it. This loss is expressed as a function of the applied current

(P=I2R). Because this loss is dependent on the current, and since the power to be

transmitted is a function of the applied volts (E) times the amps (P=IE), significant

savings can be obtained by stepping the voltage up to a higher voltage level, with the

corresponding reduction of the current value. Whether 100 amps is to be transmitted

at 100 volts (P=IE, 100 amps X 100 volts = 10,000watts) or 10 amps is to be

transmitted at 1,000 volts (P=IE, 10 amps X 1,000 volts = 10,000watts) the same

10,000 watts will be applied to the beginning of the transmission line.

• If the transmission distance is long enough to produce 0.1 ohm of resistance across

the transmission cable, P=I2R, (100 amp)2 X 0.1 ohm = 1,000 watts will be lost across

the transmission line at the 100 volt transmission level. The 1000 volts transmission

level will create a loss of P=I2R, (10 amp)2 X 0.1 ohm = 10 watts. This is where

transformers play an important role.

• Although power can be transmitted more efficiently at higher voltage levels,

sometimes as high as 500 or 750 thousand volts (kV), the devices and networks at the

point of utilization are rarely capable of handling voltages above 32,000 volts.

Voltage must be “stepped down” to be utilized by the various devices available. By

adjusting the voltages to the levels necessary for the various end use and distribution

levels, electric power can be used both efficiently and safely.

• All power transformers have three basic parts, a primary winding, secondary winding,

and a core. Even though little more than an air space is necessary to insulate an “ideal

“transformer, when higher voltages and larger amounts of power are involved, the

insulating material becomes an integral part of the transformer’s operation. Because

of this, the insulation system is often considered the fourth basic part of the

transformer. It is important to note that, although the windings and core deteriorate

very little with age, the insulation can be subjected to severe stresses and chemical

deterioration. The insulation deteriorates at a relatively rapid rate, and its condition

ultimately determines the service life of the transformer.

Core

The core, which provides the magnetic path to channel the flux, consists of thin strips of

high-grade steel, called laminations, which are electrically separated by a thin coating of

insulating material. The strips can be stacked or wound, with the windings either built

integrally around the core or built separately and assembled around the core sections. Core

steel can be hot- or cold-rolled, grain-oriented or non grain oriented, and even laser-scribed

for additional performance. Thickness ranges from 0.23 mm to upwards of 0.36 mm. The

core cross section can be circular or rectangular, with circular cores commonly referred to as

cruciform construction. Rectangular cores are used for smaller ratings and as auxiliary

transformers used within a power transformer. Rectangular cores use a single width of strip

steel, while circular cores use a combination of different strip widths to approximate a

circular cross-section. The type of steel and arrangement depends on the transformer rating as

related to cost factors such as labor and performance.

Just like other components in the transformer, the heat generated by the core must be

adequately dissipated. While the steel and coating may be capable of withstanding higher

temperatures, it will come in contact with insulating materials with limited temperature

capabilities. In larger units, cooling ducts are used inside the core for additional convective

surface area, and sections of laminations may be split to reduce localized losses.

The core is held together by, but insulated from, mechanical structures and is grounded to a

single point in order to dissipate electrostatic buildup. The core ground location is usually

some readily accessible point inside the tank, but it can also be brought through a bushing on

the tank wall or top for external access. This grounding point should be removable for testing

purposes, such as checking for unintentional core grounds. Multiple core grounds, such as a

case whereby the core is inadvertently making contact with otherwise grounded internal

metallic mechanical structures, can provide a path for circulating currents induced by the

main flux as well as a leakage flux, thus creating concentrations of losses that can result in

localized heating.

The maximum flux density of the core steel is normally designed as close to the knee of the

saturation curve as practical, accounting for required over excitations and tolerances that exist

due to materials and manufacturing processes. For power transformers the flux density is

typically between 1.3 T and 1.8 T, with the saturation point for magnetic steel being around

2.03T to 2.05 T.

There are two basic types of core construction used in power transformers: core form and

shell form.

• In core-form construction: there is a single path for the magnetic circuit. For single-

phase applications, the windings are typically divided on both core legs as shown. In

three-phase applications, the windings of a particular phase are typically on the same

core leg. Windings are constructed separate of the core and placed on their respective

core legs during core assembly.

Schematic Diagram of Shell-form Construction

• In shell-form construction, the core provides multiple paths for the magnetic circuit.

The core is typically stacked directly around the windings, which are usually

“pancake”-type windings, although some applications are such that the core and

windings are assembled similar to core form. Due to advantages in short-circuit and

transient-voltage performance, shell forms tend to be used more frequently in the

largest transformers, where conditions can be more severe. Variations of three-phase

shell-form construction include five and seven-legged cores, depending on size and

application.

Schematic Diagram of Shell-form Construction

Windings

The windings consist of the current-carrying conductors wound around the sections of the

core, and these must be properly insulated, supported, and cooled to withstand operational

and test conditions.

Copper and aluminum are the primary materials used as conductors in power-

transformer windings. While aluminum is lighter and generally less expensive than copper, a

larger cross section of aluminum conductor must be used to carry a current with similar

performance as copper. Copper has higher mechanical strength and is used almost

exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly

acceptable. In cases where extreme forces are encountered, materials such as silver-bearing

copper can be used for even greater strength. The conductors used in power transformers are

typically stranded with a rectangular cross section, although some transformers at the lowest

ratings may use sheet or foil conductors. Multiple strands can be wound in parallel and joined

together at the ends of the winding, in which case it is necessary to transpose the strands at

various points throughout the winding to prevent circulating currents around the loop(s)

created by joining the strands at the ends. Individual strands may be subjected to differences

in the flux field due to their respective positions within the winding, which create differences

in voltages between the strands and drive circulating currents through the conductor loops.

Proper transposition of the strands cancels out these voltage differences and eliminates or

greatly reduces the circulating currents. A variation of this technique, involving many

rectangular conductor strands combined into a cable, is called continuously transposed cable

(CTC).

A view of Pancake Winding

In core-form transformers, the windings are usually arranged concentrically around the core

leg, which shows a winding being lowered over another winding already on the core leg of a

three- phase transformer. Shell-form transformers use a similar concentric arrangement or an

interleaved arrangement.

With an interleaved arrangement, individual coils are stacked, separated by insulating barriers

and cooling ducts. The coils are typically connected with the inside of one coil connected to

the inside of an adjacent coil and, similarly, the outside of one coil connected to the outside

of an adjacent coil. Sets of coils are assembled into groups, which then form the primary or

secondary winding.

When considering concentric windings, it is generally understood that circular windings have

inherently higher mechanical strength than rectangular windings, whereas rectangular coils

can have lower associated material and labor costs. Rectangular windings permit a more

efficient use of space, but their use is limited to small power transformers and the lower range

of medium- power transformers, where the internal forces are not extremely high. As the

rating increases, the forces significantly increase, and there is need for added strength in the

windings, so circular coils, or shell-form construction, is used.

In some special cases, elliptically shaped windings are used. Concentric coils are typically

wound over cylinders with spacers attached so as to form a duct between the conductors and

the cylinder. As previously mentioned, the flow of liquid through the windings can be based

solely on natural convection, or the flow can be somewhat controlled through the use of

strategically placed barriers within the winding. This concept is sometimes referred to as

guided liquid flow.

A variety of different types of windings have been used in power transformers through the

years. Coils can be wound in an upright, vertical orientation, as is necessary with larger,

heavier coils; or they can be wound horizontally and placed upright upon completion. As

mentioned previously, the type of winding depends on the transformer rating as well as the

core construction.

Safety

Safety is of primary concern when working around a transformer. The substation transformer

is usually the highest voltage item in a facility’s electrical distribution system. The higher

voltages found at the transformer deserve the respect and complete attention of anyone

working in the area. A 6.6 kV system will arc to ground over 1.5 to 2.5 in. However, to

extinguish that same arc will require a separation of 15 in. Therefore, working around

energized conductors is not recommended for anyone but the qualified professional. The best

way to ensure safety when working around high voltage apparatus is to make absolutely

certain that it is de-energized.

• Although inspections and sampling can usually be performed while the transformer is

in service, all other service and testing functions will require that the transformer is

de-energized and locked out. This means that a thorough understanding of the

transformer’s circuit and the disconnecting methods should be reviewed before any

work is performed.

• A properly installed transformer will usually have a means for disconnecting both

the primary and the secondary sides; ensure that they are opened before any work is

performed. Both disconnects should be opened because it is possible for generator or

induced power to back feed into the secondary and step up into the primary. After

verifying that the circuit side-energized at the source, the area where the work is to be

performed should be checked for voltage with a “hot stick” or some other voltage

indicating device.

• It is also important to ensure that the circuit stays de-energized until the work is

completed. This is especially important when the work area is not in plain view of the

disconnect. Red or orange lock-out tags should be applied to all breakers and

disconnects that will be opened for a service procedure. The tags should be highly

visible, and as many people as possible should be made aware of their presence before

the work begins.

• Some switches are equipped with physical locking devices (a hasp or latch). This is

the best method for locking out a switch. The person performing the work should

keep the key at all times, and tags should still be applied in case other keys exist.

• After verifying that all circuits are de-energized, grounds should be connected

between all items that could have a different potential. This means that all conductors,

hoses, ladders another equipment should be grounded to the tank, and that the tank’s

connection to ground should be verified before beginning any work on the

transformer. Static charges can be created by many maintenance activities, including

cleaning and filtering. The transformer’s inherent ability to step up voltages and

currents can create lethal quantities of electricity.

• The inductive capabilities of the transformer should also be considered when working

on ade-energized unit that is close to other conductors or devices that are energized. A

de-energized transformer can be affected by these energized items, and dangerous

currents or voltages can be induced in the adjacent windings.

• Most electrical measurements require the application of a potential, and these

potentials can be stored, multiplied, and discharged at the wrong time if the proper

precautions are not taken. Care should be taken during the tests to ensure that no one

comes in contact with the transformer while it is being tested. Set up safety barriers,

or appoint safety personnel to secure remote test areas. After a test is completed,

grounds should be left on the tested item for twice the duration of the test, preferably

longer.

• Once the operation of the transformer is understood, especially its inherent ability to

multiply voltages and currents, then safety practices can be applied and modified for

the type of operation or test that is being performed. It is also recommended that

anyone working on transformers receive regular training in basic first aid, CPR, and

resuscitation.

Nameplate Data

The transformer nameplate contains most of the important information that will be needed in

the field. The nameplate should never be removed from the transformer and should always be

kept clean and legible

A Wye Delta Transformer Nameplate

Although other information can be provided, industry standards require that the following

information be displayed on the nameplate of all power transformers:

a. Serial Number: The serial number is required any time the manufacturer must be

contacted for information or parts. It should be recorded on all transformer

inspections and tests.

b. Class: The class will indicate the transformer’s cooling requirements and increased

load capability.

c. kVA Rating: The kVA rating, as opposed to the power output, is a true indication of

the current carrying capacity of the transformer. kVA ratings for the various cooling

classes should be displayed. For three phase transformers, the kVA rating is the sum

of the power in all three legs.

d. Voltage Rating: The voltage rating should be given for the primary and secondary,

and for all tap positions.

e. Temperature Rise: The temperature rise is the allowable temperature change from

ambient that the transformer can undergo without incurring damage.

f. Polarity (single phase): The polarity is important when the transformer is to be

paralleled or used in conjunction with other transformers.

g. Phasor Diagrams: Phasor Diagrams will be provided for both the primary and the

secondary coils. Phasor diagrams indicate the order in which the three phases will

reach their peak voltages, and also the angular displacement (rotation) between the

primary and secondary

h. Connection Diagram: The connection diagram will indicate the connections of the

various windings, and the winding connections necessary for the various tap

voltages.

i. Percent Impedance: The impedance percent is the vector sum of the transformer’s

resistance and reactance expressed in percent. It is the ratio of the voltage required to

circulate rated current in the corresponding winding, to the rated voltage of that

winding. With the secondary terminals shorted, a very small voltage is required on the

primary to circulate rated current on the secondary. The impedance is defined by the

ratio of the applied voltage to the rated voltage of the winding. If, with the secondary

terminals shorted, 138volts are required on the primary to produce rated current flow

in the secondary, and if the primary is rated at 13,800 volts, then the impedance is 1

percent. The impedance affects the amount of current flowing through the transformer

during short circuit or fault conditions.

j. Impulse Level (BIL): The impulse level is the crest value of the impulse voltage the

transformer is required to withstand without failure. The impulse level is designed to

simulate a lightning strike or voltage surge condition. The impulse level is a withstand

rating for extremely short duration surge voltages. Liquid-filled transformers have an

inherently higher BIL rating than dry-type transformers of the same kVA rating.

k. Weight: The weight should be expressed for the various parts and the total.

Knowledge of the weight is important when moving or un tanking the transformer.

l. Insulating Fluid: The type of insulating fl.uid is important when additional fluid

must be added or when unserviceable fluid must be disposed of. Different insulating

fluids should never be mixed. The number of gallons, both for the main tank, and for

the various compartments should also be noted.

m. Instruction Reference: This reference will indicate the manufacturer’s publication

number for the transformer instruction manual.