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BADARPUR THERMAL POWER

STATION

(A UNIT OF NTPC)

BTPS

NAME-MANSI TIWARI

BRANCH-ELECTRICALS

COLLEGE-APEEJAY COLLEGE OF

ENGINEERING,SOHNA

GURGAON

ROLL NO-072023

SEMESTER-IVTHTTHTHTHGTH

SUMMER TRAINING REPORT

SUMMER TRAINING REPORT BTPS

CERTIFICATE

This is to certify that MANSI TIWARI student of bachelor of Technology, Electricals, 3rd Year, APEEJAY COLLEGE OF ENGINEERING (SOHNA,GURGAON) has successfully completed her 34 days industrial Training at Badarpur Thermal Power Station, New Delhi from 29th June to 1st August 2009. She has completed the whole training as per the training report submitted by her.

Training Incharge

BTPS/NTPC, Badarpur,

New Delhi

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ACKNOWLEDGEMENT

With profound respect and gratitude, I take the opportunity to convey my thanks for permitting me to complete my training here.

I extend my heartfelt thanks to Ms. Rachna Singh Bahal for providing me this opportunity to be a part of this esteemed organization.

I’m extremely grateful to all the technical staff of BTPS / NTPC for their co-operation and guidance that has helped me a lot during the course of training. I have learnt a lot working under them and I will always be indebted to them for this value addition in me.

I would also like to thank the training incharge of APEEJAY COLLEGE OF ENGINEERING(SOHNA,GURGAON) and all the faculty members of electrical department for their effort of constant co-operation, which have been a significant factor in the accomplishment of my industrial training.

MANSI TIWARI

APEEJAAY COLLEGE OF ENGINEERING

SOHNA,GURGAON

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ABOUT NTPC

NTPC Limited today is one of the largest companies in India in terms of Market Capitalization and the single largest player in power sector, catering to approximately 30% of country's power needs.

Set up in 1975 by Government of India, today it is a Navratna PSU with a strong workforce of 24,447 power professionals and an annual turnover of Rs.28, 750.7 Crores.

The Company has 14 coal based and 7 gas based power plants across India with a total installed capacity of 26,404 MW. Several new projects are underway as the company has ambitious plans of achieving 75,000 MW installed capacity by 2017.

NTPC Limited is the largest power generating company of India. A public sector company, it

was incorporated in the year 1975 to accelerate power development in the country as a wholly

owned company of the Government of India. At present, Government of India holds 89.5% of

the total equity shares of the company and the balance 10.5% is held by FIIs, Domestic Banks,

Public and others. Within a span of 33 years, NTPC has emerged as a truly national power

company, with power generating facilities in all the major regions of the country.

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NTPC's core business is engineering, construction and operation of power generating plants. It also provides consultancy in the area of power plant constructions and power generation to companies in India and abroad.

Major Achievements of NTPC:

1) Largest thermal power generating company of India.

2) Sixth largest thermal power generator in the world.

3) Second most efficient utility in terms of capacity utilization.

4) One of the nine PSUs to be awarded by the status of Navratna.

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ABOUT BTPS

Badarpur thermal power station started working in 1973 with a single 95MW unit. There were 2 more units (95 MW each) installed in next 2 consecutive years. Now it has total five units with total capacity of 720 MW. Ownership of BTPS was transferred to NTPC with effect from 01.06.2006 through GOI’s Gazette Notification.

Given below are the details of unit with the year they are installed.

Address: Badarpur, New Delhi – 110 044Telephone: (STD-011) – 26949523Fax: 26949532Installed Capacity 720 MW Derated Capacity 705 MW Location New Delhi Coal Source Jharia Coal FieldsWater Source Agra CanalBeneficiary States DelhiUnit Sizes 3X95 MW

2X210 MWUnits Commissioned Unit I- 95 MW - July 1973

Unit II- 95 MW August 1974 Unit III- 95 MW March 1975 Unit IV - 210 MW December 1978 Unit V - 210 MW - December 1981

Transfer of BTPS to NTPC Ownership of BTPS was transferred to NTPC with effect from 01.06.2006 through GOI’s Gazette Notification

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Training at BTPS

I was appointed to do 5 weeks training at this esteemed organization from 29th June to 1ST August 2009. I was assigned to visit various division of the plant, which were:

Control & Instrumentation (C & I) Electrical Maintenance Department - 1 (EMD-1) Electrical Maintenance Department -2 (EMD-2)

This 5 weeks training was a very educational adventure for me. It was really amazing to see the plant by yourself and learn how electricity, which is one of our daily requirements of life, is produced.

This report has been made by my experience at BTPS. The material in this report has been gathered from my textbook, senior student reports, manuals and power journals provided by training department. The specification and principles are as learned by me from the employees of each division of BTPS.

MANSI TIWARI

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THERMAL POWER PLANT

Thermal power plant converts the heal energy of coal to electrical energy. Coal is burnt in a boiler, which converts water into steam. The expansions of steam in turbine produces mechanical power, which drives the generator or the alternator.

GENERAL STRUCTURE OF BTPS(750MW CAPACITY) :

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Schematic arrangement of steam power station

1. Coal and ash handling arrangement.

2. Steam generating plant.

3. Steam turbine.

4. Alternator.

5. Feed water.

6. Cooling arrangement.

Coal and ash handling plant :-

The coal is transported to the power station by road or rail and is stored in the coal storage plant. Storage of coal is primarily a matter of protection against coal strikes, failure of transportation system and general coal shortages. Form the coal storage plant, coal is delivered to the coal handling plant(CHP) where it is pulverized (i.e., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air. The main aim of CHP is to maintain the level of coal in the bunkers for smooth supply of coal to the boilers. The working conditions in CHP are dusty, dirty and often wet.

The pulverized coal is fed to the boiler by bell conveyors. The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal. The removal of the ash from the boiler furnace is necessary for proper burning of coal. It is worthwhile to give a passing reference to the amount of coal burnt and ash produced in a modern thermal power station. A 100 MW station operating at 5Q/ load factor may burn about 20,000 tons of coal per month and ash produced may be to the tune of IO% to 5% of coal fired i.e., 2,000 to 3,000 tons. In fact, in a thermal station, about 50% to 60% of the total operating cost consists of fuel purchasing and its handling.

Following is the systematic diagram of the coal handling plant :

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CRUSHERS:-

In CHP crusher works on the principle of combination of impact and attrition crushing. In this

type of crushing first coal breaks due to impact crushing and further it is scrubbed between

two hard surfaces to get desired coal size. Some crushers work only on the principle of impact

crushing. Generally these crushers are used before final crushers. The output size of coal

affects the performance of CHP. The two hard surfaces of crusher are crucial parts. One of

these is known as grinding plate and the other as rings, hammers etc. The link between

crusher rotor and driver assembly is also crucial, as its failure will stop the crushing process.

WAGON TIPPLERS: -

In CHP generally there are two types of wagon tippler. They are known as rota type and rotary

type. The main difference between these tipplers is that rotary type tippler has floating barrel

and rota type tippler turns between two bearings. The drive linkages undergo cyclic loading

and failure of these linkage stop the equipment operation, due to this unloading of coal cars

affects, which drops the performance of CHP.

BUNKER - FEEDING CONVEYORS: -

CHP has number of conveyors but bunker-feeding conveyors play a vital role. The main aim of

each CHP is to maintain bunker levels for smooth coal supply to boilers. As these conveyors

feed the bunker their performance affects CHP performance. The drive linkages consists of

gearbox and couplings. Failure of any part of the linkage will stop operation of feeding bunker

level. So these parts are crucial parts of bunker feeding conveyors. The conveyor pulleys are

also crucial parts.

FEEDERS: -

The performance of feeders affects the efficiency of CHP. The feeders used in CHP are Apron

Feeder, Vibrating Feeders, Roller Screens and Vibrating Screen Feeders etc. Generally vibrating

feeders used, are of electromagnetic type. The springs, coils and suspension rods are the

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crucial parts. Weak coil springs that are not generating sufficient accelerating forces can also

cause low speed and reduce the performance. In vibrating screen feeder have crucial part like

beam and its members, drive linkages etc. Apron Feeders is sturdy machine, the crucial parts

are fans, chain and rollers. The roller screens have crucial parts in drive linkages

Steam generating plant:-

The steam generating plant consists of boiler for the production of steam and other auxiliary

equipment for the utilization of flue gases.

a) Boiler:-

The heat of combustion of coal in the boiler is utilized to convert water into steam at high temperature and pressure. The flue gases from the boiler make their journey through super-heater, Economizer, air pre-heater and are finally exhausted to atmosphere through the chimney.

b) Super-heater :-

The steam produced in the boiler is wet and is passed through a super-heater where it is dried and superheated (i.e., steam temperature is increased above that of boiling point of water) by the flue gases on their way to chimney. Superheating provides two principle benefits. Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of turbine (which would cause blade corrosion) is avoided. The superheated steam from the super-heater is fed to steam turbine through the main valve.

c) Economizer:-

An economizer is essentially a feed water heater and derives heat from the flue gases for this purpose. The feed water is fed to the economizer before supplying to the boiler. The economizer extracts a part of heat of flue gases to increase the feed water temperature.

d) Air-preheater.-

An air-pre heater increases the temperature of the air supplied for coal burning by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan

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and is passed through air-pre heater before supplying to the boiler furnace. The air pre heater extracts heat from flue gases and increases the temperature of air used for coal combustion. The principal benefits of preheating the air are increased thermal efficiency and increased steam capacity per square meter of boiler surface.

Steam turbine:- The dry and superheated steam from the super heater is fed to the steam turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water

circulation.Alternator:- The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the

alternator is delivered to the bus bars through transformer, circuit breakers and isolators.Feed water:- The condensate from the condenser is used as feed water to the boiler. Some water may be lost in the cycle which is suitably made up from external source. The feed water on its way to the

boiler is heated by water heaters and economizer. This helps in raising the overall efficiency of the plant.Cooling arrangement:- In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed by means of a condenser. Water is drawn from a natural source of supply such as a river, canal or lake and is circulated through the condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used. During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled. The cold water from the cooling tower is reused in the condenser. NCHP feeds unit 4 and 5 (each of 210 MW capacity). It consists of double stream of conveyors of capacity 600 metric tones/hour. Except for stacking conveyors of 600mtph which is a single one wagon Tippler, four vibrating feeders of 300mtph each below the wagon tippler, two rotatry breakers of 600mtph each in secondary crusher house, one telescopic chute for stacking, two sets of reclaim hoppers, necessary transfer points and new rail tracks for the wagon tippler are provided. The wagon tippler is provided with integral weight bridge for recording the gross and true weight of the wagon and is located beyond the old marshalling yard, due to space Limitation only one wagon tippler is provided.

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Car pullers are used for placing the loaded wagon on the wagon tippler and removing the empty wagon.

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CONTROL & INSTRUMENTATION This division basically calibrates various instruments and takes care of any fault that occurs in any of the auxiliaries in the plant. It has the following Labs. MANOMETRY LAB.

1. PYROMETRY LAB.

2. AUTOMATION LAB.

3. PROTECTION & INTERLOCKING LAB.

4. TURBO SUPERVISORY INSTRUMENT LAB.

5. FURANCE SAFETY SUPERVISORY SYSTEM.

6. ELECTRONICS TEST LAB.

This department is the brain of the plant because from relays to transmitters followed by the electronic computation chipsets and recorders and lastly the controlling circuitry, all fall under their responsibility.

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MANOMETRY LAB

Various instruments used in this lab are:-

1. MANOMETER: -

It is a tube, which is bent, in the U shape. It is filled with a liquid. This device corresponds to a difference in the pressure across the two limbs.

2. BOURDON PRESSURE GUAGE :-

It is an oval section of tube. Its one end is fixed. It is provided with a pointer to indicate the pressure on a calibrated scale. It is of two types.

a. Spiral type-: It is used for measuring low pressure. It is more sensitive and is used where compactness is necessary.

b. Helical type-: It is used for measuring high pressure. It is most sensitive and compact. Pointer may be mounted direct on end of helix which rotates, thus eliminating backlash error and lost motion.

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PYROMETRY LAB

This lab consists of various temperature measuring instruments.

LIQUID IN GLASS THERMOMETER:-

Mercury in the glass thermometer boils at 340 degree Celsius which limits the range of temperature that can be measured it is an L shaped thermometer, which is designed to reach all inaccessible places.

1. The Bi-Metallic Strip :-

Bi-metal strips are composed of two metals, as the name implies, it is the one whose coefficients of linear expansion are dissimilar. These two metal plates are welded together as a sandwich. When heated, both metals expand, but the metal with greatest coefficient of linear expansion will expand more causing the sandwich to curl up or down depending on the position of this metal. (Refer Fig.).

2. ULTRA VOILET CENSOR:-

This device is used in furnace and it measures the intensity of ultraviolet rays present and according to the wave generated, a signal of the order of same is generated, which directly indicates the temperature in the furnace. This lab also has the responsibility of calibrating various instruments. Depending on the range of the device, the method to calibrate the device is adopted. The low range measuring devices are calibrated in the oil bath or just by using boiling water. The high temperature measuring device is calibrated in the small furnace.

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AUTOMATION LAB

This lab deals in automating the existing equipment system, and feeding routes. Earlier, the old technology dealt with only data acquisition system and was known as primary. The modern technology or the secondary system is coupled with management information system. But this lab universally applies the pressure measuring instrument as the controlling force. Suppose, if in a motor the fall in pressure of lubricating oil directly implies to the heating of the machine then similarly fall in water pressure in the water pipe line implies overload or short circuit of the feed or circulating pump motors. However, the relays provided, are used only for the protection and interlocking. Once the measure is common, the pressure control circuit can easily be designed with single chip having multiple applications. Another point is the universality of the supply, the laws of the electronics state that it can be anywhere between 12 V and 35V. At the plant all control instruments are excited by 24V DC supply (0.5-2OA).

PROTECTION AND INTERLOCK LABThere are two kinds of protection system based on the voltage level at which they

operate. These are discussed below:-

a) HIGH TENSION CONTROL CIRCUIT:-

For high-tension system the control system is excited by separate DC supply. For starting the circuit, condition should be in series with the starting coil of the equipment to energize it. The tripper is a coil, which de-energizes the start coil so that the equipment stops. The tripper derives signal from the tripping coil of the high tension tripping system. It should be noted that for tripping all the conditions should be in parallel.

b) LOW TENSION CONTROL CIRCUIT: -

For low-tension circuits the control circuits are directly excited from the 0.415KV AC supply. The same circuit achieves both excitation and tripping. Here the tripping coil is provided for emergency tripping if the interconnection fails. This system is more prone to failure as it is excited from 415V of AC.

I NTERLOCKING : -Page 19


It is basically interconnecting two or more equipment so that if one equipment fails other can perform the task. This type of interdependence is also created so that all the equipment connected together are started and shutdown in specific sequence to avoid damage. For protection of equipments tripping are provided for all the equipments. Tripping can be considered as the series of the instruction connected through OR gate. When a fault occurs and any one of the tripping is, satisfied a signal is sent to the relay, which trips the circuit. The main equipment of this lab are relays and circuit breakers. Some of the instruments used for protection are:-

1. RELAY : -

It is a protective device. It can detect wrong condition in electrical circuit by constantly measuring the electrical quantities flowing under normal & faulty condition. Some of the electrical quantities are voltage, current, phase angle and Velocity. After detecting the fault, the relay operates to complete the trip circuit, which results in the break up of circuit caused due to circuit breakers, and disconnects the faulty circuit. Two types of relays used in this lab are given below.

a) Current Relay : - It gets energized when the rated amount of current flows through it. It is always connected in series.

b) Potential Relay : - It gets energized by voltage and has copper winding as the coil. It is always connected in parallel.

2. FUSE : -

It is a short piece of metal inserted in the circuit, which melts if heavy current flows through it and thus breaks the circuit. Usually sliver is used as a fuse material because:-

a) The expansion coefficient of sliver is very small. As a result, no critical fatigue occurs and thus the continuous full capacity normal current rating is assured for the long time.

b) The conductivity of the silver is unimpaired by the surges of the current that produces temperatures just near the melting point.

c) Silver fusible elements can be raised from normal operating temperature to vaporization much quicker than any other.

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3. MINIATURE CIRCUIT BREAKERS : -

They consist of current carrying contacts, one is movable and the other is fixed. When a fault occurs, the contacts separate and an arc is struck between them. They are used with combination of the control circuit to:-

a) Enable the starting of plant and distributors.

b) Protect the circuit in case of a fault.

The miniature circuit breakers that are used, employ three tripping mechanism as given below:-

a) Manual Trip : - Manual trip implies that the miniature current breaker can be operated as an electrical switch.

b) Thermal trip : - Thermal trip is the hi-metallic tripper when the circuit draws the excess current, the contact between the two metals breakers rises due to temperature.

c) Short Circuit Trip :-The short circuit or dec. Mag. Trip works in the case of short circuit as the short circuit current is many times the rated current, it is allowed to flow through coil with movable aluminum core attached to the tripping spring. This on being energized with sufficiently large current pulls the core, which in turn releases the spring.

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FURANCE SAFETY SUPERVISORY SYSTEM LAB

This lab has the responsibility of starting the fire in the furnace to enable the burning of coal. For first stage coal burners are in the front and rear of the furnace and for the second and third stage corner firing is employed. Unburnt coal is removed using forced draft fan. The temperature inside the boiler is 1100 degree Celsius and its height is 18 to 40m. It is made up of mild steel. An ultra violet sensor is employed in the furnace to measure the intensity of the ultra violet rays inside the furnace and according to it, signal in the same order of same mV is generated, which directly indicates the temperature of the furnace.

For firing the furnace a 10 KV spark plug is operated for 10 seconds over a spray of diesel fuel and pre- heated air along each of the feeder mills. The furnace has six feeder mills each separated by warm air pipes fed from forced draft fans. In first stage indirect firing is employed that is feeder mills are not fed directly from coal but are fed from pulverized coal from bunkers. The furnace can operate on the minimum feed from three feeders but under no circumstances should any one of the intermediate mills be left out under operation, to prevent creation of pressure difference with in the furnace, which theaters to blast it.

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TURBO SUPERVISORY INSTRUMENTS LAB

This lab takes of all the instruments, which are part of the turbo supervisory system to allow safe and proper operation of the turbine. Some of the turbine parameters that have to be monitored are as follows:-

a) TURBINE SPEED :-

The speed of the turbine is to be kept constant so that the frequency of the generated electricity is close to 50 Hz. The indicator of the speed gives us a remote indication of the speed when barring gear rotates the rotor. It gives a local and remote digital indication of the turbine speed, which in turn is given by the photoelectric pick up system. There is a white dot on the turbines, which reflect the light given by the photoelectric pick up device. The rate at which light is sensed is use to calculate the speed of turbine. There is indicator, which also set up alarm signal at 10% and 16% over speed.

b) AXIAL SHIFT OF ROTOR : -

During the rotation of the turbine at high speeds where there is the wearing down of bearing, there is axial shift. Depending on the bearing which have become worn, thrust collar is given with respect to working pads, if this parameter is not monitored properly, then severe bubbling and mechanical interfaces can take place. The position of the thrust collar is taken by detector, which has two elements. There is variable type transducer and a bridge configuration.

c) SHAFT ECCENTRICITY: -

Eccentricity is the deviation of the mass centre from the geometrical centre of the bearing case. It usually occurs in the rotor when there is a shut down. If it becomes large, then there will be a variation, which can be dangerous. To measure the eccentricity a passive and active magnetic reluctance type transducer in combination with bridge circuit in balance condition is used. In this case tolerance is of the order of 10 to 500 microns.

d) BEARING VIBRATIONS :-

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This vibration is to and fro motion of the machine under the influence of oscillatory force caused by unbalanced masses in the rotating system. This is one of the most vital parameter of the turbine and it has to be monitored continuously.

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EMD-I

The EMD-I section in BTPS is engaged in maintenance of motors and switchgears.

MOTORS:-

Motors play a very important role in the operation of a power plant as most devices are run by it. The objective of this Chapter is to give an insight into the fundamentals of motor operation and starting .Motor are of various types. There are DC as well as AC motors. Both are used at different places according to the work required. These are further classified according to their construction.

DC Motor Classification:-

> Series motor

> Shunt motor

> Compound motor

AC Motor Classification:-

> Squirrel cage motor

> Wound motor

> Slip ring induction motor

In a modern thermal power plant normally a three-phase squirrel cage Induction motor is used but some times a double wound motor is also used when we need a high starting torque e.g. in ball mills.

THREE PHASE INDUCTION MOTOR:-

In a three phase induction motor, stator is connected to a three phase supply which produces a rotating magnetic field. Speed of rotation is proportional to main frequency and inversely proportional to the number of pairs of poles. N sync - 60 x supply frequency Pairs of poles. Stator can have concentric single layer windings with each coil side occupying one stator slot. In practice many types of stator winding may be encountered. Two of the most common

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types are illustrated in Fig. 58.A three phase Induction motor stator connected a 3 phase supply produces a rotating magnetic field whose speed is given by :-

Ns=120*f/p

STATOR :-

Stator can have concentric single layer winding with each coil side occupying one stator slot. Two most common types of windings are.

(1) Distributed winding: - This type of winding is distributed over a number of slots.

(2) Double layer winding:- Each stator slot contains sides of two separate coils.

BEARING AND LUBRICATION :-

A good bearing is needed for trouble free operation of the motor. Since it is a very costly part of motor, a lot of care has to be taken by checking it at regular intervals. Damaged bearing may severally affect a motor, so lubrication plays important role. Two types of lubrication are used.

(1) Oil Lubrication

(2) Grease lubrication

INSULATION:-

Since winding play an important role in the operation of a motor. It has to be properly insulated from being short circuited and causing unnecessary damage. Previously class B insulation was being used but now better class F insulator is used.

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TYPES OF INSULATION

TYPES OF INSULATION TEMPRATURE WITHSTANDING CAPACITY (0 C)

YAEBFHC

90105120130155180

<180

ROTOR :-

There are two types of rotors used in three phase induction motors as given below.

1) Squirrel Cage.2) Wound Rotor Motors:

Squirrel cage and wound rotor motors have the same basic mode of operation. Rotor inductors cut the rotating stator magnetic field, an e.m.f. is induced across rotor windings, current flows, a rotor magnetic field is produced, which interacts with the stator field causing a turning motion. The rotor does not rotate at synchronous speed; its speed varies with the applied load. The slip speed is just enough to enable sufficient induced rotor current to produce the power dissipated by motor load and motor losses. The torque at any speed can be made maximum by arranging it for the ideal situation to be achieved. The induction motor has poor efficiency and power factor (mainly inductive reactance). On full load a typical efficiency of 85% and power factor of 0.8 can be achieved. Fig. 59 shows a simplified squirrel cage motor.

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SYNCHRONOUS MOTORS:-

Conventional three phase stator, speed of rotation is dependant on the number of pairs of stator poles. Rotor winding is D.C. excited or of variable reluctance. True synchronous motor is not self starting. Squirrel cage or wound motor is required to accelerate it to a synchronous speed. Rotor 'pulls in' and rotates with stator field. On load, the rotor has a 'load angle' with respect to the rotating magnetic field. The speed of the rotor is constant over the load range of the motor. As the load increases, load angle increases and power drawn from the supply increases. When on excessive overload, rotor 'pulls out' of synchronism. When operating at synchronous speed the power factor of the motor can be changed by varying the degree of excitation. A conventional cylindrical wound rotor is as shown in figure 60.

MOTOR STARTING:-

The methods employed in starting a motor are extremely varying, depending upon size, type, starting repetition and environment etc. Probably the simplest and most common method is to connect the supply directly to the motor and allow it to accelerate normally to its running speed. This method is referred to as Direct-on-line Starting (D-O-L).

Direct-On-Line Starting:

The choice of this method may depend upon a number of conditions. If for example the load has high inertia, then D.O.L. would not be ideal because the prolonged starting current could be six times the normal running current. This in turn (assuming that the switch used could carry this large current) would put an excessive "drain" upon the supply

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system. In fact, for large horse power motors the supply authorities will not usually permit D.O.L starting. D.O.L. staring is however used extensively in modern power stations one reason being that it is less expensive than other methods. Excessive current surge is more tolerable in power stations than on consumer premises.

Direct -On Line-Starters : - The simplest electric motor drive consists of a squirrel-cage motor switched direct-on-line, and an associated automatic motor starter consists basically of a contactor to connect the motor to the supply and an overload relay to prevent over-heating of the motor.

Reduced Voltage Methods :-

When it is necessary to limit the starting current drawn by a motor, reduced voltage starting methods may be considered. The methods available are star/Delta, Auto-transformer, and primary resistance. Of these, Star/ Delta is by far the most common. It must be remembered however, that reduced voltage also means reduced torque, and in the case of star/delta starting, the initial torque is only one third of that available with D-O-L starting. This is adequate for most drives.

Star/Delta starters :-

This method involves starting the motor from rest, with the stator windings connected in star configuration. In this condition the effective voltage across each winding is 1/3 of the line voltage, or 59%. The current and torque are reduced to 33% of the values obtained if the motor was started D-O-L. After a period of acceleration the supply is removed from the motor, and by a suitable switching process the windings are connected in delta

configuration. Before the motor has chance to deaccelerate, the supply is re-established with full voltage across each winding. Some deacceleration does takes place in practice, and

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mechanical shock is introduced to the drive on re-connection at full voltage. This would not be practical on large units but can be accepted on many small and medium drives. (Refer Fig-61).

Auto-Transformer Starters:-

In its simple form the motor is initially started at reduced voltage by connections to a tapped auto-transformer. The transformer is usually provided with three sets of tapping which allow site adjustments to suit most applications. Typical values are 50%, 65%, and 80%. Because the transformer is short rated, the number of starts is usually limited to five per hour. The method involves disconnection of the motor from supplies before reconnection is made at

full line voltage. Connections are arranged as shown below (Fig. 63).

The switching sequence is as follows:

o Contactor I closes to connect motor to autotransformer tapping. When motor has achieved suitable speed, contactor I opens .

o Immediately following this, contactor II closes to connect the motor to full line voltage.

Due to the open circuit transition from starting to running position, it is possible to obtain transient currents between twenty to thirty times. When making the running connection, the heaviest transients occur when the starter is on the high tap position (80%). An improved method of starting without open transition is described next.

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Induction Regulators: -

It is worth a consideration that induction regulators, whilst being, primarily, speed controllers, can be used to provide a smooth voltage variation 1 from about 60% to full voltage, ensuring a smooth and gradual start without transients. The frequency of starts is limited and the devices are very expensive for ordinary staring purposes. It is not proposed to discuss them at this stage but they will be dealt with in further text under "motor speed control"

D.C. Starters:-

As a general case a D.C. Starter consists of a variable resistance place in series with the motor armature, which is gradually reduced to zero as the speed rises. As the motor accelerates a back e.m.f. is produced which limits the current so that at running speed the motor requires no external resistance (Refer Fig. 64-A). The value of resistance is usually varied in small steps by a manually operated wiper arm, When the "Full on" position is reached an electromagnet attracts the arm and holds it against spring tension. This is known as the hold in coil and if the supply fails or drops below a certain value, the magnetic field is insufficient to hold the arm, which returns to the "off" position

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INDUSTRIAL MOTORS:-

There are various types and sizes of motors used in a power station. These are used for various purposes as prime movers. Apart from the simple motors used in different areas, there are HT Motors used in conjunction with various heavy-duty equipments. These are FD, ID, PA and other fans, Boiler Feed pumps, CW pumps etc. These motors have certain special features like cooling, auto starting, interlocks and controls. Critical auxiliaries have stand-by's driven by DC motors, speed control, wherever required it is achieved using either hydraulic couplings, eddy current couplings or double frequency power controllers.

FAILURE OF MOTORS & CAUSES:-

Number of motor failures have taken place due to:-

1) High number of starts in an hour.2) Incorrect setting of thermal overload relays/other motor predictive relays. 3) The use of incorrect fuses.4) The failure to switch on the space heaters when the motors are made off (cooling

two motors especially).5) Incorrect oil levels in fluid couplings.6) Non calibration of protective relays as pre schedules.

MAIN MOTOR USED IN THE BOILER AND OFF-SIDE AREA:-

(1) ID FAN

(2) FU FAN

(3) PA FAN

(4) MILL FAN

(5) BALL MILL FAN

(6) RC FEEDER

(7) SLAG CRUSHER

(8) LUBRICATING OIL PUMP

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(9) PC FEEDERS

(10) BOWL MILL FAN

TECHNICAL SPECIFICATION OF MOTORS

Boiler feed pump motor (100 MW unit)

MAIN MOTOR SPECIFICATIONS VALUESKW TYPE MAKERS NUMBER RPM RATING FREQUENCY EFFICIENCYSTATOR KV AMPERE EFFICIENCY POWER FACTOR YEAR

1600ATP-1600/66007300613/X-122980CONTINUOUS50Hz95.6%STAR CONNECTION6.6KV10395.30.91971

Boiler feed pump motor (210 MW unit)

MAIN MOTOR SPECIFICATIONS VALUESKW RPM RATING FREQUENCY STATOR KV AMPERE EFFICIENCY POWER FACTOR PEAK LOAD TYPE OF BEARING

400 kw1483CONTINUOUS50HzSTAR CONNECTION6.6KV421 A95.3%0.85200%SLEEVE

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CONDENSER WATER PUMP MOTOR

MAIN MOTOR SPECIFICATIONS VALUESRPM RATING FREQUENCY EFFICIENCYSTATOR KV AMPERE POWER FACTOR

424CONTINUOUS50Hz92.8%STAR CONNECTION6.6KV79 A0.80

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EMD-II

The ELECTRICAL MAINTENANCE DEPARTMENT -2 (EMD-2) is engaged in maintenance of Generators, Transformers and Switchyard.

GENERATOR

The transformation of mechanical energy into electrical energy is carried out by the Generator. This Chapter seeks to provide basic understanding about the working principles and development of Generator.

HISTORY :-

The first A.C. Generator concept was enunciated by Michael Faraday in 1831. In 1889 Sir Charles A. Parsons developed the first AC turbo-generator. Although slow speed AC generators have been built for some time, it was not long before that the high-speed generators made its impact. Development contained until, in 1922, the increased use of solid forgings and improved techniques permitted an increase in generator rating to 20MW at 3000rpm. Up to the out break of Second World War, in 1939, most large generators were of the order of 30 to 50 MW at 3000 rpm. During the war, the development and installation of power plants was delayed and in order to catch up with the delay in plant installation, a large number of 30 MW and 60 MW at 3000 rpm units were constructed during the years immediately following the war.

The first-cooled generator, a 60MW machine, was installed in UK in 1949. This was a conventionally cooled generator wherein hydrogen replaced air as cooling medium. In 1955 the first 100MW generators were commissioned and from the same design followed the 120 M W machines which came into service from 1958. The 200 MW generators were installed in 1959. The advantages of direct cooling were further emphasized when hydrogen was superseded by the use of water for cooling the stator windings, and ratings of generators rapidly increased from 275 MW to 500 MW. The next decisive stage must be the development of single shaft generators in output range 750-1000 MW.

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WORKING PRINCIPLE

The A.C. 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 lines of magnetic flux (via magnetic field) cut through the stator windings. This induces an electromagnetic force (e.m.f.) in the stator windings. The magnitude of this e.m.f. is given by the following expression.

E = 4.44 / Ø F N volts

Where Ø = Strength of magnetic field in Weber’s.

F = Frequency in cycles per second or Hertz.

N = Number of turns in a coil of stator winding

And Frequency F = P n/120

Where P = Number of poles

n = revolutions per second of rotor.

From the expression it is clear that for the same frequency, number of poles increases with decrease in speed and vice versa. Therefore, low speed hydro turbine drives generators having 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 construction.

CONSTRUCTION

Generator can be divided into three parts in constructional point of view:

1. Stator

2. Rotor

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3. Generator Cooling System

1) STATOR

a) Stator Frame

A grid frame is required to arrest forces and torques aroused during operation and give mechanical support. It comprises of an inner frame and outer frame. The outer frame is a rigid structure of welded steel plates, within this shell is a fixed case of grider built circular and axial ribs. The ribs divide yoke in the components through which hydrogen flows for cooling. The inner cage is fixed into yoke by an arrangement of springs.

b) Stator core

Stator core is built up of large no. of laminations of thin CRG0 steel to contribute the reduction in weight. Each lamination segment is 0.5mm thick steel with varnish on both the sides. To obtain maximum compression and elimination under setting during the operation, the laminations are hydraulically compressed and heated during the stacking procedure and the complete stack is kept under pressure locate in the frame by the means of clamping bolt and pressure plate. The use of cold rolled grain-oriented steel can contribute to reduction in the weight of stator core for two main reasons:

I. There is an increase in core stacking factor with improvement in lamination cold rolling and in cold buildings techniques.

II. The advantage can be taken of the high magnetic permeance of grain-oriented steels of work the stator core at comparatively high magnetic saturation without fear or excessive iron loss of two heavy a demand for excitation ampere turns from the generator rotor.

c) Stator windings

The three-phase stator winding is a fractional pitch two layer type. These are made up of copper tubes wound with insulated type, which is impregnated with varnish, dried under vacuum and hot pressed. Water is fed into the windings (copper tubes) for cooling purposes. Water is fed to the windings through the plastic (Teflon) tubes. There is a 54° transposition in the lot portion. In the end-winding portion no transposition is there instead windings are short circuit. Two flexible core suspensions are located directly adjacent to the

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point where the frame is supported on foundation. The spring takes care of forces due to weight and short circuit.

2) ROTOR

The electrical rotor is the most difficult part of the generator to design. It revolves in most modern generators at a speed of 3,000 revolutions per minute. It is also an electromagnet and to give it the necessary magnetic strength the windings must carry a fairly high current. The passage of the current through the windings generates heat but the temperature must not be allowed to become so high, otherwise difficulties will be experienced with insulation. The rotor shaft is made of high quality heat-treated steel forged from a vacuum cast steel ingot and it is further forged and machined. Very often a hole is bored through the centre of the rotor axially from one end of the other for inspection. Slots are then machined for windings and ventilation.

a) Rotor winding

Silver bearing, de-oxidized copper is used for the windings with mica as the insulation between the conductors. Hollow conductors with slots or holes arranged for circulation of the cooling gas through the actual conductors. An axial flow time located in the turbine and the shaft journal circulates the cooling gas. At high speed, centrifugal force tries to lift the winding out of the slots, and hence these contained by wedges. The two ends of windings are connected to the slip rings, made of forged steel, and mounted on insulated sleeves.

b) Rotor balancing

When completed the rotor must be tested for mechanical balance, which means that a check is made to see if it will run up to normal speed without vibration. To do this it would have to be uniform about its central axis and it is most unlikely that this will be so to the degree necessary for perfect balance. Arrangements are therefore made in all designs to fix adjustable balance weights around the circumference at each end.

3) GENERTOR COOLING SYSTEM

The 210MW generator is provided with an efficient cooling system to avoid excessive heating and wear and tear of components during operation.

a) Rotor cooling system

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The rotor is cooled by means of gap pickup cooling; wherein the hydrogen gas in the air gap is sucked through the scoops on the rotor wedges and directed to flow along the ventilating canals mild on the hot zone of the rotor. This method not only gives uniform distribution of temperature but also eliminates the deformation of copper due to high temperatures. Hydrogen is used as cooling medium because of its high heat carrying capacity and low density. But in view of its forming an explosive mixture with oxygen, proper arrangement for filling, purging and maintaining its purity inside the generator have to be made. Also, in order to prevent escape of hydrogen forming the generator casing, shaft-system sealing is used to provide oil sealing.

b) Stator cooling system:-

The stator winding is cooled by distillate, which is fed from one end of the machine by Teflon tube and flows through the upper bar and returns back through the lower end of the other slot. The stator winding is cooled by circulating the dematerialized water (DM water) used for the cooling of the stator winding calls for the use of very high quality of cooling water. For this purpose DM water of specific resistance is selected. The system is designed to maintain a constant rate of cooling water flow to the stator winding at a normal inlet water temperature of 40°C.

4) GENERATOR SEALING SYSTEM

Sealings are employed to prevent leakage of hydrogen from the stator at the point of rotor exit. A continuous film between the rotor collar and seal liner is maintained by the means of oil at a pressure, which is about O.5 atm above the casing of hydrogen gas pressure. The thrust pad provides a positive maintenance of the oil film thickness.

SEAL OIL SYSTEM

This system follows the following path:

a. Main Oil Tank

b. Main Oil Pump

c. Seal Oil Cooler

d. Seal Oil Filter

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e. Damper Tank

f. DPR (Differential Pressure Regulator)

g. Hydrogen Seal

h. Seal Oil Fans

i. Main Oil Tank

This system has two more pumps:

a. AC Seal Oil Pump

b. DC Seal Oil Pump

The pressure of seal is 4.2kg per sq Cm. while the pressure of hydrogen is 3.5 kg per sq. cm. The purpose of this system is to provide seal to the hydrogen used for cooling of stator. If the seal breaks, the system also breaks down.

MOT(main oil tank):-

It contains 28000ltrs of turbine oil. It supplies oil to all bearings and seal.

MOP(main oil pump):-

It takes from the main oil tank and supplies further.

Seal oil cooler:-

It contains clarified water and distilled mineral water. The outer covering area is called housing. It covers an assembly of about 200 tubes assembled on plates. Air gaps are found between the tubes (25mm above, below and on each side of each tube).These gaps are filled using hydrogen which cools the stator.

Seal oil filters:-

These are rounded mesh like structures kept over each other. There are about 34 filters present.

Damper tank:-

It is kept at 14mtrs of height above the ground. If MOP fails A/C or D/C pump works and if these fail then damper pump comes into action. Here the hydrogen pressure is .7%

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to .9% less than the oil pressure because if the hydrogen pressure increases, the seal would break down.

DPR(differential pressure regulator):-

The function of DPR is to regulate the pressure of the oil and balance it. Whenever main oil pump fails, the AC seal oil pumps starts automatically .DC seal pump fails but it can supply the oil up to an hour. The generator is kept at a height of 8meters so that oil is supplied by pressure.

5) BEARINGS

Bearings are lubricated type bearings. Ring oiling is supplemented by recirculation of externally cooled oil. An emergency supply of oil is also maintained as a standby for failure of main supply.

6) EXCITATION SYSTEM

The excitation currents for typical 500 MW generators range from 3700 to 5100 amps and sliding electric contracts and components need frequent attention and maintenance due to heating caused by brush friction and brush losses. The electric power generators require direct current excited magnet for its field system. The excitation field system should be reliable, stable in operation and must respond quickly to excitation current requirements. The field excitation system is employed is static excitation system. The voltage and current ratings are 310 V & and 2600 amp dc. The excitation is controlled by automatic voltage regulator (AVR). The start up power is taken using the station battery to provide initial alternator field current.

a) High Frequency Excitation

The high frequency excitation system, adopted in 200/210 MW TG is based on the principle of separate excitation with the help of a 500 C/S A/C main and 400 C/S pilot exciter in conjunction with the static rectifying unit. Both the exciters are directly mounted on the TG and four field windings on the stator. There is no winding on the rotor to enhance its reliability and maintenance. The pilot exciter (PEX) is a permanent magnet type and serves as a source of stable supply to the power magnetic amplifiers of AVR and the manual excitation of flux at various operational conditions. The rectifying unit is water cooled, three phase static

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converter, which rectifies the HFEX output and free the turbo-generator fields. The main exciter HFEX is an inductor type generator which has three phase AC winding.

ELECTRICAL SPECIFICATIONS OF GENERATOR

RATING TURBO GENERATORS ONE,TWO AND THREE (95 MW each)

Power KW 95000 KW

Capacity 1175000 KV

Voltage 10500 V

Speed 6000 Rpm

Hydrogen 2.5 Kg/Cm2 (Gauge)

Stator Winding Connection Star-Star

Phase 3

Year 1972

Power Factor 0.85(Lag)

Stator Current 6475 A

Frequency 50 Hz

Cooling Hydrogen

Maker BHEL(Haridwar)

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Rating Of Turbo Generators Four And Five [Units TG4 and TG5] (210MW each)

Generator --KVA Rating 247000kvaKW Rating 210000kwVoltage 15750 V- Stator

310 V - RotorAmperes 9050 A- Stator

2600 A - RotorRated Power Factor 0.85 Lag Phases 3Connection Star-StarCoolant Water And HydrogenGas Pressure 3.5 Kg/Cm3Insulation Class BType Thw-210-2Maker & Year BHEL (Haridwar) 1986-87Rated Speed 3000 RpmRated Frequency 50 C/SPhase Connection Double StarRotor Cooling

A) H2 Pressure 3.5kg/Cm2

B) Purity 97%C) Gas Volume 66 M3

Stator Cooling

A) Water Pressure 3.5 Kg/Cm2

B) Qty Of Water 130 M3/HrGenerator Seal Oil System Sealing Data:A) Seal Oil Pressure 4.1 To 4.4 Kg/Cm2

B) Quantity 80 Lt/MinC) Type Radial Flow, Double Chamber Thrust TypeAC Seal Oil PumpA) Type Multistage Vertical Split Centrifugal TypeB) Capacity 3.34 Lit/Se At 12kg/Cm2DC Seal Oil PumpA) Type Multistage Vertical Split Centrifugal TypeB) Capacity 4.16 Liters At 12 Kg/Cm2Resistance Of Stator Winding/Phase At 20 Degrees 0.00 155 OhmResistance Of Rotor Winding/Phase At 20 Degrees 0.08% Ohm

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Capacitance Of Stator Winding In Hot Position 0.6 Micro FaradLine Charge Capacity 75 MVARBasic Impulse Insulation Level Between Turns 49000 VBasic Impulse Insulation Level Wrt. Body 49000 VGenerator Excitation Data

Ratio Of Cooling Voltage To The Excitation Voltage 2.0Ratio Of Field Current To The Nominal Current 1.6Duration Of Field Forcing 60 Sec.Response Time 100 Sec.Short Circuit Current

Sub-Transient Current On The Three Phase Short Circuit 10 PUTransient Current In Three Phase Short Circuit 3.3 PUSteady State Current In Three Phase Short Circuit 1.4 PUResistance

Direct Axis Sub Transient Resistance 0.2 14 PUZero Sequence Reactance 0.105 PUNegative Phase Sequence Reactance 0.26 PUTemperature

Maximum Temperature Of Stator Core 105 DegreesMaximum Temperature Of Stator Winding 115 DegreesPermissible Rise Of Temperature Rise Of Rotor 71 DegreesMaximum Temperature Of Hot Gas 75 DegreesMaximum Stator Winding Temperature 75 Degrees

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SWITCHYARD AND TRANSMISSION EQUIPMENTS

The switchyard links the generating station to the supply grid and constitutes the efficient switching arrangement so that it provides control over power flow from generating station to the grid and grid to station as required. The switchgear is basically switching network to the switch and the power from the generator to the grid via common bus connecting all the generators in parallel.

In BTPS switchyard is placed and installed in straight to transformer in MCBR. The switchyard is installed at the starting of BTPS gate. The switchyards not only controls the incoming but also feeds the outgoing feeder there are various equipments installed in the switchyard in order to protect the switchyard from voltage surge.

The various components of switchyard are listed below:

Transformer Isolators Circuit Breakers Lightening Arrestors Current Transformer Potential Transformer Earth Switches Bus Bars and Clamp Fittings Supporting Structures for Hanging Buses Control Relay Panel Fire Fighting equipments Power Cables and Control Cables

TRANSFORMERS

Transformers are used to step up or step down the voltage levels. The main transformers in concern with switchyard are:

a) Switchyard Service Transformer

It supplies the auxiliary load for switchyard i.e. AC. Systems, lighting etc.

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b) Generator transformers

It steps up the generated voltage level of 16.5KV to 220KV.

c) Station transformers

It is used for supplying the station auxiliary load and start up for power station via ICTs.

d) Interconnecting transformers

These are single-phase auto transformer and supply three-phase power to the grid in a group of three. The ICT steps up 220KV voltage to 400KV voltage before feeding the grid.

ISOLATORS

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 purposes of isolating a certain portion when required for maintenance etc. Switching isolators are capable of interrupting transformer magnetized currents, interrupting line charging current, load transfer switching. Its main application is in connection with transformer feeder as this makes it possible to switch out one transformer while the other one is still on load.

CIRCUIT BREAKERS

A circuit breaker is one, which can make and break the circuit on load and even on faults. The equipment is most important and is heavy-duty equipment mainly utilized for the protection of the various circuits and operation at load. Normally isolators accomplish circuit breakers. Main types of circuit breakers are:

Minimum oil CB Bulk Oil CB Air Blast CB SF6 Gas CB

For 220KV switchyard, SF6 type CB has been employed and 400KV switchyard air blast and SF6 CBs have been employed.

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LIGHTENING ARRESTORS

LA’s are provided at the terminals of the transformer for protection against lightening or any surges developing in the system. The station shielding against direct lightening stoke is provided through earth wires located at structures’ peaks.

CURRENT TRANSFORMER

The CTs used are single-phase oil immersed type. The secondary current is generally 1A, but also 5A in certain cases.

POTENTIAL TRANSFORMER

Since we are dealing with voltages 220KV and above, capacitor voltage transformers (CVT, a type of Fr) are used. The secondary voltage is 110/3 volts.

CTs and PTs are mainly used for three advantages:

Metering Protection Controlling

EARTH SWITCHES

Earth switches in the switchyard are the simple mechanically operated switches. The purpose of these is to earth the bus if required for the purpose of eliminating induced voltage in a particular bay on account of parallel running live conductors. These are always accompanied by an auxiliary switch to provide interlock and indication contact.

THE BUS BAR ARRANGEMENT

For 220 KV and 400KV switchyard different bus bar arrangements have been adopted. The type of arrangements to be adopted depends upon the reliability of supply from the substation. For 220KV switchyard, two main and one transfer bus arrangement has been provided whereas for 400KV switchyard, 1½ bus bar arrangement has been provided.

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a) Two main and one transfer bus arrangement

In this arrangement there are two main buses and one transfer bus. The main buses are so named because they are provided with main elaborate protection and other things and normally load is taken through them. Whenever hilt occurs, the load can interchange the loads through bus couplers. Under fault conditions the fault circuit can be disconnected and repair and maintenance can be done when required. This is done from the reliability point of view, because it is necessary to insure proper transmission of power from generator to the grid.

b) 1½ Bus Bar Arrangement

This type of bus arrangement has been adopted for 400KV switchyard. This arrangement is more reliable and costly than the previous one as it has more circuit breakers and isolators for given feeders. By using this type of arrangement, even if there is a case of failure of both buses, power can be returned from line 1 to2 or vice versa without affecting the rest of the system.

BUS BARS SUPPORTING STRUCTURES

The bus bar supporting structures used for hanging bus bars are generally of steel lattice type.

CONTROLLING RELAYS

These panels consist of a no. of various types of relays installed for Protection against over currents, over voltages, earth faults etc. As soon as these receive an indication of some fault from CT, PT etc, these relays trip concerned circuits, with or without time lag (as designed), Hence saving the system from damage.

FIRE FIGHTNING EQUIPMENT

Soap pits are provided in respect of all the transformers where the quantity of oil exceeds 2000 LTR. Besides, supportable type of fire fighting equipment such as dry powder type, chemical foam type are also provided in adequate quantities for protection of electrical equipments. Automatic types of emulsifier system (using water) have also been used for the protection of equipment against fire.

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CONTROL ROOM

The control room building for a sub-station includes the panels, PLCC equipment, & battery etc. The control room can be truly said to be the heating of a sub-station, where the people concerned continuously monitor the working conditions of equipment/system for its proper functioning. Malfunctioning of any equipment can be immediately brought to notice the person operating in the control room by the means of certain indications on the panel and the fault can be easily eliminated without much damage or tripped automatically or manually from the control room itself.

POWER CABLES AND CONTROL CABLES

The control cables are armored type and are laid in trenches covered with RCC covers. These trenches are also housing the power cables.

TRANSMISSION LINE EQUIPMENT

Transmission lines are required for transmitting power form generating stations to the load centers. The important components of the transmission lines are:

1) Conductor and accessories 2) Insulators and hardware3) Supporting structure and accessories4) Earth wire and accessories

The optimum voltage is determined purely from techno economic considerations. The voltage standardized for the transmission line in the country are 66KV, 132KV, 220KV and 400KV. The optimum power and distance over which these voltages an adopted an as given below :

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The permissible voltages regulations are + 6 to –9 %( for HV lines and + 12 ½% for EHV lines).

1) Conductors & Accessories

The lines may be single circuit or double circuit either in vertical or horizontal configurations. A double circuit line carries double the power than that of a single circuit line. The conductors used for the transmission lines are aluminum conductors steel reinforced, conforming to IS-398. The size standardized for the transmission lines are

'Dog' ACSR (0.1 sq inch copper equivalent ) for 66KV line 'Panther' ACSR (0.2 sq inch copper equivalent) for 132 KV line 'zebra' ACSR ( 0.4 sq inch copper equivalent) for 220KV line and Twin 'Moose' ACSR (2 x 0.5 sq inch copper equivalent) for 400 KV lines.

All aluminum conductors are used at 11 KV and lower voltage lines For 33 KV lines ACSR conductors are used.

2) Insulator & Hardware

The steel structures support these conductors attached through insulator strings. The string consists of a number of hard wares namely suspension tension clamps, socket clevis, ball clevis, anchor shackles, etc, under standard atmospheric conditions.

Line voltage No. of discs insulators Electromechanical strength of the discs insulator

66KV 5 to 6 5000 Kg132 KV 9 to 10 7500 kg220 KV 13 to 14 11500 kg400 KV 21 to 22 16500 kg

3) Supporting Structure & Accessories

The Supporting structures are normally of bolted steel lattice type. Angle sections are bolted together to form a square base lattice structure. The structures are provided with cross arms through which insulator strings are hung for supporting the conductors. The base of these towers may vary from 2.5 meters to 6 meters. The maximum distance from the

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ground is normally maintained in accordance with the stipulations made in the Indian Electricity Rules. These clearances are as follows:

The super structures are placed at an interval called the span length the maximum span in a line is influence by:-

(i) strength and size of conductor, (ii) height and strength of the structure,(iii) wind pressure,(iv) maximum temperature and(v) spacing and circuit configuration.

These structures are used for the straight run as well as for the points where the line deviates from the straight run. Span lengths and phase to phase distances of lines are given as under

Line voltage span length phase to phase distance66KV 250 m 2 m

132 KV 330 m 4 m220 KV 350 m 5 m400 KV 400 m 7.8 m

The structures used in deviation points are called. Angle towers. Experience has shown that 4 types of deviations viz

up to 2°, from 2° to 15°, from 15° to 30° , From 30° to 60°.

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The structures are designed for the maximum wind loadings occurring in that region and also for the maximum and minimum temperatures in that region. The design loadings and the permissible loadings for those structures are covered in the Indian standard IS 802- 1973.

4) Earth wires & Accessories

Earth wire is provided for giving protection to the transmission line against lightning strikes. These wires are supported in such a manner as to provide a shield angle of 30° to the conductors for 400KV lines, 2 ground wires are used with reduced shield angle of 20°. These earth wires are of galvanized stranded steel, high tensile steel quality. Earth wire is connected to the tower steel. The tower footings are further earthed thorough a suitable earthing arrangement.

Failures and causes of Bus faults in switchyard :-

Failures and causes of Bus faults in switchyard have taken place due to:-

Loose contact in isolators. While bus switching of feeders. Attributable to improper setting or auxiliary contacts or isolates. Loose sheets from boiler roof and other high rise structures near to switchyard

flying in dust storm and land on the live posts in the switchyard.

An uncleared fault on a live bus is quite serious as it has potential to cause grid collapse and in order to avoid such an occurrence, in addition to following proper and adequate maintenance practices, it is essential not to bypass the bus bar protection.

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TRANSFORMERS

Transformer is a static (or stationary) piece of apparatus by means of which electric power in one circuit is transformed to electric power of the same frequency in another circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease or increase in current. This Chapter deals with the basic theory, constructional features and types of major transformers found in a power station. In its simplest form, it consists of two inductive coils, which are electrically separated by magnetically linked through a path of low reluctance as shown in Fig 53. The two coils possess high mutual inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up in the laminated core and it produces mutually induced e.m.f. If the second coil circuit is closed, a current flows in it and so electrical energy is transferred from the first coil to the second coil.

The first coil, in which electric energy is fed from the a.c supply mains, is called primary winding, while the second coil is known as secondary winding. The necessity of the transformer arises when voltages are required to be changed. For example, the generated voltage of the alternators will be around 15 KV.

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MAJOR TRANSFORMER IN THE POWER STATION

a) GENERATOR TRANSFORMER

The generator is connected to this transformer by means of isolated bars. This transformer is used to step up the generating voltage of 16.5 KV to 200 KV. There is one GT per unit. It provides with off load tap changer on high voltage side. This transformer has elaborate cooling system consisting of number of oil pumps and cooling fans apart from various accessories.

SPECIFICATIONS:-

Make BHEL

Cooling OFAFRating HV 270 MVA

LV 27OMVA

No Load Voltage HV 235 KV LV 16.5KV

Line Current HV 664.12 ALV 9458.75 A

Temperature rise oil 50°C

Temperature rise winding 55°C

Phases 3

Frequency 50 Hz

Connection Symbol STAR - DELTA

Impedance volts 270 MVA

Normal tap% 13.75%

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STATION TRANSFORMERS

There are two station transformers in the power station that draw the power from the grid via 220 KV bus. Station transformer is a three winding transformer having two LV winding and one HV winding of voltages 6.6 KV and 220KV respectively. From the station transformer four switchgears are fed. It is required to feed power to the auxiliaries during start-ups. It is also provided with on load tap changer to cater high fluctuating voltages of the grid.

SPECIFICATIONS:-

Make NGEF

Cooling ONAN/ONAF/OFAFRated voltage HV 220 KV

LV1 6.6KVLV 2 6.6KV

Rating HV 55 MVALV1 22.5 MVALV2 22.5 MVA

Rated current HV 144.3 ALV1 230ALV2 230A

Temperature rise oil 50°C


Phases 3

Frequency 50 Hz

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UNIT AUXILARY TRANSFORMER

The UAT draws its input from main bus-duct connecting generator of the GT. There are two UAT per unit. It is used to step down the voltage level from 16.5 KV to 6.6 KV and gives supply to the unit auxiliary switchgear. The on load/off load Tap changer have been provided for operational requirements.

SPECIFICATIONS

Make BHEL

Cooling ONAF / ONANRating ONAF 1600 KVA

ONAN 1250 KVA

No Load Voltage HV 16.5 KVLV 6.6KV

Line Current HV LVONAF 559.8 A 1338.8 AONAN 437.3 A 1045.9 A

Max. Ambient Temp. 50°C


Phases 3

Frequency 50 Hz

Symmetrical SC current 14.3 x I Amps

Impedance volts 270 MVA

Normal tap% 13.75%

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TEST FOR TRANSFORMERS

OIL TEST

The oil test is performed at 33 KV and oil if filled in the container and 33KV is given to two electrodes. When at 33 KV sparking takes place and oil extinguishes the spark, oil gets black. If the oil quenches the arc and does not get black, it suites for transformer.

INSULATION TEST

The test is performed to cheek the insulation of the transformer and done by megger. These tests are done between

a) High voltage to earthb) Low voltage to earthc) High voltage to low voltage firstly of 15 sec. And then 60 sec. And then for 600 sec.

NO LOAD TEST

This test is done in order to determine the no load or core loss and no-load 10. In this secondary or load side is disconnected.

PROTECTION OF TRANSFORMERS

Buchholz relay is a gas actuated relay installed in oil immersed transformers for protection against all kinds of faults. It is mainly installed in the pipe connecting the conservator to the main tank. It is used for transformers more than 750 KVA.

Operation1) In case of fault within the transformer the heat due to fault causes the

decomposition of the same transformer oil in the main tank. The product of decomposition contains more than 70% of hydrogen. It gets trapped in upper part of relay chamber. When the gas accumulates it exerts sufficient pressure on the float to cause it to tilt and close the contact of mercury switch attached thus completing the alarm circuit to sound the alarm.

2) If a series fault occurs then an enormous amount of gas is generated in the tank. The oil in the tank rushes towards conservator via the Buchholz relay and in doing so tilts the flap to close the contact of mercury switch. This completes the trip circuit to open the circuit breaker.

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