Ntpc tranning report

A

TRAINING REPOTOn

“TRAINING AND VISIT TO PLANT”

Submitted to

CHHATTISGARH SWAMI VIVEKANAND TECHNICAL

UNIVERSITY

BHILAIIn partial fulfillment of requirement for award

Of

BACHELOR IN ENGINEERING

In

ELECTRICAL AND ELECTRONICS

By

VIMLESH DEWANGAN

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ACKNOWLEDGEMENT

It is very difficult to prepare a project report of such a nature because of limited time. But

every time we feel encouraged because the whole staffs and executives of the company

who have helped us by providing the much-required information about the company, its

operations and have helped in structuring and completion of the project.

we feel deep sense gratitude towards Operation department, electrical department who

has Provided us invaluable help cooperation, all sorts of guidance and continuous

advice from time to time without which it would have been impossible to complete this

training.

Our special thanks to all members and staff of the NTPC Limited Electrical Depts. For

their competent guidance and cooperative nature and friendly spirit that supported us

throughout the whole length of the project work. Without their help this project would

not have possible.

And above all a heart full thanks to, our beloved Parents and our Teachers for

providing us support and cooperation in completion of this project.

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PREFACE

Theoretical knowledge is the fundamental weapon for any management student. But apart

from theoretical studies we need to experience a deeper insight into the practical aspects

of those theories by working as a part of organization during our summer training.

Training is a period where a student can apply his theoretical knowledge on practical

field.

Primarily practical knowledge and theoretical knowledge have a very vast difference. So

this training has high importance as to know how both the aspects can be applied

together.

The training session helps to get details about the working process in the organization. It

has helped me to know about the organizational management and discipline, which has

its own importance. The training is going to be a lifelong experience.

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CONTENTSINTRODUCTION TO KSTPP...........................................................................................

ABOUT THE COMPANY ……………………………………………………………..

The company

Installed Capacity

Globalization

WORKING OF A POWER PLANT ……………………………………………………

Fuel Processing

Feed Water Heating and Dearation

Conversion of Water to Steam by boiler

Generation of Electricity by

1. TURBINE

Steam Condensing

Steam-Water Cycle

2. GENERATOR

3. ELECTRICAL SYSTEM

Switchgear / Switch yard

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ABOUT NTPC Limited

NTPC Limited is the largest power generation company in India. Forbes Global 2000

for 2009 ranked it 317th in the world. It is an Indian public sector company listed on the

Bombay Stock Exchange although at present the Government of India holds 84.5%(after

divestment the stake by Indian government on 19 october,2009) of its equity. With a

current generating capacity of 43,128 MW, NTPC has embarked on plans to become a

1,28,000 MW company by 2032. It was founded on November 7 1975. NTPC's core

business is engineering, construction and operation of power generating plants and

providing consultancy to power utilities in India and abroad.

The total installed capacity of the company is 311 MW (including JVs) with 15 coal

based and 7 gas based stations, located across the country. In addition under JVs, 3

stations are coal based & another station uses naphtha/LNG as fuel. By 2017, the power

generation portfolio is expected to have a diversified fuel mix with coal based capacity of

around 53000 MW, 10000 MW through gas, 9000 MW through Hydro generation, about

2000 MW from nuclear sources and around 1000 MW from Renewable Energy Sources

(RES). NTPC has adopted a multi-pronged growth strategy which includes capacity

addition through green field projects, expansion of existing stations, joint ventures,

subsidiaries and takeover of stations.

NTPC has been operating its plants at high efficiency levels. Although the company has

18.79% of the total national capacity it contributes 28.60% of total power generation due

to its focus on high efficiency. NTPC’s share at 31 march 2013of the total installed

capacity of the country was 24.51% and it generated 29.68% of the power of the country

in 2008-09. Every fourth home in India is lit by NTPC. 170.88BU of electricity was

produced by its stations in the financial year 2005-2006. The Net Profit after Tax on

March 31, 2006 was INR 58,202 million. Net Profit after Tax for the quarter ended June

30, 2006 was INR 15528 million, which is 18.65% more than for the same quarter in the

previous financial year, 2005.

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Pursuant to a special resolution passed by the Shareholders at the Company’s Annual

General Meeting on September 23, 2005 and the approval of the Central Government

under section 21 of the Companies Act, 1956, the name of the Company "National

Thermal Power Corporation Limited" has been changed to "NTPC Limited" with effect

from October 28, 2005. The primary reason for this is the company's foray into hydro and

nuclear based power generation along with backward integration by coal mining.National

Thermal Power (NTPC) the 138 position in 2009, 10 Indian companies make it to FT's top

500.

Future Goals

The company has also set a serious goal of having 50000 MW of installed

capacity by 2012 and 75000 MW by 2017. The company has taken many steps like step-

up its recruitment, reviewing feasibilities of various sites for project implementations etc.

and has been quite successful till date.

Power Burden

India, as a developing country is characterized by increase in demand for

electricity and as of moment the power plants are able to meet only about 60-75% of this

demand on a yearly average. The only way to meet the requirement completely is to

achieve a rate of power capacity addition (Implementing power projects) higher than the

rate of Demand addition. NTPC strives to achieve this and undoubtedly leads in sharing

this burden on the country.

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NTPC Headquarters

NTPC devided in 7 Headquarters

.

Sr. No. Headquarters City

1 NCRHQ Noida

2 ER-I, HQ Patna

3 ER-II, HQ Bhubaneshwar

4 NER Luchknow

5 SR HQ Hyderabad

6 WR HQ I Mumbai

7 WR HQ II Raipur

NTPC Plants1.Thermal based

Sr.

No.City State MW

1 Singrauli Uttar Pradesh 2,000

2 Korba Chhattisgarh 2,600

3 Ramagundam Andhra Pradesh 2,600

4 Farakka West Bengal 2,100

5 Vindhyachal Madhya Pradesh 4,260

6 Rihand Uttar Pradesh 3,000

7 Kahalgaon Bihar 2,340

8 NCTPP, Dadri Uttar Pradesh 1,820

9 Talcher Kaniha Orissa 3,000

10 Unchahar Uttar Pradesh 1,050

11 Talcher Thermal Orissa 460

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12 Simhadri Andhra Pradesh 2,000

13 Tanda Uttar Pradesh 440

14 Badarpur Delhi 705

15 Sipat Chhattisgarh 2980

16 Mauda Maharashtra 1000

17 Barh Bihar 660

TOTAL 33,015

2.Coal Based (Owned by JVs)

Sr. No. City State MW

1 Durgapur West Bengal 120

2 Rourkela Orissa 120

3 Bhilai Chhattisgarh 574

4 Kanti Bihar 220

5 Jhajjar Haryana 1500

6 Vallur Tamil Nadu 1500

Total 4,034

3.GAS based

Sr. No. City State MW

1 Anta Rajasthan 419.33

2 Auraiya Uttar Pradesh 663.36

3 Kawas Gujarat 656.20

4 Dadri Uttar Pradesh 829.78

5 Jhanor Gujarat 657.39

6 Rajiv Gandhi Kerala 359.58

7 Faridabad Haryana 431.59

Total 4017.23

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NTPC Hydel

The company has also stepped up its hydel projects implementation. Currently the

company is mainly interested in the North-east India wherein the Ministry of power in

India has projected a Hydel power feasibility of 3000 MW. Run of the river Hydro

Project

There are few run of the river hydro projects are under construction on tributary of

Ganga. In which 3 are being made by NTPC Limited. These are:

1. Loharinag Pala Hydro Power Project by NTPC Ltd: In Loharinag Pala Hydro Power

Project with a capacity of 600 MW (150 MW x 4 Units). The main package has been

awarded. The present executives' strength is 100+. The project is located on river

Bhagirathi(Tributory of Ganga) in Uttarkashi district of Uttarakhand state. This is 1st

project in downstream from origin of Ganges at Gangotri.

2. Tapovan Vishnugad 520MW Hydro Power Project by NTPC Ltd: In joshimath city.

3. Lata Tapovan 600MW Hydro Power Project by NTPC Ltd: Also in Joshimath (Under

Environmental Revision).

4. Koldam Hydro Power Project 800MW in Himachal Pradesh (130 km from

Chandigarh).

5. Amochu in Bhutan.

AWARDS AND ACCOLADES

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

has identified NTPC as one of the jewels of Public Sector ‘Maharatnas’ – a potential

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global giant. Inspired by its glorious past and vibrant present, NTPC is well on its way to

realize its vision of being “A world class integrated power major, powering India’s

growth, with increasing global presence”.

NTPC has received the International Project Management Award 2005 for its

Simhadri Project at the International Project Management Association World

Congress. NTPC is the only Asian company to receive this award.

NTPC was recipient of Golden Peacock Environment Management Award

instituted by the “World Environment Foundation” for the year 2006.

NTPC was ranked as Third Great Place to Work for in India for the second time

in succession by a survey conducted by Grow Talent and Business World 2005.

NTPC was awarded MOU Award for Excellence in performance for 2003-04 and

ranked first amongst the top ten Public Sector Enterprises.

NTPC has received the award for Innovative HR Practices at world HR Congress

in February, 2006.

NTPC has bagged the Platt’s global energy award 2005 for the “Community

development Program of the Year”.

NTPC has bagged the BML Munjal Award for encouraging Learning and

Development and using it as a strategic HR tool.

NTPC Korba

NTPC Korba Super Thermal Power Project is one of the most prestigious

flagships of NTPC striving ahead to bridge the country generation gap especially in the

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western region. . NTPC is the sixth largest thermal power generator in the World and

second most efficient utility in terms of capacity utilization based on data of 1998.

The station is located in Korba district in Chhattisgarh in the east south side of the

country. It has secured ISO 14001 and ISO 9002 certificate in the field of environment

and power generation but also in various other fields.).

It has won number of awards from Government of India for proper utilization and

consumption and has bagged the safety awards presented by U.S.A and British Safety

Council.

Coal Source - Kusmunda block, gevra mines

Fuel Oil Source - Indian Oil Corporation (IOC), COLD (Customer operated

lubricant and oil deposit).

Water Source - Hasdeo River

Beneficiary States -Madhya Pradesh, Chattisgarh, Maharashtra, Gujarat,

Goa, Daman, Diu & Nagar Haveli

Units Commissioned

Unit -I 200 MW March 1983

Unit -II 200 MW October 1983

Unit -III 200 MW March 1984

Unit -IV 500 MW May 1987

Unit -V 500 MW March 1988

Unit -VI 500 MW March 1989

Unit -VII 500 MW December 2010

GLOBALISATION

Globalisation has brought significant advantages to countries and business around the

world but the benefits have spread unequally both within and among countries. While the

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rules favouring global market expansion have grown more robust, the rules intended to

promote equally valid social objectives viz. in the areas of human rights, labour standards

and environment lag behind and in some cases actually have become weaker.

In order to promote Corporate Social Responsibility and citizenship in the new global

marketplace, UN Secretary General, Mr. Kofi Annan first proposed the Global Compact

at Davos in Jan'99. It was thus created to help organisations redefine their strategies and

course of actions so that all people can share the benefits of globalisation, not just a

fortunate few.

The Global Compact’s operational phase was launched at UN Headquarters in New York

on 26 July 2000. and has since then focussed its efforts on achieving practical results and

fostering the engagement of business leaders in the direction.

Through the power of collective action, the Global Compact seeks to promote responsible

corporate citizenship so that business can be part of the solution to the challenges of

globalisation. In this way, the private sector – in partnership with other social actors – can

help realize the Secretary-General’s vision: a more sustainable and inclusive global

economy.

The Global Compact is a network. At its core are the Global Compact Office and six UN

agencies:

Office of the High Commissioner for Human Rights

United Nations Environment Programme

International Labour Organization

United Nations Development Programme

United Nations Industrial Development Organization

United Nations Office on Drugs and Crime

WORKING OF A POWER PLANT

ENERGY GENERATION:

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Typical diagram of a coal-fired thermal power station

1. Cooling tower 10. Steam Control valve 19. Super heater

2. Cooling water pump11. High pressure steam

turbine

20. Forced draught (draft)

fan

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

4. Step-up transformer (3-phase) 13. Feed water heater 22. Combustion air intake

5. Electrical generator (3-phase) 14. Coal conveyor 23. Economizer

6. Low pressure steam turbine 15. Coal hopper 24. Air preheater

7. Condensate pump 16. Coal pulverizer 25. Precipitator

8. Surface condenser 17. Boiler steam drum 26. Induced draught (draft)

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fan

9. Intermediate pressure steam

turbine18. Bottom ash hopper 27. Flue gas stack

A modern boiler has capacity of burning pulverized coal at rates up to 200 tones an hour

(32000 metric ton per day). From the coal store, fuel is carried on a conveyor belt and

discharged by means of a coal tipper into the bunker. It then falls perhaps through a

weigher into the coal pulverizing mill where it is grounded to a powder as fine as flour.

The mill usually consists of a round metal table on which large steel rollers or balls are

positioned. The table revolves, forcing the coal under the rollers or balls which crush it.

Air is drawn from the top of the boiler house by the Forced Draught (FD) Fan and

passed through the air preheaters, to the hot air duct. From here some of the air passes

directly to the burners and the remainder is taken through the Primary Air (PA) Fan to

pulverizing mill, where it is mixed with powdered coal, blowing it along pipes to burners

of the furnace. Here, it mixes with the rest of the air and burns with great heat.

The boiler consists of a large number of tubes extending the full height of the structure

and the heat produced raises the temperature of the water circulating in them to create

stem which passes to the steam drum at very high pressure. The steam is then heated

further in the super heater and fed through the outlet valve to the high pressure cylinder

of the steam turbine. It may be hot enough to make the steam pipe glow a dull red

(around 540°C).

When the steam has been through the first cylinder (High Pressure) of the turbine, it is

returned to the boiler and reheated before being passed through the other cylinder

(Intermediate and Low Pressure) of the turbine.

From the turbine the steam passes into a condenser to be turned back into water called

‘condensate’. This is pumped through feed heaters (where it may be heated to about

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250°C) to the economizer where the temperature is raised sufficiently for the condensate

to be returned to the lower half of the steam drum of the boiler.

The flue gases leaving the boiler are used to reheat the condensate in the economizer and

then passes through the air –preheater, to the Electrostatic Precipitor (ESP). Finally,

they are drawn by the Induced Draught (ID) Fan into the main flue and to the chimney.

The ash is either sold for use in road and building constructions or piped as slurry of ash

and water to a settling lagoon, where the water drains off. Once this lagoon (which may

originally have been a worked out gravel pit) has been filled, it can be returned to

agricultural use, or the ash removed for other purposes.

The electrostatic precipitator consists of metal plates which are electrically charged .Dust

and Grit in the flue gases are attracted on to these plates, so that they do not pass up the

chimney to pollute the atmosphere. Regular mechanical hammer blows cause the

accumulations of ash, dust and grit to fall to the bottom of the precipitator, where they

collect in a hopper for disposal. Additional accumulations of ash also collect in the

hoppers beneath the furnace.

Conversion of Steam to mechanical power :

From the boiler, a steam pipe conveys steam to the turbine through a stop valve (which

can be used to shut off steam in an emergency) and through control valves that

automatically regulate the supply of the steam to the turbine. Stop valve and control

valves are located in a steam chest and a governor, driven from the main turbine shaft,

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operates the control valves to regulate the amount of steam used. (This depends upon the

speed of the turbine and the amount of electricity required from the generator).

Steam from the control valves enters the high pressure cylinder of the turbine, where it

passes through a ring of stationary blades fixed to the cylinder wall. These act as nozzles

and direct the steam onto a second ring of moving blades mounted on a disc secured to

the turbine shaft .This second ring turns the shafts as a result of the force of the steam.

The stationary and moving blades together constitute a ‘stage’ of the turbine and in

practice many stages are necessary, so that the cylinder contains a number of rings of

stationary blades with rings of moving blades arranged between them. The steam passes

through each stage in turn until it reaches the end of the high pressure cylinder and in its

passage some of its heat energy is changed into mechanical energy.

The steam leaving the high pressure cylinder goes back to the boiler for reheating and

returns by further pipe to the intermediate pressure cylinder. Here it passes through

another series of stationary and moving blades.

Finally ,the steam is taken to the low pressure cylinders, each of which it enters at the

centre flowing outwards in opposite directions through the rows of turbine blades – an

arrangement known as double flow – to the extremities of the cylinder. As the steam

gives up its heat energy to dive the turbine, its temperature and pressure fall and it

expands .Because of this expansion and blades are much larger and longer towards the

low pressure ends of the turbine.

The turbine shaft usually rotates at 3000 revolutions per minute. This speed is

determines by the frequency of the electricity system used in this country and is the speed

at which a 2- pole generator must be driven to generate alternating current at a frequency

of 50 /cycles per second.

When as much energy as possible has been taken from the steam it is exhausted directly

to the condenser. This runs the length of the low pressure part of the turbine and may be

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beneath or on either side of it. The condenser consists of a large vessel containing some

20,000 tubes, each about 25 mm in diameter. Cold water from river, estuary, sea or

cooling tower is circulated through these tubes and as the steam from the turbine passes

round them it is rapidly condensed into water – condensate .Because water has a much

smaller comparative volume than steam, a vacuum is created in the condenser. This

allows the steam to be used down to pressures below that of the normal atmosphere and

more energy can be utilized.

From the condenser, the condensate is pumped through low pressure feed heaters by the

extraction pump, after which its pressure is raised to boiler pressure by the boiler feed

pump. It is passed through further feed heaters to the economizer and the boiler for

reconversion into steam.

Where the cooling water for power station s is drawn from large rivers, estuaries or the

coast, it can be returned directly to the source after use. Power stations situated on

smaller rivers and inland do not have such vast water resources available, so the cooling

water is passed through cooling towers (where its heat is removed by evaporation) and

re- used.

A power station generating 2000000kw of electricity required about 227,500 cubic

meters water an hour for cooling purposes. Where cooling towers are used, about one

hundredth part of its source to carry away any impurities that collect. Most of it, however,

is recalculated.

Switching and transmission:

The electricity is usually produced in the stator windings of large modern generators at

about 25000 volts and is fed through terminal connections to one side of a generator

transformer, that steps up the voltage to 132kv or 400kv. From here conductors carry it

to a series of three switches comprising an isolator, a circuit –breaker (CB) and another

isolator.

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The circuit- breaker, which is heavy – duty switch capable of operating in a fraction of a

second, is used to switch off the current flowing to the transmission lines. Once the

current has been interrupted the isolators can be opened. These isolate the CB from all

outside electrical sources, so that there is no chance of any high voltages being applied to

its terminal s. maintenance or repair work can then be carried out in safety.

From the CB the current is taken to the bus bars – conductors which run the length of the

switching compound- and then to another CB with its associated isolators, before being

fed to the grid .Each generator in a power station has its own transformer, CB and

associated isolators but the electricity generated is fed on to a common set of bus bars.

CB’s work like combined switches and fuses but they have certain special features and

are very different from the domestic switch and fuse. When electrical current is switched

off by separating two contacts, an arc is created between them. At the voltage used in the

home, this arc is very small and only lasts for a fraction of a second but at very high

voltage s used for transmission ,the size and power of the arc is considerable and it must

be quickly quenched to prevent damage.

One type of CB has its contact immersed in insulating oil so that when the switch is

opened ,either by powerful electrical coils or mechanically by springs the arc is quickly

extinguished by the oil .Another type works by compressed air which operates the

switch and at the same time ‘blows out ’the arc.

Three wires are used in a three phase system for large power transmission as it is cheaper

than two wire ‘single phase’ system that supplies the home. The centre of the power

station is control room .Here engineer monitor the output of the electricity, supervising

and controlling the operation of generating plant and high voltage switch- gear and

directing power to the grid system as required .Instruments on the control panels show

the output and conditions which exist on all the main plant and a miniature diagram

indicates the precise state of the electrical system.

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Coal handling plant:-

As we all know, the coal and water are the main inputs for power generation. the thermal

energy of coal is processed and converted to electricity .For 2260 MW VSTPP stage-I

&II, we need on an average 34000 MT of coal a day; which means we are entrusts the

tandem task of handling, processing and feeding approx.11 million MT of coal in a year.

In CHP, coal is received at track hopper from mines through BOBR Wagons. The

unloaded coal is scooped into conveyor & subjected to further process of removal of

extraneous material & crushing to -20 mm size. After crushing, the coal again screened

for elimination of extraneous materials, weighed and sent to boiler bunkers. Excess coal,

if any, is sent to coal yard for stacking.

During this process, the coal is passed through suspended magnet, magnetic separators,

metal detectors, belt weighers to ensure that sized coal, free of foreign material is

supplied to the power station.

The coal supply is from mines of Northern Coal Fields Ltd., coal industry being labour

intensive and open cast mining is done, the coal supply varies over a wide band through-

out the year. During summer, under scorching sun and in rainy season due to water entry

in mines and slippery road, the coal production goes down and remains highly unstable.

Coal production is at peak normally during November-March. However, the coal

requirement for the

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power station is more or less uniform. This makes the job of coal handling plant,

challenging.

The coal yard is open .In peak time, the coal stock goes up to 8 lacks MT. The coal is

known for spontaneous combustion. To prevent this, coal yard management has to be

done properly. The coals heaps are sprayed with water and compacted by running

Dozers. This prevents air pockets in coal heaps, helps in fire protection and preserve

volatile materials to maintain calorific value of the fuel.

The coal conveyors work as a chain. The start & stop of conveyors are linked with

preceding/succeeding conveyors. If a conveyor trips, all the preceding conveyors have to

get tripped immediately. Any failure of protection or delayed tripping will result in huge

coal spillage. This makes the protections and interlocks more vital and important in CHP.

It is worthwhile to mention that total conveyor length is above 10 KMs and manual

surveillance everywhere is quiet difficult and cumbersome job and heights.

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ROTARY PLOUGH FEEDERS:

The function of the rotary plough feeder is to feed coal to conveyor from the flow table of

track hopper at controlled rate.

To begin operation of plough feeder hydraulic system, the main electric motor and cooler

electric motor are turned on. Both hydraulic pumps should be in neutral position or no

flow condition-when electric motors are switched on. The rotor and traverse are

stationary.

The rotor pump is controlled via rotary servo levers, catching spring-return cylinder is

connected to the servo lever. This stroking cylinder is controlled by two solenoid valves

which are mounted on common manifold. The rotor pump stroke valve is a double

solenoid directional valve which controls the rotor acceleration and de-acceleration. The

rotor fast stop valve is used to stop the rotor immediately at any time.

A small amount of oil flow is taken from the rotor pumps integral charge pump and used

to control the stroking cylinder, when the stroke valve B-solenoid is energized and oil

flow is directed towards the base end of the stroking cylinder. The extension of the

cylinder acts on the rotor pump lever and brings the pump on stroke and this in turn

provides oil flow to the rotor hydraulic motor. A sun needle valve is sandwiched

underneath the stroke valve to control the amount of oil flow to the cylinder. This

controlled flow allows for a certain rate of extension of cylinder, which results in a

metered increase of the rotor pump flow. The acceleration of the rotor is therefore,

controlled by the needle valve and the rotor speed is determined by the length of time the

stroke valve B-solenoid is energized. The longer the solenoid is energized, the further the

stroking cylinder extends and the higher the pump flow.

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Once the stroke valve B-solenoid is energized long enough for the required rotor speed,

that solenoid is de energized, bringing the stroke valve spring centered to its neutral

position. The oil flow that has been delivered to extend the stroking cylinder to its

required position is then locked in place by a pilot operated check valve. This valve is

also sandwiched underneath the stroke valve. Since this oil is trapped with the stroking

cylinder in the required position, the rotor will continue at the specified rate.

The rotor may be de accelerated in the same way to lower speed as it is accelerated.

During de-acceleration the stroke valve A-solenoid is energized. This opens the check

valve and allows the oil that is in stroking cylinder to flow to the tank, retracting the

cylinder, resulting in decreasing the pump flow and slowing down the rotor speed. The

rate of de-acceleration is controlled by needle valve. The speed to which rotor slows

down is determined by how long the stroke valve A-solenoid is energized. Continued

energization of the stroke valve A-solenoid will bring the rotor pump back to its neutral

and no flow condition and stop the rotor

When the rotor is running at certain speed and is required to be stopped immediately, the

rotor fast stop valve is energized, which in turn drains oil from stroking cylinder to the

tank directly, bypassing the needle valve. When the fast stop solenoid is energized, the

rotor will stop at once.

When the main electric motor is stopped or when the motor pump has been brought to its

neutral position by either the stroke valve A-solenoid or the cast stop solenoid, the stroke

valve B-solenoid must be energized to accelerate the rotor back up to the desired speed.

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Boiler:-

Boiler is device for generating steam for power processing or heating purposes. Boiler is

designed to transmit heat from an external combustion source contained within the boiler

itself.

Boilers may be classified on the basis of any of the following characteristics:

1. Uses: The characteristics of the boiler vary according to the nature of service

performed. Customarily Boilers are called either stationary or mobile.

2. Pressure: To provide safety control over construction features, all boilers must be

constructed in accordance with the Boiler Codes which differentiates boilers as

per their characteristics.

3. Materials: Selection of construction materials is controlled by boiler code material

specifications.

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4. Size: Rating core for boilers standardize the size and ratings of boilers based on

heating surfaces. The same is verified by performance tests

5. Tube Contents: In addition shell type of boiler, there are 2 general steel boiler

classifications, the fire tube and water tube boilers.

6. Firing: Te boiler may be a fired or unfired pressure vessel.

7. Heat Source: The heat may be derived from

1. The combustion of fuel

2. The hot gases of other chemical reactions

3. The utilization of nuclear energy

8. Fuel: Boilers are often designated with respect to the fuel burned.

9. Fluid: The general concept of a boiler is that of a vessel that is to generate a

steam.

10. Circulation: The majority operate with natural circulation. Some utilize positive

circulation in which the operative fluid may be forced ‘once through’ or

controlled with partial circulation.

11. Furnace position: The boiler is an external combustion device in that the

combustion takes place outside the region of boiling water. The relative location

of the furnace to the boiler is indicated by the description of the furnace as being

internally or externally fired. The furnace is internally fired if the furnace region

is completely surrounded by water cooled surfaces. The furnace is externally fired

if the furnace is auxiliary to the boiler.

12. Categorization of boilers: Boilers are generally categorized as follows:

A. Steel Boilers

I. Fire Tube type

II. Water Tube type

i. Natural Circulation

ii. Positive Circulation

III. Shell type

B. Cast Iron Boilers

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C. Special Design Boilers

D. Nuclear Reactors

13. Boiler classification according to end use.

Boilers can be classified into 2 categories viz,

i. Utility Boilers

ii. Industrial Boilers

Boiler accessories:-

Boiler furnace: A boiler furnace is that space under or adjacent to a boiler in which fuel

is burned and from which the combustion products pass into the boiler proper. It provides

a chamber in which the combustion reaction can be isolated and confined so that the

reaction can be isolated and confined so that the reaction remains a controlled force. It

provides support or enclosure for the firing equipments

Boiler Drum: The function of steam drum is to separate the water from the steam

generated in the furnace walls and to reduce the resultant solid contents of the steam to

below the prescribed limit of 1ppm. The drum is located on the upper front of the boiler.

Economizer: The purpose of the economizer is to preheat the boiler feed water before it

is introduced into the steel drum by recovering the heat from the fuel gases leaving the

boiler. The economizer in the boiler rear gas passes below the rear horizontal super

heater.

Super Heater: There are 3 stages of super heater besides the side walls and extended side

walls. The first stage consists of horizontal super heater of convection mixed flow type

with upper and lower banks located above economizer assembly in the rear pass. The 2 nd

stage super heater consists of pendant platen which is of radiant parallel flow type. The

3rd stage super heater pendant spaced is of convection parallel flow type the outlet

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temperature and pressure of the steam coming out form the super heater is 540 ºC and

157 kg/cm2.

Pre-heater: The function of preheater is to reheat the steam coming out from high

pressure turbine to a temperature of 540 ºC.

Burners: there are total 24 pulverized coal burners for corner fired C.E. type boilers and

12 oil burners provided each in between 2 pulverized fuel burners.

Igniters: There are 12 side Eddy plate oil/ H.E.A igniters per boiler. The atomizing air for

igniters is taken from plant air compressor at 7 kg/cm. There are 2 igniter air fans supply

air for combustion of igniter oil. Mainly 2 types of igniters are used:-

Eddy Plate Igniter

High Energy Arc Type Igniter

HT & LT SYSTEM:-

High Tension System:

It involves the operation of various ht motors. These 6.6 KV motors are feeded by the HT

buses charged from UTA or the station transformers (when the unit is tripped). They

consumes around 8-10% of the total MW generated w.r.t. one unit. The various HT

equipments are listed below:

Primary Air (PA) Fan: Its function is to blow the crushes coal from ball mill to furnace

through pipes. It operates using three type of air:- hot air, cold air and atmospheric air.

Forced Draft (FD) Fan: Its function is to enable easy combustion of grounded coal in

furnace. It sucks from the atmospheric air which gets heated in the air heaters and then

26

sent to the boiler .It also supplies hot air to PA Fan if required as per the atmospheric

conditions.

Induced Draft (ID) Fan: The air heater receives heat from the boiler and hence the air

accompanied there contains a huge amount of ash. This air is then passed through ESP

and finally exhausted through the chimney with the help of ID fans.

Boiler Feed Pump(BFP):Its function is to lift the condensed steam ,passed through the

Low Power Heaters (LPH) and Deaerater up to the boiler drum via High Power Heaters

(HPH) and Economizer, for the redistribution in water walls.

Circulating Water Pump (CWP): Its function is to circulate cold water from the

cooling tower in the condenser.

Condensate Extraction Pump: Its function is to extract the condensed steam from the

condenser.

Make Up Water Pump: Its function is to maintain an optimum level of cooling water in

the condenser.

Bowl Mill: Its function is to grind the crushed coal from the bunkers for the supply to the

boiler.

Low Tension System:

It involves the operation of various LT motors. These are generally 415 V motors and are

feeded by the LT buses. These LT buses are charged from the HT switchgear via 6.6/

0.415 KV transformers. Since these buses feed many of those motors, which should never

be shut down, therefore they have arrangement for charging from the HT switchgear of a

different unit (when a particular unit is tripped) or from the DG room (when there is

complete black out). Some of the LT equipments are listed below:

27

Seal Oil Pump: Its function is to prevent the hydrogen filled between the stator and the

rotor for cooling purpose from leaking out by maintaining a pressure higher than that of

filled hydrogen in the surrounding.

Barring Oil Pump: It lubricates the barring gear required for rotating the turbine rotor,

with a speed of 3-5 RPM minimum, when the unit is tripped. It functions only when the

unit tripped.

Starting Oil Pump: Its function is to provide lubrication at the time of starting of a unit

because it deals with around 18-20 kg of oil and hence also serve the purpose of

providing a starting torque.

Main Oil Pump: It provides lubrication after the required speed of 3000 RPM is attained

i.e. when the unit starts functioning healthily.

28

TURBO GENERATOR (TG):-

The AC generator or alternator is based upon the principle of electromagnetic induction

and consists generally of a stationary part called stator and a rotating part called rotor.

The stator housed the armature windings. The rotor houses the field windings. D.C.

Voltage is applied to the field windings through slip rings. When the rotor is rotated, the

line of magnetic flux cut through the stator windings. This induces an electromagnetic

force in the stator windings. The magnitude of this e.m.f is given by:

29

E = 4.44φ FN volts

Φ = strength of magnetic field in Weber.

F = frequency in cycles per second or hertz.

N = number of turns in a coil of stator winding

F = frequency = Pn/120

Where P=number of poles

n= revolution per second of rotor.

From the expression it is clear that for the same frequency, number of poles increase s

with decrease in speed and vice versa. Therefore, low speed hydro turbine drives

generators have 14 to20 poles where as high speed steam turbine driven generators have

30

generally 2 poles. Pole rotors are used in low speed generators, because the cost

advantage as well as easier construction.

Armature windings

Direct current machines are always constructed with armature windings of the closed

type, such as the grammering winding or lap- or wave- wound drum winding. Such

windings may also be used in a.c. machines, as when such a closed winding is provided

with taps and slip rings for single phase or polyphase operation.

In general ,however the windings of ac machines are of the open type three windings

spaced 120° electrical degrees apart and connected in Y, constitute an open-type winding

and this is the arrangement generally used; but if the three coils were connected in Δ, the

winding would be of the closed type.

The chief characteristics of a.c. windings are defined by such features as:

the number of phase;

the number of circuits in parallel per phase, which may be one or more;

the connections between phases, which may be star or mesh(Y or Δ in three phase

machines);

the number of coil layers per slot, which may be either one or two-layer type

predominating;

the angular spread of the consecutive conductors belonging to a given phase belt;

the pitch of the individual coils comprising the winding ; and

the arrangement of the end connections.

1. Stator Frame

The stator frame consists of a cylindrical center section and two end shields which are

gas tight and pressure-resistant. 

The stator frame accommodates the electrically active parts of the stator, i.e. the stator

core and the stator windings. Both the gas ducts and a large number of welded circular

ribs provide for the rigidity of the stator frame Ring-shaped supports for resilient core

31

suspension are arranged between the circular ribs The generator coolers subdivided into

cooler sections arranged vertically in the turbine side stator end shield. In addition, the

stator end shields contain the shaft Seal and bearing components.

2. Stator Core

The stator core is stacked from insulated electrical sheet steel laminations and mounted in

supporting rings over insulated dovetailed guide bars Axial compression of the stator core

is obtained by clamping fingers, pressure plates, and nonmagnetic through-type clamping

bolts, which are insulated from the core The supporting rings form part of an inner frame

cage, This cage is suspended in the outer frame by a large number of separate flat springs

distributed over the entire core length, The flat springs are tangentially arranged on the

circumference in sets with three springs each, i.e. two vertical supporting springs on both

sides of the core and one horizontal stabilizing spring below the core. The, springs are so

arranged and tuned that forced vibrations of the core resulting from the magnetic field

will not be transmitted to the frame add foundation. 

The pressure plates and end portions of the stator core are effectively shielded against

stray magnetic fields, The flux shields era cooled by a flow of hydrogen gas directly over

the assembly.

3. Stator winding

Stator bars, phase connectors and bushings are designed for direct Water cooling. In

order to minimize the stray losses, the bars are composed of separately insulated strands

which are transposed by 540° in the slot portion and bonded together with epoxy resins in

heated moulds. After bending, the end turns are likewise bonded together with baked

synthetic resin fillers. 

The bars consist of hollow and solid strands distributed over the entire bar cross-section

so that good heat dissipation is ensured. At the bar ends, all the solid strands are jointly

brazed into a Connecting sleeve and the hollow strands into a water box from which the

32

cooling water enters and exits via Teflon insulating hoses connected to the annular

manifolds. The electrical connection between top and bottom bars is made by a bolted

connection at the connecting sleeve.

The water manifolds are insulated from the stator frame, permitting the insulation

resistance of the water-filled winding to be measured. During operation, the water

manifolds are grounded.

High-voltage insulation is provided according to the proven Micalastic system. With this

insulating system, several half-overlapped continuous layers of mice tape are applied to

the bars. The mice tape is built up from large area mica splitting which are sandwiched

between two polyester backed fabric layers with epoxy as an adhesive. The number of

layers, i.e., the thickness of the insulation depends on the machine voltage. The bars era

dried under vacuum and impregnated with epoxy resin which has very good penetration

properties due to its low viscosity. After impregnation under vacuum, the bars are

subjected to pressure, with nitrogen being used as pressurizing medium (VPI process).

The impregnated bars are formed to the required shape in moulds and cured in an even at

high temperature. The high-voltage insulation obtained is nearly void-free and is

characterized by its excellent electrical, mechanical and thermal properties in addition to

being fully Waterproof and oil-resistant. To minimize corona discharges between the

insulation and the slot wall, a final coat of semi-conducting varnish is applied to the

surfaces of all bars within the slot range. In addition, all bars are provided with an end

corona protection to control the electric field at the transition from the slot to the end

winding and to prevent the formation of creepage spark concentrations.

4. Rotor Shaft

The rotor shaft is a single piece mild forging manufactured from a vacuum casting. Slots

for insertion of the field winding are milled into the rotor body. The longitudinal slots are

distributed over the circumference so that two solid poles are obtained. The rotor poles

are designed with transverse slots to reduce twice system frequency rotor vibrations

caused by deflections in the direction of the pole and neutral axis.

33

  5. Rotor Winding

The rotor winding consists of several coils which are inserted into the slots and series

connected such that two coil groups form one pole. Each coil consists of several series

connected turns, each of which consists of two half turns which are connected by brazing

in the end Section. 

The rotor winding consists of silver-bearing de-oxidized copper hollow conductors with

two lateral cooling ducts. L-shaped strips of laminated epoxy glass fibre fabric with

Nomex filler are used for slot insulation. The slot wedge, are made of high-conductivity

material and extend below the shrink seat of the retaining ring. The seat of the retaining

ring is silver-plated to ensure a good electrical contact between the Slot wedges and rotor

retaining rings. This system has long proved to be a good damper winding.

6. Retaining Rings

The centrifugal forces of the rotor end windings are contained by single-piece rotor

retaining rings. The retaining rings are made of non-magnetic high-strength steal in order

to reduce stray losses. Each retaining ring with its shrink-fitted insert ring is shrunk onto

the rotor body in an overhung position. The retaining ring is secured in the axial position

by a snap ring.

SPECIFICATION OF 500MW GENERATOR USED IN KORBA PLANT:

KVA 588000

For stator

Volt 2000

Amp 16200

For rotor

Volt 340

Amp 4040

Rpm of stator 3000

34

Frequency 50

Connection x x

Coolant water and hydrogen

Working period 3-4 year

Gas pressure 3.5 bar

Rotor cooling hydrogen (forced)

Stator water cooling(forced)

HYDROGEN COOLING SYSTEM

The hydrogen is circulated in the generator interior in a closed circuit by one multi-stage

axial - flow fan arranged on the rotor at the turbine end. Hot gas is drawn by the fan from

the air gap and delivered to the coolers, where it is recooled and then divided into three

flow paths after each cooler:

Flow path 1 is directed into the rotor at the turbine end below the fan hub for cooling of

the turbine end half of the rotor

Flow path II directed from the coolers to the individual frame compartments for cooling

of the stator cam.

Flow path III is directed to the stator end winding space at the exciter end through guide

ducts in the frame for cooling of the exciter end half of the rotor and of the core end

portions,

The three flows mix in the air gap. The gas is then returned to the coolers via the axial-

flow fan.

35

The cooling water flow through the hydrogen coolers should be automatically controlled

to maintain a uniform generator temperature level for various loads and cold water

temperatures,

A. Cooling of Rotor 

For direct cooling of the rotor winding, cold gas is directed to the rotor end windings at

the turbine and exciter ends. The rotor winding is symmetrical relative to the generator

center line and pole axis. Each coil quarter is divided into two cooling zones. The first

cooling zone consists of the rotor end winding and the second ore of the winding portion

between the rotor body end and the mid-point of the rotor. Cold gas is directed to each

cooling, zone through separate openings directly before the rotor body end. The hydrogen

flows through each individual conductor in closed cooling ducts. The heat removal

capacity is selected such that approximately identical temperatures are obtained for all

conductors. The gas of the first cooling zone is discharged from the coils at the pole

center into a collecting compartment within the pole area below" the end winding. From

there the hot gas passes into the air gap through pole few slots at the end of the rotor

body. The hot gas of the second cooling zone is discharged into the air gap at mid length

of the rotor body through radial openings in the hollow conductors and wedges.

B. Cooling of Stator Core

For cooling of the stator core, cold gas is admitted to the individual frame compartments

via separate dealing gas ducts. 

From these frame compartments the gas then flows into the air gap through slots in the

core where it absorbs the heat from the core. To dissipate the higher losses in the core

ends, the cooling gas slots we closely spaced in the core end sections to ensure effective

cooling. These ventilating ducts are supplied with cooling gas directly from the end

winding space. Another flow path is directed from the stator end winding space pat the

36

clamping fingers between the pressure plate and core end section into the air gap. A

further flow path passes into the air gap along either side of the flux shield. 

All the flows mix in the air gap and cool the rotor body and state, bare surfaces. The gas

is then returned to the coolers via the axial flow fan to ensure that the cold gas directed to

the exciter end cannot be directly discharged into the air gap, an air gap choke is arranged

within the range of the stator end winding cover and the rotor retaining ring at the exciter

end.

Primary Cooling Water Circuit in the Generator 

The treated water used for cooling of the stator winding, phase connectors and bushings

is designated a primary water in order to distinguish it from the secondary coolant (raw

water, condensate, etc). The primary water is circulated in a closed circuit and dissipates

the absorbed heat to the secondary cooling water in the primary water cooler. The pump

is supplied with hot primary water from the primary water tank and delivers the water to

the generator via the coolers. The cooled water flow is divided into two flow paths m

described in the following paragraphs.

Flow-path 1 cools the stator windings. This flow path first passes to a water manifold

on the exciter end of the generator and from them to the stator bars via insulated hoses.

Each individual bar is connected to the manifold by a separate hose. Inside the bars the

cooling water flows through hollow strands. At the turbine end, the water is passed

through similar h~% to another water manifold and then returned to the primary water

tank. Since a single pass water flow through the stator is used, only a minimum

temperature rise is obtained for both the coolant and the ban. Relative movements due to

different thermal expansions between the top and bottom bars are thus minimized.

Flow path 2 cools the phase connectors and the bushings. The bushing and phase

connectors consist of thick, walled copper tubes through which the cooling water is

circulated. The six bushings and the phase connectors arranged in a circle around the

37

stator end winding are hydraulically interconnected m that three parallel flow paths are

obtained. The primary water enters three bushings and exits from the three remaining

bushing.

The secondary water flow through the primary water cooler should be controlled

automatically to maintain a uniform generator temperature level for various loads and

cold water temperatures.

BEARING

The sleeve bearings are provided with hydraulic shaft lift oil during startup and turning

gear operation. To eliminate shaft currents, all bearings are insulated from the stator and

base plate, respectively. The temperature of the bearings is monitored with

thermocouples embedded in the lower bearing sleeve so that the measuring points are

located directly below the babbitt. Measurement and any required recording of the

temperatures are performed in conjunction with the turbine supervision. The bearings

have provisions for fitting vibration pickups to monitor bearing vibrations.

DC SUPPLY SYSTEM

The dc supply requirement can be classified in two categories depending upon the type of

loads:

1. For emergency auxiliary rise which are not in operation while the unit is running but

have to be switched on in case of A.C. supply failure. The requirement for d.c. lub oil

pump, seal oil pump, jacking oil pump, scanner air fan etc. Along with dc emergency

lighting can be classified in this category.

2. Loads in which continuous supply is required control and protection supply for switch

gear, indications, annunciation system, communication systems and DAS etc.

38

Float charger is sized to supply the continuous DC load in addition to the float current

requirement of batteries. Current requirement is met by the batteries in case of DC motor

operation. In case of AC

supply failure the battery system will automatically feed the load .to charge the batteries

initially or in case of complete discharge of (after feeding DC load during AC supply

failure), the batteries are required to be charges with boost charger ,the boost changer is

capable of meeting high voltage /current requirements of batteries . In addition to this the

periodic equalizing charge requirement is also met by boost chargers. It is essential to

disconnect the batteries from load prior to boost charging so as to avoid the damage to

DC equipments due to high voltage supplied by boost charger.

39

EXCIATATION SYSTEM OF THE TURBO GENRATOR (TG)

Excitation energy for the Turbo Generator (TG) is obtained from a separate excitation

source using a thyristor exciter, which provides the controlled rectifier current to feed the

field winding. The thyristor (service) exciter consists of an auxiliary a.c. generator

mounted on the TG shaft and two thyristor converters cooled by distillate from the TG

stator cooling circuit.

Either of the converters is arranged in a 3-φ bridge circuit. The converters are connected

in parallel and they function simultaneously. Each converter has its own individual

thyristor firing control system, which is interconnected through the circuits for

synchronizing of firing pulses applied to the two converter arms. Due to this

interconnection, uniform sharing of load current between the parallel–operating

converters is provided and besides, each thyristor firing control system duplicates the

other if loss of supply voltage occurs.

If the service exciter fails, the TG field excitation can be provided from the standby

exciter. For this purpose, the use is made of a separately installed set consisting of a d.c.

generator and an a.c. driving motor.

The auxiliary generator is of the self –excitation type. Power to the field winding is

obtained from the rectifier transformer connected to generator stator winding and the

thyristor converter cooled by natural air circulation. Application of the auxiliary

generator field flashing is accomplished by means of short- time connection of a 220 V

separate power source. When applied, the separate power source provides a build up of

the generator terminal voltage up to 10-20 % of the rated value, there upon the connected

thyristor bridge starts to function and promotes a build up generator terminal voltage up

to the rated value.

Changing automatically or manually the firing angle of the thyristors in the converters

accomplishes control of the TG filed excitation. With a decrease of the auxiliary voltage

40

to 80 % of the rated value or in the case of its complete loss, the regulator and thyristor

firing control system are supplied with back up power from a 220 V storage battery. With

the restoration of the auxiliary voltage to 84% of the rated value, the back-up power

supplies are blocked.

41

42

TransformerThe transformers used in a power station have its sides abbreviated as Low Voltage (LV)

and High Voltage (HV) rather than primary and secondary.

Major transformers in a power station

Generator transformer (GT):- The generator is connected to this transformer by means

of isolated bus duct. This transformer is used to step up the generating voltage of 15.75

KV or 21 KV (depending on the generator) to grid voltage normally 400 kV. This

transformer is generally provided with OFAF cooling.

Unit Auxiliary Transformer (UAT):-

The UAT draws its input from the main bus duct connecting generator to the generator

transformer. It is used for the working of large devices such as boilers, heavy motors etc.

The total kVA capacity of UAT required can be determined by assuming 0.85 p.f. and

η=0.9 for total auxiliary motor load. For large units, it has become necessary to use more

43

than one auxiliary transformer. It uses the generated 15.75kV or 21 kV to covert into 6.6

kV.

The maximum short circuit currents on auxiliary bus should be limited with in the

maximum switch gear rating available. The maximum permissible voltage dip while

starting the largest single auxiliary motor, usually boiler feed pump, shall remain within

acceptable limits.

Station Transformer: The station transformer is used to feed the power to the auxiliaries

during the start UPS. This transformer normally rated for the initial auxiliary load

requirements of unit. In physical cases this load is of order of 60% of the load at full

generating capacity. It is also provided with on load tap changer to cater to the fluctuating

voltage of the grid.

ICT (Inter Connecting Transformer): It connects 400KV substation to 132 KV

substation.

CPT (Construction Power Transformer): This is the transformer which gives the

output for construction in which the voltage required is 220 V.

Cooling of transformer:

Heat is produced in the winding due to the current flowing in the conductors (I2R) and in

the core on account of eddy currents and hysterisis losses. In small dry type transformer

heat is dissipated directly to the atmosphere. In oil immersed transformer heat is

dissipated by thermo siphon action.

The purpose of using oil is:-

1. Cooling: Provides a better cooling and helps in exchanging heat

2. Insulation: A non conductor of electricity so good insulator.

The oil used is such that its flash point is pretty high so that it doesn’t have any

possibility to catch fire.

There various types of cooling:-

44

AN - Air Natural

ON - Oil Natural

AF - Air forced

OF - Oil forced

ONAF - Oil natural Air forced

OFAN -Oil forced Air natural

OFAF - Oil forced Air forced

The oil serves as the medium for transferring the heat produced inside the transformer to

the outside transformer. Thermo Siphon action refers to the circulating currents set up in

a liquid because of temperature difference between one part of the container and other.

When oil gets heated up the oil with greater temperature goes to the upper side of the

transformer. Now, if it is Oil natural it is cooled in it as is whereas in Oil Forced, a

radiator is being constructed and a pump is being attached to it to pull the oil from the

upper part of the transformer.

Now this oil in the chamber gets cooled either by direct heat exchanging through the

atmosphere which is called Air Natural or by forced air draft cooling by a radiator with

many electric fans which are automatically switched on and off depending upon the

loading of transformer which is known as Air Forced cooling.

As the oil gets cooled it becomes heavier and sinks to the bottom.

45

Transformer accessories:

i. Conservator: With the variation of temperature there is corresponding variation

in the oil volume. To account for this an expansion vessel called conservator is

added to the transformer with a connecting pipe to the main tank. It is also used to

store the oil and make up of the oil in case of leakage.

ii. Breather: In conservator the moisture from the oil is excluded from the oil

through breather it is a silica gel column, which absorbs the moisture in air before

it enters the conservator air surface. Normally dehydrating gel is blue in

appearance after the saturation it turns into red.

46

iii. Radiator: This a chamber connected to the transformer to provide cooling of the

oil. It has got fans attached to it to provide better cooling.

Cause of Failure of Transformer:-

Insulation Failures – Insulation failures were the leading cause of failure in this study.

This category excludes those failures where there was evidence of a lightning or a line

surge. There are actually four factors that are responsible for insulation deterioration:

pyrolosis (heat), oxidation, acidity, and moisture. But moisture is reported separately. The

average age of the transformers that failed due to insulation was 18 years.

Design /Manufacturing Errors - This category includes conditions such as: loose or

unsupported leads, loose blocking, poor brazing, inadequate core insulation, inferior short

47

circuit strength, and foreign objects left in the tank. In this study, this is the second

leading cause of transformer failures.

Oil Contamination – This category pertains to those cases where oil contamination can

be established as the cause of the failure. This includes sludging and carbon tracking.

Overloading - This category pertains to those cases where actual overloading could be

established as the cause of the failure. It includes only those transformers that

experienced a sustained load that exceeded the nameplate capacity.

Fire /Explosion - This category pertains to those cases where a fire or explosion outside

the transformer can be established as the cause of the failure. This does not include

internal failures that resulted in a fire or explosion.

Line Surge - This category includes switching surges, voltage spikes, line

faults/flashovers, and other T&D abnormalities. This significant portion of transformer

failures suggests that more attention should be given to surge protection, or the adequacy

of coil clamping and short circuit strength.

Maintenance /Operation - Inadequate or improper maintenance and operation were a

major cause of transformer failures, when you include overloading, loose connections

and moisture. This category includes disconnected or improperly set controls, loss of

coolant, accumulation of dirt & oil, and corrosion. Inadequate maintenance has to bear

the blame for not discovering incipient troubles when there was ample time to correct it.

Flood – The flood category includes failures caused by inundation of the transformer due

to man-made or natural caused floods. It also includes mudslides.

Loose Connections - This category includes workmanship and maintenance in making

electrical connections. One problem is the improper mating of dissimilar metals, although

this has decreased somewhat in recent years. Another problem is improper torquing of

48

bolted connections. Loose connections could be included in the maintenance category,

but we customarily report it separately.

Lightning - Lightning surges are considerably fewer in number than previous studies we

have published. Unless there is confirmation of a lightning strike, a surge type failure is

categorized as “Line Surge”.

Moisture - The moisture category includes failures caused by leaky pipes, leaking roofs,

water entering the tanks through leaking bushings or fittings, and confirmed presence of

moisture in the insulating oil. Moisture could be included in the inadequate maintenance

or the insulation failure category above, but we customarily report it separately.

TRANSFORMER AGING :-

Notice that we did not categorize "age" as a cause of failure. Aging of the insulation

system reduces both the mechanical and dielectric-withstand strength of the transformer.

As the transformer ages, it is subjected to faults that result in high radial and compressive

forces. As the load increases, with system growth, the operating stresses increase. In an

aging transformer failure, typically the conductor insulation is weakened to the point

where it can no longer sustain mechanical stresses of a fault. Turn to turn insulation then

suffers a dielectric failure, or a fault causes a loosening of winding clamping pressure,

which reduces the transformer's ability to withstand future short circuit forces.

Oil contamination can be regularly checked with periodic test (DGA) and regular

monitoring of data.

Transformer Protection:-

There are two types of protections:

Mechanical

Electrical

49

Mechanical Protection:

I. Pressure regulating valve: Transformer tank is a pressure vessel as the inside

pressure can group steeply whenever there is a fault in the windings and the

surrounding oil is suddenly vaporized. Tanks as such are tested for the pressure

with stand capacity of 0.35 kg/cm to prevent bursting of tank and thus the

catastrophe; these tanks in addition are provided with expansion vents with a thin

diaphragm made of bakelite/copper/ glass at the end. This diaphragm is the

Pressure Relief Device/ Expansion Vent which senses the pressure and releases

the valve when the pressure is more than the specified limit.

II. Bucholz’s relay: This has 2 floats, one of them with surge catching baffle and gas

collecting space at top. This is mounted in the connecting pipe line between

conservator and main tank. Gas evolution at a slow rate, which is associated with

minor fault inside the transformers, gives rise to the operation or top float whose

contacts are wired for alarm. There is a glass window with marking to read the

volume of gas collected in the relay. Any major fault in the transformer creates a

surge element in the relay trips the transformer, size of the relay varies with oil

volume in the transformer and the mounting angle also is specified for proper

operation of the relay.

50

III. Temp. Indicators: Most of the transformers are provided with indicators that

displace oil temperature and winding temperature there are thermometers pockets

provided in the tank top cover which hold the sensing bulls in them. Oil

temperature measured that of the top oil, where as the winding temperature

measurement is indirect. This is done by adding the temperature rise due to the

heat produced in a heater coil when a current proportional to that following in

windings is passed in it to that or top oil. For proper functioning of OTI and WTI

it is essential to keep the thermometers pocket clean and filled with oil.

Nowadays, the temp. in the transformer is measured by a device called RTD

(Resistance Temp. Detector). This works on the principle that the change in

resistance is directly proportional to the change in temp. And thus, the temp. is

monitored by keeping track of the resistance.

IV. Other protections: Other protections are also there like oil level, oil temp, oil

flow, pressure etc.

51

Electrical Protection:

I. Magnetizing current: The magnetizing current is the minimum amount of

current required to setup the required flux or in other words the min. current to

overcome the permeability of the winding. Now this test is done to check the

healthiness of the winding, if the amount of current is in the specified limit, then

the coil is said to be healthy.

II. Core Balance: In this test a voltage (400V) is applied to one of the phases of the

winding. Now on the same side the voltage is checked for the other two phases,

they should be in the specified limit and the more imp. Point to be noted is that

they should sum up to the applied voltage as the total mmf is const. The same test

is repeated for all the 3 phases of both sides. This also checks the healthiness of

the coil.

III. Insulation Resistance: This test is done to check whether the insulation of the

windings is proper or not. The resistance of the insulation of the winding is

measured and checked with the specified values. If there is damage in the

insulation, it can be easily tracked by checking the resistance value for the

insulation of that winding.

IV. Winding Resistance: This test is done to check whether there is any internal fault

in the winding. If there is any short circuit in the winding of any of the phases, the

value of the resistance will get decreased for that winding. Having a short in one

of the phases will result into unequal voltages in the 3 phases which is not

desirable.

V. Transformer turns ratio test: In this test we measure the transformer turns ratio.

A 400 V supply is given to one of the LV side of the transformer and the voltage

is noted on the HV side, now by the relation N2/N1 = V2/V1 we check the turn ratio

52

N2/N1. This is a very important test because if the turns ratio is not correct then the

output voltage would deflect from the desired value.

VI. Tanδ test: This is not the power angle δ rather this is the load angle that is the

angle between the load and the resistive part. So this value is desired to be very

low.

Tanδ for

transfer

10°C 20°C 30°C 40°C 50°C 60°C 70°C

Upto 220

kV(%)

1.8 2.5 3.5 5 7 10 14

Upto 500

kV (%)

1 1.3 1.6 2 2.5 3.2 4

The transformer used in the stage 1 (210MW) of the power plant is a 3 – phase

transformer with Δ – Υ connection i.e. Δ on L.V. side and Υ on H.V. side. The reason

for doing so is that the 3rd harmonic component of the voltage doesn’t appear in the line

voltage in a 3 – phase Υ connection.

The type of cooling used in the transformer is OFAF

Rating HV - 250MVA

Rating LV - 250 MVA

No load voltage HV – 420KV

No load voltage LV – 15.75 KV

Line current HV – 343.66 A

Line current LV – 9164.29 A

Oil quantity – 48790 L || 42450kg

The transformers used in stage 2 are single phase transformers that 3 single phase

transformers. The rating there is 600 MVA out of which the real power output is 500

53

MW. The input in this case is 21 kV. The reason for using 3 different transformers in this

case is due to the high power rating.

To reduce the losses the core is made up of a special type of material which is CRGO

(Cold Rolled Grain Oriented) steel which is further laminated to reduce the eddy current

losses.

DISOLVED GAS ANALYSIS (DGA)

When performing DGA, it is important to differentiate between combustible gases and

non-combustible gases. Though significant amounts of non-combustible gases and the

problems they create are common in transformers when fluids are exposed to air in the

headspace in the tank, they do not pose a safety hazard. On the other hand, large

quantities of combustible gases in transformer fluid and the headspace above the fluid

could cause fire and explosion. In most cases, combustible gases, or fault gases, occur in

very small quantities when oil or paper insulation breaks down. However, when thermal

and electrical stresses exceed the design or operational limits, fault gases can form in

significant volumes. The type and severity of the abnormal condition have the greatest

impact on what kind of fault gases form and how quickly they accumulate.

Insulating materials within transformers and related equipment break down to liberate

gases within the unit. The distribution of these gases can be related to the type of

electrical fault and the rate of gas generation can indicate the severity of the fault. The

identity of the gases being generated by a particular unit can be very useful information

in any preventative maintenance program. This technique is being used quite successfully

throughout the world. This paper deals with the basics underlying this technique and

deals only with those insulating fluids of mineral oil origin.

54

Obvious advantages that fault gas analyses can provide are:

1. Advance warning of developing faults

2. Advance warning of developing faults

3. Status checks on new and repaired units

4. Convenient scheduling of repairs

5. Monitoring of units under overload

The following sections will deal with the origins of the fault gases, methods for their

detection, interpretation of the results, and philosophies on the use of this technique.

Some limitations and considerations that should be kept in mind concerning the use of

this technique will also be discussed.

Fault Gases

The causes of fault gases can be divided into three categories; corona or partial discharge,

pyrolysis or thermal heating, and arcing. These three categories differ mainly in the

intensity of energy that is dissipated per unit time per unit volume by the fault. The most

severe intensity of energy dissipation occurs with arcing, less with heating, and least with

corona.

A partial list of fault gases that can be found within a unit are shown in the following

three groups:

1. HYDROCARBONS AND HYDROGEN

Methane CH4

Ethane C2H6

Ethylene C2H4

Acetylene C2H2

Hydrogen H2

2. Carbon oxides

Carbon monoxide CO

55

Carbon dioxide CO2

3. Non-fault gases

Nitrogen N2

Oxygen O2

These gases will accumulate in the oil, as well as in the gas blanket of those units with a

head space, as a result of various faults. Their distribution will be effected by the nature

of the insulating materials involved in the fault and the nature of the fault itself.

The major (minor) fault gases can be categorized as follows by the type of material that is

involved and the type of fault present:

1. Corona

a. Oil H2

b. Cellulose H2 , CO , CO2

2. Pyrolysis

a. Oil

Low temperature CH4 , C2H6

High temperature C2H4 , H2 ( CH4 , C2H6 )

b. Cellulose

Low temperature CO2 ( CO )

3. Arcing H2, C2H2 (CH4, C2H6, C2H4)

56

INTERPRETATION OF DGA RESULTS AND DIGNOSTICS METHODS

This technique of incipent fault diagnosis is by far the most accurate and reliable. The

various methods of data interpretation are being regularly refined and are received and

discussed with enthusiasm professional gatherings. The latest developments have been

published in technical periodicals.

Review of the most commonly used gas-in-oil diagnostic methods:

1) IEEE C57.104-1991

2) Doernenburg Ratios

3) Rogers Ratios Method

4) IEC 599

5) Duval Method

6) GE Method

Catching small problems before they become big is critical to keeping your transformers

operational, and dissolved gas analysis is an increasingly viable option for preventing

failure in liquid-cooled transformers. Although advances in preventative maintenance

have yet to yield a technique as reliable for dry-type transformers, DGA is making

transformer maintenance easier and more effective at uncovering potential failure.

57

CONTINUOUS MONITORING OF KEY FAULT GASES (H2 AND CO2)

Hydrogen (H2) and carbon monoxide (CO) are common denominators to faults causing

the breakdown of dielectric oil and cellulosic insulation.

The continuous monitoring of these two gases provides a basic element in the monitoring

and management of the life and performance of transformers.

The HYDRAN technology, developed in 1974, proven and used worldwide for the first

and only effectively on line fault gas monitoring. it provides the necessary real –time

protection from rapid, short-term evolving type faults .it used proven techniques which

continuously monitor the two key fault gases (H2 for the detection of fault degrading oil

and co for fault degrading cellulose) and generates alarm output when preset gas alarm

levels are reached .these alarm levels are determined from a previously established DGA

baseline for H2+CO.

HYDRAN technology is an IEEE recognized (IEEE std .C57.104-1991) method of

monitoring for incipient fault characteristics in power transformers. In the 20 years this

technology has been commercially available and successfully applied to power

transformers in the field, it has saved an estimated $200M in transformer capital in

vestments and countless $ millions in lost revenues and collateral damages.

58

Switchgear

The equipment which normally fall in this category are

Isolators

Switching Isolators

Circuit Breakers (CB)

Load Break Switches

Earth Switches

An isolator is one which can break an electric circuit when the circuit is to be switched

on load. These are normally used in various circuits for the purpose of isolating a certain

portion when required for maintenance etc.

59

Switching isolators are capable of

I. Interrupting transformer magnetized currents

II. Interrupting line charging current and

III. Load transfer switching

Its main application is in connection with transformer feeders as the unit makes it

possible to switch out one transformer while the other is still on load.

A circuit breaker (CB) is one which can break or make the circuit on load and even on

faults. The equipment is most important and is a heavy duty equipment mainly utilized

for protection of the various circuits and operation at load. Normally circuit breakers are

installed accompanied by isolators.

Load break switches are those interrupting devices which can makes or break ckts at 8*

rated current. These are normally installed on the same circuit or on the circuits which are

backed up by circuit breakers.

Earth switches are devices which are normally used to earth a particular system to avoid

accident, which may happen due to induction on account of live adjoining ckts. These do

not handle any appreciable current at all.

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ISOLATOR

The most common form of isolators is the rotating centre post type in which each phase

has three insulators post, with the outer posts carrying fixed contacts & connections while

the centre post having the contact arm which is arranged to move through 90° on its axis.

The isolators are driven by an operating mechanism box normally installed near the

ground level. The box has the operating mechanism in addition to its control ckt, and

auxiliary contacts. The operating mechanism may be solenoid operated pneumatic or

simple motorized system. Motorized operating mechanism generally consists of a.c. three

phase motor or d.c. motor transmitting through a sturdy spur gear to the torsional shaft of

the isolator.

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Center Break Isolator

CIRCUIT BREAKER (CB)

There are different ways of classifying CB. These are:

Medium method

i. Bulk oil CB

ii. Minimum oil CB

iii. Air blast CB

iv. Sulphur hexa-fluoride (SF6)CB

Air blast circuit breaker

Operating mechanism

i. Spring operated ckts

ii. Solenoid operated CB

iii. Pressure operated CB

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Arc interruption

The main requirement of a CB is that it shall be capable of making and breaking the

current associated with any dimensions. These requirements are met by interrupters. Its

two types are:

1) Air blast interrupter

2) Oil interrupter

Air blast interrupter: The power for extinguishing the arc is drawn from an external

source and its magnitude must be such as to interrupt the maximum current. As such if

the magnitude of fault is less the same should be interrupted even before the current

reaches its natural zero, here heat is conducted away from the arc until current zero,

causing very rapid de-ignition and ultimately replacing arc path by a column of

compressed air of very high di-electric strength.

Oil breaker interrupter: In this type, extinguishing power is obtain from the arc itself.

The arc decomposed the oil and vaporized it into hydrogen, acetylene, and small

proportion of other hydrocarbon .Hydrogen, because of its high thermal conductivity and

de- igniting property, assist in cooling the arc at the same time as the pressure within the

enclosure is built up due to the restricted venting. These final arc extinctions are achieve

by rapidly cooling and de-igniting of the gas and expelling the arc product from the

control device, resulting in the rapid built up of dielectric strength.

Oil breaker

These CB normally are of single break type. These comprise of two sections. One upper

compartment the arc control device and fixed and moving contacts and a lower

supporting compartment to arc control device is contact in a blacklisted paper enclosure

which is in turned housed in a porcelain insulator.

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Air blast breakers

In this, the interrupters are insulated from earth, by means of parcelin insulator. The

number being determined by the system voltage .To air supply blast pipe to the interrupt

unit is placed inside the support insulator the interrupter unit may be mounted on above

the other and fed via by pass blast pipes or own braches from a common point at the top

of the support insulator. The whole of the operating mechanism of the ckt form an

electrically operated trip coil Isolation .In this type of is achieved by keeping the

interrupter open and the contact gas is permanently pressurized the loss of air in

pressurized cb will result in either its reclosure or loss or dielectric strength across the

open contact such an occurrence could prove disastrous to the system and it , as therefore

been arranged that an isolator associated the pressurized cb opens automatically after the

cb has been tripped.

64

Sulphur hexafluoride (SF6) CB

The principle of current interruption is similar to that of an air blast CB it does not,

therefore, represent a new conception of circuit breaking but simply employs a new arc

extinguishing medium namely SF6. The success of the cb depends solely on the high arc

interrupting performance of this gas i.e. when it is broken down under electrical stress it

will very quickly reconstitute itself. It is five times heavier than air and has

approximately twice the di-electric strength. The CB is completely sealed and operates as

a closed system which means that no flame is emitted during operation and the noise

level is considerably reduced.

65

Earth switches:

Earth switches in the switch yard are simple mechanically operated switches, the

purpose of which is to earth the bus if required for the purpose eliminating induced

voltage in the particular bay on account of parallel running live conductors. It is always

accompanied by an auxiliary switch to provide interlock and indication contact.

The following interlocks are provided with isolators:

Isolators cannot operate unless the breaker is open.

Bus I & II isolators cannot be closed simultaneously.

This interlock can be by-passed in the event of closing of bus coupler breaker.

No isolator can be operated when corresponding earth switch is on.

Only one bay can be taken on bypass bus.

Switchgear protection

Voltage Transformer Supervision (VTS)

The VTS feature is used to detect failure of the ac voltage inputs to the relay. This may

be caused by internal VT faults, overloading, or faults on the interconnecting wiring to

relays. This usually results in one or more VT fuses blowing. Following a failure of the

ac voltage input there would be a misrepresentation of the phase voltages on the power

system, as measured by the relay, which may result in mal-operation.

The VTS logic in the relay is designed to detect the voltage failure, and automatically

adjust the configuration of protection elements whose stability would otherwise be

compromised. a time –delayed alarm output is also available.

There are three main aspects to consider regarding the failure of the VT supply. These are

defined below:

1) Loss of one or two phase voltages

2) Loss of all three phase voltages under load conditions

3) Absence of three phase voltages upon line energisation

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Directional earth fault protection (DEF)

Method of directional polarizing selected is common to all directional earth fault

elements, including channel aided element. There are two options available in relay

menu:

1) Zero sequence polarizing: Relay performs directional decision by comparing phase

angle of residual current w.r.t. inverted residual voltage:

(- Vres= - (Va+Vb+Vc)) derived by relay

2) Negative sequence polarizing: Relay performs a directional decision by comparing

phase angle of derived NPS current w.r.t. derived NPS voltage. Even though directional

decision is based on phase relationship of I2 w.r.t. V2, operating current quantity for DEF

elements remains derived residual current.

Application of Zero sequence polarizing

This is conventional option applied where there is not mutual coupling with parallel line

and where power system is not solidly earthed close to relay location. As residual voltage

is generated during earth fault condos this quantity is used to polarize DEF elements.

Relay internally derives this voltage from 3-φ voltage input which must be supplied from

either a 5-limb or 3 single phase VTs. These types of VT design allow presence of

residual flux and permit relay to derive required residual voltage. In addition, primary

star point of VT must be earthed. A 3 limb VT has no path for residual flux and is

therefore not compatible with use of zero sequence polarizing. Typical settings are:

Resistance earthed systems use a 0° RCA setting i.e. for a forward earth fault residual

current is in phase with inverted residual voltage.

When protecting solidly earthed distribution systems or cable feeders, a -45° RCA setting

should be set.

When protecting solidly earthed transmission systems, a -60° RCA setting is set.

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Application of negative sequence polarizing

In certain applications, the use of residual voltage polarization of DEF may either be

difficult to achieve, or may be problematic. An example of the former case would be

where a suitable type of VT is unavailable, for e.g. if only a 3 limb VT were fitted. An

example of latter case will be an HV/EHV parallel line application where problems with

Zero sequence mutual coupling may exist. In either of cases, the problem may be solved

by the use of NPS quantities for polarization. This method determines the fault direction

by comparison of nps voltage to nps current. The operate quantity, however, is still

residual current.

When negative sequence polarizing is used relay requires that Characteristic Angle is set.

The Application Notes section for NPS overcurrent protection better describes how angle

is calculated- typically set at -45° (I2 lags –V2).

Under Voltage Protection

Under voltage conditions may occur on a power system for a variety of reasons, some of

which are outlined below:-

Increased system loading .Generally, some corrective action would be

taken by voltage regulating equipment such as AVR’s or On Load Tap

Changers, in order to bring the system voltage back to it’s nominal value.

If the regulating equipment is unsuccessful in restoring healthy system

voltage, then tripping by means of and undervolatge relay will be required

following a suitable time delay.

Faults occurring on the power system result in a reduction in voltage of

the phases involved in the fault, the proportion by which the voltage

decreases in directly dependent upon the type of fault, method of system

earthing and its location with respect to the relaying point consequently,

co-ordination with other voltage and current –based protection devices is

essential in order to achieve correct discrimination.

68

Power swing blocking (PSB)

Power swings are oscillations in power flow which can follow a power system

disturbance .they can be caused by sudden removal of faults, loss of synchronism across a

power system or changes in direction of power flow as a result of switching. such

disturbances can cause generators on the system to accelerate or decelerate to adapt to

new power flow conditions, which in turn lead s to power swinging .a power swing may

cause the impedance presented to a distance relay to move away from the normal load

area and into one or more of its tripping characteristics .in the case of a stable power

swing, it is important that the relay should not trip. The relay should also not trip during

loss of stability since there may be a utility strategy for controlled system break up during

such as event.

Protection of overhead lines and cable circuits

Overhead lines are amongst the most fault susceptible items in plant in a modern power

system. It is therefore essential that the protection associated with them provides secure

and reliable operation for distribution systems, continuity of supply is of paramount

importance. The majority of faults on overhead lines are transient or semi-permanent in

nature, multi-shot autoreclose cycles are commonly used in conjunction with

instantaneous tripping elements to increase system availability. Thus, high speed fault

clearance is often a fundamental requirement of any protection scheme on a distribution

network. The protection requirements for sub-transmission and higher voltage system s

must also take into account system stability .Where systems are not auto enclosure is

commonly used. This in turn dictates the need for high speed protection to reduce overall

fault clearance times.

Underground cables are vulnerable to mechanical damage, such as disturbance by

construction work or ground subsidence. Also, faults can be caused by ingress of ground

moisture into the cable insulation .or its buried joints. Fast fault clearance is essential to

limit extensive damage and avoid the risk of fire, etc.

69

Many power systems use ear thing arrangements designed to limit the passage of earth

fault current. Methods such as resistance earthing make the detection of earth faults

difficult. Special protection elements are often used to meet such onerous protection

requirements.

Physical distance must also be taken in to account. over head lines can be hundreds of

kilometers in length .If high speed, discriminative protection is to be applied it will be

necessary to transfer information between the line ends .this not only puts the event of

loss so this signal. Thus, back up protection is an important feature of any protection

scheme. In the event of equipment failure, may be of signaling equipment or switch gear,

it is necessary to provide alternative forms of fault clearance. It is desirable to provide

backup protection which can operate with minimum time delay and yet discriminate with

the main protection and protection elsewhere on the system.

Broken Conductor Detection

The majority of faults on a power system occur between one phase and ground or two

phases to ground .These are known as shunt faults and arise from lightning discharges

and other overvoltage which initiate flashovers. Alternatively, they may arise from other

causes such as birds on overhead lines or mechanical damages to cables etc. Such faults

result in an appreciable increase in current and hence in the majority of applications are

easily detectable.

Another type of unbalanced fault which can occur on the system is the series or open

circuit fault. These can arise from broken conductors, mal-operation of single phase

switch-gear, or the operation of fuses. Series faults will not cause an increase in phase

current on the system and hence are not readily detectable by standard overcurrent relays.

However, they will produce an unbalance and a resultant level of Negative Phase

Sequence (NPS) current, which can be detected.

70

It is possible to apply a NPS overcurrent relay to detect the above condition. However on

a lightly loaded line, the NPS current resulting from a series fault condition is close to

full load steady state unbalance arising from CT errors, load unbalance etc. A negative

sequence element therefore would not operate at low load levels.

The relay incorporates an element which measures ratio of NPS to Positive Phase

Sequence (PPS) current (I2/I1).This will be affected to a lesser extent than the

measurement of NPS current alone since ratio is constant with load variations in load

current. Hence, a more sensitive setting may be achieved.

Circuit Breaker Fail Protection (CBF)

Following inception of a fault one or more main protection devices will operate and issue

a trip output to the circuit breaker(s) associated with the faulted circuit.

Operation of the circuit breaker is essential to isolate the fault, and prevent

damage/further damage to the power system. For transmission /sub –transmission

systems,

Slow fault clearance can also threaten system stability .it is therefore common practice to

install circuit breaker failure protection, which monitors that the circuit breaker has

opened within a reasonable time .if the fault current has not been interrupted following a

set time delay from circuit breaker trip initiation ,breaker failure protection (CBF) will

operate.

CBF operation can be used to backtrip upstream CB to ensure that the fault is isolated

correctly. CBF operation can also rest all start output contacts, ensuring that any blocks

asserted on upstream protection are removed.

71

Negative Sequence Overcurrent Protection (NPS)

When applying traditional phase over current protection, the overcurrent element s must

be set higher than maximum load current, there by limiting the element’s sensitivity.

Most protection schemes also use an earth fault element operating from residual current,

which improves sensitivity for earth faults. However, certain faults may arise which can

remain undetected by such schemes.

Any unbalanced fault condition will produce negative sequence current of some

magnitude .thus a negative phase sequence over current element can operate for both

phase – to – phase and phase to earth faults.

The following section describes how negative phase sequence overcurrent protection may

be applied in conjunction with standard over current and earth fault protection in order to

alleviate some less common application difficulties.

Negative phase sequence over current elements give greater sensitivity to resistive phase

–to –phase faults, where phase over current may not operate.

In certain applications, residual current may not be detected by earth fault relay due to the

system configuration .For example, an earth fault relay applied on the delta side of a delta

–star transformer is unable to detect earth faults on the star side. However, negative

sequence current will be present on both side of the transformer for any fault condition,

irrespective of the transformer configuration. Therefore, an negative phase sequence

overcurrent element may be employed to provide time delayed back up protection for any

uncleared asymmetrical fault downstream.

Where rotating machines are protected by fuses, loss of a fuse produces a large amount of

negative sequence current .This dangerous condition for the machine due to the heating

effects of negative phase sequence current and hence an upstream negative phase

sequence overcurrent element may be applied to provide back up protection for dedicated

motor protecting relays.

72

It may be required to simply alarm for the presence of negative phase sequence currents

on the system .Operators may then investigate the cause of unbalance.

Note that in practice ,if the required fault study information is unavailable, the setting

must adhere to minimum threshold previously outlined, employing a suitable time delay

for co-ordination with downstream devices. This is vital to prevent unnecessary

interruption of the supply resulting from in adherent operation of this element.

Where P = number of poles

N = revolution per second of rotor.

From the expression it is clear that for the same frequency, number of poles increase s

with decrease in speed and vice versa. Therefore, low speed hydro turbine drives

generators have 14 to 20 poles where as high speed steam turbine driven generators have

generally 2 poles. Pole rotors are used in low speed generators, because the cost

advantage as well as easier contruction…..

73

CONCLUSION

The training season was very educational and informative. Being a

BHARAT NAVARATNA, this NTPC have good harmonic

relationship and coordination between the staff members. As the

vocational training seem laborious job to get in touch with the

activities. It was nobility of people to provide the information and required theoretical

background at their continuous job hour.

Most of the equipments were technically strong for huge production. Doing training in

NTPC, I hope it would be useful in my future not only in academic but also in

professional carrier. Electricity is much more than just another commodity. It is the life-

blood of the economy and our quality of life. Failure to meet the expectations of society

for universally available low cost power is simply not an option. As the world moves into

the digital age, our dependency on power quality will grow accordingly. The

infrastructure of our power delivery system and the strategies and policies of our ensures

must keep pace with escalating demand.

Unfortunately, with the regulators driving toward retail competition, the utility

business priority is competitiveness (and related cost-cutting ) and not reliability.

74