`

by

ABHINAW KUMAR RAI

ELECTRICAL ENGINEERING

BANKURA UNNAYANI INSTITUTE OF ENGINEERING

FOR THE PERIOD OF THREE WEEKS FROM 16.06.15 TO 07.07.15 at

(DAMODAR VALLEY CORPORATION)

P.O. MEJIA, DIST. BANKURA WEST BENGAL-722183

A TRANING REPORT ON

MEJIA THERMAL POWER STATION

ACKNOWLEDGEMENT

The dissertation has been prepared based on the vocational training undergone in a highly esteemed organization of Eastern region, a pioneer in Generation Transmission & Distribution of power, one of the most technically advanced & largest thermal power stations in West Bengal, the Mejia Thermal Power Station (M.T.P.S), under DVC. I would like to express my heartfelt gratitude to the authorities of Mejia Thermal Power Station and BANKURA UNNAYANI INSTITUTE

OF ENGINEERING for providing me such an opportunity to undergo training in the thermal power plant of DVC, MTPS. I would also like to thank the Engineers, highly experienced without whom such type of concept building in respect of thermal power plant would not have been possible. Some of them are:

1) Mr. Parimal Kumar Dubey

2) Mr.

3) Mr.

4) Mr.

FOR THE PERIOD OF THREE WEEKS FROM 16.06.15 TO 07.07.15 at

(DAMODAR VALLEY CORPORATION)

P.O. MEJIA, DIST. BANKURA WEST BENGAL-722183

INTRODUCTIONDamodar Valley Corporation was established on 7th July 1948.It is the most reputed company in the eastern zone of India. DVC in established on the Damodar River. It also consists of the Durgapur Thermal Power Plant in Durgapur. The MTPS under the DVC is the second largest thermal plant in West Bengal. It has the capacity of 2340MW with 4 units of 210MW each, 2 units of 250MW each & 2 units of 500 MW each. With the introduction of another two units of 500MW that is in construction it will be the largest in West Bengal. Mejia Thermal Power Station also known as MTPS is located in the outskirts of Raniganj in Bankura District. It is one of the 5 Thermal Power Stations of Damodar Valley Corporation in the state of West Bengal. The total power plant campus area is surrounded by boundary walls and is basically divided into two major parts, first the Power Plant area itself and the second is the Colony area for the residence and other facilities for MTPS employees.

MEJIA THERMAL POWER STATION

(4 X 210MW+2 X 250MW+2 X 500M W)

TECHNICAL SPECIFICATION OF MTPS INSTALLED CAPACITY: -

1) Total number of Units : - 4 X 210 MW(unit 1 to 4) with Brush Type Generators, 2 X 250 MW(unit 5 and 6) with Brush less Type Generators, 2*500 MW(unit 7 and 8) Generators.

2) Total Energy Generation: - 2340 MW

3) Source of Water: - Damodar River

4) Sources of Coal: - B.C.C.L and E.C.L, also imported from Indonesia

Station Unit no. Capacity(MW)

Boiler Make

Turbine Make

MEJIA TPS 1,2,3&45&6

210250

BHELBHEL

BHELBHEL

MEJIA TPS 1&2 500 BHEL BHEL

In a Thermal Power generating unit, combustion of fossil fuel (coal, oil or natural gas) in Boiler or fissile element (uranium, plutonium) in Nuclear Reactor generates heat energy. This heat energy transforms water into steam at high pressure and temperature. This steam is utilised to generate mechanical energy in a Turbine. This mechanical energy, in turn is converted into electrical energy with the help of an Alternator coupled with the Turbine. The production of electric energy utilising heat energy is known as thermal power generation. The heat energy changes into mechanical energy following the principle of Rankine reheat-regenerative cycle and this mechanical energy transforms into electrical energy based on Faraday’s laws of electromagnetic induction. The generated output of Alternator is electrical power of three-phase

alternating current (A.C.). A.C. supply has several advantages over direct current (D.C.) system and hence , it is preferred in modern days. The voltage generated is of low magnitude (14 to 21 KV for different generator rating) and is stepped up suitably with the help of transformer for efficient and economical transmission of electric power from generating stations to different load centres at distant locations.

OVERVIEW OF THERMAL POWER PLANTA thermal power plant continuously converts the energy stored in the fossil fuels(coal, oil, natural gas) into shaft work and ultimately into electricity. The working fluid is water which is sometimes in liquid phase and sometimes in vapour phase during its cycle of operation. Energy released by the burning of fuel is transferred to water in the boiler to generate steam at high pressure and temperature, which then expands in the turbine to a low pressure to produce shaft work. The steam leaving the turbine is condensed into water in the condenser where cooling water from a river or sea circulates carrying away the heat released during condensation. The water is then fed back to the boiler by the pump and the cycle continues. The figure below illustrates the basic components of a thermal power plant where mechanical power of the turbine is utilised by the electric generator to produce electricity and ultimately transmitted via the transmission lines.

COAL HANDLING PLANT

Coal transported from mines by railway wagons

Unloaded to a moving underground conveyor belt

Crusher house

(20mm)

COAL HANDLING PLANT PROCEDURE

Generally most of the thermal power plants uses low grades bituminous coal. The conveyer belt system transports the coal

Dead storage (40 days)

Live storage (8

hrs)

(BOILER HOUSE)

COAL MILL

Pulverized coal (200 mesh)Boiler (hot

air+coal dust)

Combustion

from the coal storage area to the coal mill. Now the FHP (Fuel Handling Plant) department is responsible for converting the coal converting it into fine granular dust by grinding process. The coal from the coal bunkers. Coal is the principal energy source because of its large deposits and availability. Coal can be recovered from different mining techniques like

• shallow seams by removing the over burnt expose the coal seam

• underground mining.

The coal handling plant is used to store, transport and distribute coal which comes from the mine. The coal is delivered either through a conveyor belt system or by rail or road transport. The bulk storage of coal at the power station is important for the continues supply of fuel. Usually the stockpiles are divided into three main categories.

• live storage

• emergency storage

• long term compacted stockpile.

The figure below shows the schematic representation of the coal handling plant. Firstly the coal gets deposited into the track hopper from the wagon and then via the paddle feeder it goes to the conveyer belt#1A. Secondly via the transfer port the coal goes to another conveyer belt#2B and then to the crusher house. The

coal after being crushed goes to the stacker via the conveyer belt#3 for being stacked or reclaimed and finally to the desired unit. ILMS is the inline magnetic separator where all the magnetic particles associated with coal get separated.

WATER TREATMENT PLANTRaw water supply:

Raw water received at the thermal power plant is passed through Water Treatment Plant to separate suspended impurities and dissolved gases including organic substance and then through De-mineralised Plant to separate soluble impurities.

Deaeration:

In this process, the raw water is sprayed over cascade aerator in which water flows downwards over many steps in the form of thin waterfalls. Cascading increases surface area water to facilitate easy separation of dissolved undesirable gases (like hydrogen sulphide, ammonia, volatile organic compound etc.) or to help in oxygenation of mainly ferrous ions in presence of atmospheri oxygen to ferric ions.

Coagulation:

Coagulation takes place in clariflocculator. Coagulant destabilises suspended solids and agglomerates them into heavier floc, which is

separated out through sedimentation. Prime chemicals used for coagulation are alum, poly-aluminium chloride (PAC).

Filtration:

Filters remove coarse suspended matter and remaining floc or sludge after coagulation and also reduce the chlorine demand of the water.

Chlorination:

Neutral organic matter is very heterogeneous i.e. it contains many classes of highmolecular weight organic compounds. Humic substances constitute a major portion ofthe dissolved organic carbon from surface waters. They are complex mixtures of organic compounds with relatively unknown structures and chemical composition.

DM (Demineralised Water) Plant

In De-mineralised Plant, the filter water of Water Treatment Plant is passed through the pressure sand filter (PSF) to reduce turbidity and then throughactivated charcoal filter (ACF) to adsorb the residual chlorine and iron in filter water.

BOILERBoiler is an enclosed vessel in which water is heated and circulated until the water is turned in to steam at the required pressure. Coal is burned inside the combustion chamber of boiler. The products of combustion are

nothing but gases. These gases which are at high temperature vaporize the water inside the boiler to steam.

Types of Boiler:

1.Fire tube 2.Water tube

WATER TUBE BOILLER

Important parts of Boiler & their functions Economizer: Feed water enters into the boiler through economizer. Its function is to recover residual heat of flue gas before leaving boiler to preheat feed water prior to its entryinto boiler drum The drum water is

passed through down-comers for Circulation through the water wall for absorbing heat from furnace. The economizerre circulation line connects down-comer with the economizer inlet header through an isolating valve and a non-return valve to protect economizer tubes from overheating caused by steam entrapment and starvation. This is done to ensure circulation of water through the tubes during initial lighting up of boiler, when there is no feed water flow through economizer.

Drum: Boiler drum is located outside the furnace region or flue gas path. This stores certain amount of water and separates steam from steam-water mixture. The minimum drum water level is always maintained so as to prevent formation of vortex and to protect water wall tubes (especially its corner tubes) from steam entrapment / starvation due to higher circulation ratio of boiler.

Super heater: Super heaters (SH) are meant for elevating the steam temperature above the saturation temperature in phases; so that maximum work can be extracted from high energy (enthalpy) steam and after expansion in Turbine, the dryness fraction does not reach below 80%, for avoiding Turbine blade erosion/damage and attaining maximum Turbine internal efficiency. Steam from Boiler Drum passes through primary super heater placed in the convective zone of the furnace, then through platen super heater placed in the radiant zone of furnace and thereafter, through final super heater placed in the convective zone. The superheated steam at requisite pressure and temperature is taken out of boiler to rotate turbo-generator.

Reheater: In order to improve the cycle efficiency, HP turbine exhaust steam is taken back to boiler to increase temperature by reheating process. The steam is passed through Reheater, placed in between final superheater bank of tubes & platen SH and finally taken out of boiler to extract work out of it in the IP and LP turbine.

De-superheater (Attemperator): Though superheaters are designed to maintain requisite steam temperature, it is necessary to use de-superheater to control steam temperature. Feed water, generally taken before feed water control station, is used for de-superheating steam to control its temperature at desired level.

TECHNICAL DATA OF THE BOILER

TURBINE

A steam turbine is a prime mover which continuously converts the energy of highpressure, high temperature steam supplied by the boiler into shaft work with lowpressure, low temperature steam exhausted to a condenser.

Cooling tower

Cooling towers cool the warm water discharged from the condenser and feed the cooled water backto the condenser. They thus reduce the cooling water demand in the power plants. Wet coolingtowers could be mechanically draught or natural draught. In M.T.P.S the cooling towers are I.D.type for units 1-6 and natural draught for units 7&8.

CHIMNEYA chimney may be considered as a cylindrical hollow tower made of bricks or steel. In MTPS the chimneys of eight units are made of bricks. Chimneys are used to release the exhaust gases (coming from the furnace of the boiler)high up in the atmosphere. So, the height of the chimneys are made high.

ELECTROSTAIC PRECIPITATORThe principal components of an ESP are 2 sets of electrodes insulated from each other. First set of rows are electrically grounded vertical plates called collecting electrodes while the second set consists of wires called discharge electrodes. the negatively charged fly ash particles are driven towards the collecting plate and the positive ions travel to the negatively charged wire electrodes. Collected particulate matter is removed from the collecting plates by a mechanical hammer scrapping system.

ELECTRIC GENERATORIn M.T.P.S. there are 6 electric generators for units 1 to 6. These are 3 phase turbo generators, 2 pole cylindrical rotor type synchronous machines which are directly coupled to the steam turbine. The generator consist of 2 parts mainly the stator and the rotor. Stator: The stator body is designed to withstand internal pressure of hydrogen-air mixture without any residual deformation. The stator core is built up of segmental punching of high permeability, low loss CRGOS steel and are in interleaved manner on spring core bars to reduce heating and eddy current loss. The stator winding has 3 phase double layer short corded bar type lap winding having 2 parallel paths. The winding bars are insulated with mica thermosetting insulation tape which consists of flexible mica foil, fully saturated with a synthetic resin having excellent electrical properties. Water cooled terminal bushings are housed in the lower part of the stator on the slip ring side. Rotor: Rotor is of cylindrical type shaft and body forged in one piece from chromium nickel molybdenum and vanadium steel. Slots are machined on the outer surface to incorporate windings. Winding consists of coil made from hand drawn silver copper with bonded insulation. Generator casing is filled up with H2 gas with required pressure, purity of gas is always maintained>97%. Propeller type fans are mounted on either side of the rotor shaft for circulating the cooling gas inside the generators.

TRANSFORMERS

The electricity thus produced by the generator then goes to the generating transformer where the voltage is increased for transmission of electricity with minimized copper losses. In general a transformer consists of primary and secondary windings which are insulated from each other by varnish. In M.T.P.S. all are either oil cooled or air cooled. Some of the transformer accessories are:

1. Conservator tank

2. Buccholz relay

3. Fans for cooling

4. Lightning arrestors

5. Transformer bushings

6. Breather and silica gel

Generating transformer 1, 2, 3, 4 MVA: 150/200/250 (H.V.) MVA: 150/200/250 (L.V.)

Volts at no load: 240000 (H.V.) Volts at no load: 15750 (L.V.)

Ampere line value: 361/482/602 (H.V.)

Ampere line value: 5505/7340/9175 (L.V.)

Phase-3 frequency: 50 Hz.

Mass of core and windings: 139000 kg.

Mass of oil: 38070 kg.

Mass of heaviest package: 164000 kg.

Connection: YNd1 connection.

Generating transformer 5 and 6 MVA: 189/252/315 (H.V.)

MVA: 189/252/315 (L.V.)

Volts at no load: 16.5kV (L.V.)

Volts at no load: 240kV (H.V.)

Ampere line value: 757.57 (H.V.)

Ampere line value: 11022.14 (L.V.)

Phase-3 frequency: 50 Hz.

Mass of core and windings: 155000 kg.

Mass of oil: 53070 kg.

Mass of heaviest package: 18000 kg.

Connection: YNd1 connection.

AUXILIARY TRANSFORMRERS Station Service Transformers Normal source to the station auxiliaries and standby source to the unit auxiliaries during start up and after tripping of the unit is station auxiliary transformer. Quantity of station service transformers and their capacity depends upon the unit sizes and nos. Each station supply transformer shall be one hundred percent standby of the other. Station service transformers shall cater to the simultaneous load demand due to start up power requirements for the largest unit, power requirement for the station auxiliaries required for running the station and power requirement for the unit auxiliaries of a running unit in the event of outage of the unit source of supply. The no. and approximate capacity of the SST depending upon the no. and MW rating of the TG sets are indicated below.

Unit Auxiliary Transformer The normal source of HV Power to unit auxiliaries is unit auxiliary transformer. The sizing of the UAT is usually based on the total connected capacity of running unit auxiliaries i.e., excluding the stand by drives. It is safe anddesirable to provide about 20% excess capacity than calculated. The no. and recommended MVA rating of unit auxiliary transformers are as shown in the above table: The UATs shall have Ddo(ungrounded system) or Dy1 (for grounded system) connection with on load tap changer to provide +10 % variation in steps of 1.25 %. Usual cooling arrangement to unit auxiliary transformers are ONAN. Radiators are usually divided in two equal halves.

SpecificationMVA: 12.5/16 Manufacturer: Atlanta Electricals

Volts at no load: 15750 (H.V.) Volts at no load: 6900 (L.V.)

Ampere line value: 458.2/586.5 (H.V.)

Ampere line value: 1045.9/1338.8 (L.V.)

Phase-3 frequency: 50 Hz.

Mass of core and windings: 14300kg.

Mass of oil: 8600kg

Mass of heaviest package: 25000kg.

Total weight: 30,500 kg.

Unit auxiliary transformer #5 & 6

Type of cooling: ONAN/ONAF (oil natural/ oil natural air force)

Rating (H.V.): 20/16 MVA Rating (L.V.): 20/16 MVA

No load voltage: 13.5 kV (H.V.) No load voltage: 6.9 kV (L.V.)

Line current: 1673.479/1336.783 amp.

Temperature rise of winding: 55*C

Insulation level: 931 KVI 38kV r.m.s (H.V.) 60kVI 20Kv r.m.s (L.V.)

CONTROL ROOM UNIT:

The above figure shows the power line diagram in the control room. It clearly shows how the electric power generated by the generator is transmitted through the generating transformers into the bus and the distribution of power by the unit auxiliary transformers.

EXCITATION SYSTEM The purpose of excitation system is to continuously provide the appropriate amount of D.C. field current to the generator field winding. The excitation system is required to function reliably under the following conditions of the generator and the system to which it is connected.

Functional components of an excitation system :

A good excitation system consists of properly co-ordinated functional components which are

a) Excitation Power source

b) Semiconductor Rectifier

c) Voltage controller

d) Protective, limiting and switching equipments

e) Monitoring, Metering and indicating equipments and

f) Cooling system

Types of Excitation System :

In earlier days DC excitation system was in use. Increase in generator capacity in turn raised the demand of excitation power which was notachievable by the DC exciters. This led to the accelerated development of AC excitation system in pace with generator capacity. With the maturing of solid state semiconductor technology AC excitation system found to be superior technically as well as economically. Excitation system can be categorized and subdividedinto the following :

a) D.C. excitation system

i) Pilot Main Exciter excitation system

ii) Rotating Amplifier excitation system.

b) A.C. excitation system

i) Rotating High Frequency excitation system

ii) Static excitation system

iii) Brushless excitation system

SWITCHYARD SECTION A switchyard is essentially a hub for electrical power sources. For instance, a switchyard will exist at a generating station to coordinate the exchange of power between the generators and the transmission lines in the area. A switchyard will also exist when high voltage lines need to be converted to lower voltage for distribution to consumers. Here in MTPS there is a big switch yard section for the units one to six, and also for seven & eight there also a switch yard. Some of the operation of the components of the switch yard is sometimes done from the control rooms of respective units. That is the switch yard under each unit is sometimes control from the control rooms of each unit respectively

A switchyard may be considered as a junction point where electrical power is coming in from one or more sources and is going out through one or more circuits. This junction point is in the form of a high capacity conductor spread from one end to the other end of the yard. As the switchyard handles large amount of power, it is necessary that it remains secure and serviceable to supply the outgoing transmission feeder seven under conditions of major equipment or bus failure. There are differentschemes available for bus bar and associated equipment connection to facilitate switching operation. The important points which dictate the choice of bus switching scheme are –

a. Operational flexibility.

b. Ease of maintenance.

c. System security.

d. Ease of sectionalizing.

e. Simplicity of protection scheme.

f. Installation cost and land requirement.

g. Ease of extension in future.

The basic components of a switchyard are as follows:

1.Circuit breaker:

A circuit breaker is an equipment that breaks a circuit either manually or automatically under all conditions at no load, full load or short circuit. Oil circuit breakers, vacuum circuit breakers and SF6 circuit breakers are a few types of circuit breakers.

2.Isolator:

Isolators are switches which isolate the circuit at times and thus serve the purpose of protection during off load operation.

3.Current Transformer :

These transformers used serve the purpose of protection and metering. Generally the same transformer can be used as a current or potential transformer depending on the type of connection with the main circuit that is series or parallel respectively. In electrical system it is necessary to

a) Read current and power factor

b) Meter power consumption.

c) Detect abnormalities and feed impulse to protective devices.

4.Potential transformers :

In any electrical power system it is necessary to

a) Monitor voltage and power factor,

b) Meter power consumption,

c) Feed power to control and indication circuit and

d) Detect abnormalities

(i.e. under/over voltage, direction of power flow etc) and feed impulse to protective device/alarm circuit. Standard relay and metering equipments does not permit them to be connected directly to the high voltage system.Potential transformers therefore play a key role by performing the following functions.

a) Electrically isolating the instruments and relays from HV side.

b) By transferring voltage from higher values to proportional standardized lower values.

POWER TRANSFORMER:

The use of power transformer in a switchyard is to change the voltage level. At the sending and usually step up transformers are used to evacuate power at transmission voltage level. On the other hand at the receiving end step down transformers are installed to match the voltage to sub transmission or distribution level. In many switchyards autotransformers are used widely for interconnecting two switchyards with different voltage level (such as 132 and 220 KV)

1-Main tank

2-Radiator

3-Reservoir tank

4-Bushing

5-WTI & OTI Index

6-Breather

7-Buccholz relay

Insulator The live equipments are mounted over the steel structures or suspended from gantries with sufficient insulation in between them. In outdoor use electrical porcelain insulators are most widely used. Following two types of insulators are used in switchyard.

a. Pedestal type

b. Disc type Pedestal type insulators

are used on steel structures for rigid supporting of the pipe bus bars, for holding the blade and the fixed contacts of the isolators.

Electric power is generated by the generator which is circulated to the main bus 1 or 2 and accordingly the respective isolator is closed. In case of any fault in the circuit breaker the power from the generator goes via the transfer bus into the main bus by means of the bus coupler. A bus tie represents the connection between the two main buses. Two 80MVA transformers draw power from the main buses and transfer the voltage to 33kV and the power goes to 33kV switchyard. A station service transformer supplies power to the auxiliary load.

The above figure shows the power flow diagram of 33kV switchyard.

The electric power after voltage transformation to 33kV by 80MVA transformers goes to the main bus of the 33kV switchyard from where power is fed to various industries and other nearbystations. There are two earthing transformers in the yard. From the bus the power is fed to two 5MVA transformers which step down the voltage level to 11kV and is thus distributed to the locality.

THE TYPE OF RELAYS USED IN MTPS FOR PROTECTION OF POWER SYSTEM COMPONENTS

• Auxiliary relay for isolations

• Fail accept relay

• Directional over current relay

• Master trip relay

• Multi relay for generator function

• Supervision relay

• Instantaneous relay

• Bus bar trip relay

• Lock out relay

• Numerical LBB protection relay

• Transformer differential protection relay

• Circulating differential protection relay

• Contact multi-relay

• Auxiliary relay

• Trip circuit R-Phase relay

• EUS section relay

• DC fail accept relay

• Trip circuit R-phase super relay Y-phase B-phase

• LBB protection relay.

SOME PUMP & MOTOR IS USED IN MTPS

PUMP :-

Service water pump -360Kw

Primary air fan(PA fan) -800Kw

Coal mill motor -2250Kw

Condense extraction pump -500Kw

MOTOR :-

Boiler feed pump motor -3500Kw

ID fan motor -1500Kw

FD fan motor -1000Kw

CW pump motor -1200Kw

SWITCHGEAR

HV SWITCHGEARS

Indoor metal clad draw out type switchgears with associated protective and control equipments are employed (fig. 2). Air break, Air Blast circuit breakers and Minimum Oil circuit breakers could still be found in some very old stations. Present trend is to use SF6or vacuum circuit breakers. SF6 and vacuum circuit breakers requires smaller size panels and thereby reasonable amount of space is saved. Fig. 2: General arrangement of 6.6 KV switchgear panels The main bus bars of the switchgears are most commonly made up of high conductivity aluminium or aluminium alloy with rectangular cross section mounted in side the switchgear cubicle supported by moulded epoxy, fibre glass or porcelain insulators. For higher current rating copper bus bars are sometimes used in switchgears.

LV SWITCHGEARS

LV switchgears feed power supply to motors above 110 KW and upto160 KW rating and to Motor Control Centers (M.C.C). LV system is also a grounded system where the neutral of transformers are solidly connected to ground. The duty involves momentary loading, total load throw off, direct on line starting of motors and under certain emergency condition automatic transfer of loads from one source of supply to the other. The switchgear consists of metal clad continuous line up of multi tier draw out type cubicles of simple and robust construction. Each feeder is provided with an individual front access door. The main bus bars and connections shall be of high grade aluminium or aluminium alloy sized for the specified current rating. The circuit breakers used in the LV switchgear shall be air break 3 pole with stored energy, trip free shunt trip mechanism. These are draw out type with three distinct position namely, Service, Test and Isolated. Each position shall have mechanical as well as electrical indication. Provision shall be there for local and remote electrical operation of the breakers. Mechanical trip push button shall be provided to trip manually in the event of failure of electrical trip circuit. Safety interlocks shall be provided to prevent insertion and removal of closed breaker from Service position to Test position and vice versa.

SWITCHING SCHEMES

One Main Bus and Transfer Bus scheme

This scheme is used in switchyards up to 132 KV. Under normal condition all feeders arefed through their respective circuit breakers from the main bus bar. During shutdown or outage of any feeder breaker, that feeder can be transferred to transfer bus and diverted through bus coupler breaker. In that case the protection shall be transferred to the bus coupler circuit breaker by changing the position of the trip transfer switch located at the switchyard control panel. This diversion of the feeder from its own circuit breaker to bus coupler circuit breaker and the vice versa is possible even in live condition without any interruption of supply to that feeder. In case of any main bus fault the entire switchyard will collapse. To avoid such total collapse of the switchyard a bus section circuit breaker is provided in the middle position of the main bus.

Two Main Bus and One transfer Bus scheme

In this scheme there is an arrangement for a duplicate main bus (MB). All the feeders in the yard may be connected to either MB # 1 or MB # 2 or may be divided in two groups and distributed in two buses. In case of outage of any circuit breaker that feeder can be diverted through bus coupler breaker. Bus tie breaker is used to tie up MB #1 & MB # 2.

One and Half Breaker Scheme

In one and half breaker scheme (Fig. 4) under normal condition all the circuit breakers will remain closed. At the time of maintenance of feeder breaker, only that breaker would be kept open and isolated. During maintenance of bus, all the breakers connected to that bus would remain open to isolate the bus. At that time, the power supply may be maintained through other bus. All equipments in the switchyard except the line side isolators can be maintained without taking shut down of any feeder. This scheme has gained popularity in many 400 KV switchyards in our country.

GENERATOR PROTECTION The purpose of generator protection is to provide protection against abnormal operating condition and during fault condition. In the first case the machine and the associated circuit may be in order but the operating parameters (load, frequency, temperature) and beyond the specified limits. Such abnormal running condition would result in gradual deterioration and ultimately lead to failure of the generator.

Protection under abnormal running conditions

a) Over current protection: The over current protection is used in generator protection against external faults as back up protection. Normally external short circuits are cleared by protection of the

faulty section and are not dangerous to the generator. If this protection fails the short circuit current contributed by the generator is normally higher than the rated current of the generator and cause over heating of the stator, hence generators are provided with back up over current protection which is usually definite time lag over current relay.

b) Over load protection: Persistent over load in rotor and stator circuit cause heating of winding and temperature rise of the machine. Permissible duration of the stator and rotor overload depends upon the class of insulation, thermal time constant, cooling of the machine and is usually recommended by the manufacturer. Beyond these limits the running of the machine is not recommended and overload protection thermal relays fed by current transformer or thermal sensors are provided.

c) Over voltage protection: The over voltage at the generator terminals may b e caused by sudden drop of load and AVR malfunctioning. High voltage surges in the system (switching surges or lightning) may also cause over voltage at the generator terminals. Modern high speed voltage regulators adjust the excitation current to take care against the high voltage due to load rejection. Lightning arresters connected across the generator transformer terminals take care of the sudden high voltages due to external surges. As such no special protection against generator high voltage may be needed. Further protection provided against high magnetic flux takes care of dangerous increase of voltage.

d) Unbalance loading protection: Unbalance loading is caused by single phase short circuit outside the generator, opening of oneof the contacts of the generator circuit breaker, snapping of conductors in the switchyard or excessive single phase load. Unbalance load produces –ve phase sequence current which cause overheating of the rotor surface and mechanical vibration. Normally 10% of unbalance is permitted provided phase currents do not exceed the rated values. For –ve phase sequence currents above 5-10% of rated value dangerous over heating of rotor is caused and protection against this is an essential requirement.

f) Loss of prime mover protection: In the event of loss of prime mover the generator operates as a motor and drives the prime mover itself. In some cases this condition could be very harmful as in the case of steam turbine sets where steam acts as coolant, maintaining the turbine blades at a constant temperature and the failure of steam results in overheating due to friction and windage loss with subsequent distortion of the turbine blade. This can be sensed by a power relay with a directional characteristic and the machine can be taken out of bar under this condition. Because of the same reason a continuousvery low level of output from thermal sets are not permissible.

Protection under fault condition a) Differential protection: The protection is used for detection of internal faults in a specified zone defined by the CTs supplying the differential relay. For an unit connected system separate differential relays are provided for generator, generator transformer and unit auxiliary transformer in addition to the overall differential protection. In order to restrict damage very high differential relay sensitivity is demanded but sensitivity is limited by C.T errors, high inrush current during external fault and transformer tap changer variations.

b) Back up impedance protection: This protection is basically designed as back up protection for the part of the installation situated between the generator and the associated generator and unit auxiliary transformers. A back up protection in the form of minimum impedance measurement is used, in which the current windings are connected to the CTs in the neutral connection of the generator and its voltage windings through a P.T to the phase to phase terminal voltage. The pick up impedance is set to such a value that it is only energized by short circuits in the zone specified above and does not respond to faults beyond the transformers.

c) Stator earth fault protection: The earth fault protection is the protection of the generator against damages caused by the failure of insulation to earth. Present practice of grounding the generator

neutral is so designed that the earth fault current is limited within 5 and 10 Amp. Fault current beyond this limit may cause serious damage to the core laminations. This leads to very high eddy current loss with resultant heating and melting of the core.

d) 95% stator earth fault protection: Inverse time voltage relay connected across the secondary of the high impedance neutral grounding transformer relay is used for protection of around 95% of the stator winding against earth fault.

e) 100% stator earth fault protection: Earth fault in the entire stator circuits are detected by a selective earth fault protection covering 100% of the stator windings. This 100% E/f relay monitors the whole stator winding by means of a coded signal current continuously injected in the generator winding through a coupling. Under normal running condition the signal current flows only in the stray capacitances of the directly connected system circuit.

f) Rotor earth fault protection: Normally a single rotor earth fault is not so dangerous as the rotor circuit is unearthed and current at fault point is zero. So only alarm is provided on occurrence of 1st rotor earth fault. On occurrence of the 2nd rotor earth fault between the points of fault the field winding gets short circuited. The current in field circuit increases, resulting in heating of the field circuit and the exciter. But the more dangerous is disturbed symmetry of magnetic circuit due to partial short circuited coils leading to mechanical unbalance.

MOTORS FOR THERMAL POWER PLANT

All the motors in Thermal Power Stations shall be of the 3-ph. A.C. squirrel cage type except for some auxiliaries, which are emergent in nature,for which DC motors shall be used. For some small valves, single phase motors may be used. All A.C. motors shall be suitable for direct on line starting.

Battery Bank

Normally D.C. power is supplied by the float charger and the batteries are kept in float condition at 2.15 V per cell to avoid discharging. The charger consists of silicon diode or thyristor rectifiers preferably working on 3 ph. 415 V supply in conjunction with an automatic voltage regulator. When there is a failure in the A.C. supply the batteries will come into operation and in this process the batteries run down within few hours. After normalization of A.C. power the batteries are charged quickly by using the boost charger at 2.75 V per cell. During this time the float chargeris isolated and load is connected through the tap off point. After normalization of battery voltage these are again put back into the float charging mode. The output from the battery as well as the charger is connected to the D.C. distribution board. From D.C. distribution board power supply is distributed to different circuits. D.C. system being at the core of the protection and control mechanism very often two 100% capacity boards with individual chargers and battery sets are used from the consideration of the reliability and maintenance facility. These two boards are interconnected by suitable tie lines.

CONCLUSION The practical experience that I have gathered during the overview training of large thermal power plant having a large capacity of 2340 MW for Unit# I to VIII in three

weeks will be very useful as a stepping stone in building bright professional career in future life. It gave me large spectrum to utilize the theoretical knowledge and to put it into practice. The trouble shooting activities in operation and decision making in case of crisis made me more confident to work in the industrial atmosphere. Moreover, this overview training has also given a self realization & hands-on experience in developing the personality, interpersonal relationship with the professional executives, staffs and to develop the leadership ability in industry dealing with workers of all categories. I would like to thank everybody who has been a part of this project, without whom this project would never be completed with such ease.