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Embed code for: Guru Nanak Dev Thermal Power Plant
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Guru Nanak Dev Thermal Power Plant is a coal-based plant. The requirement of coal for four units based on specific fuel consumption of 0.60 kg / kWh. The conveying and crushing system will have the same capacity as that of the unloading system. The coal comes in as large pieces. This coal is fed to primary crushers, which reduce the size of coal pieces from 400mm to 150mm. Then the coal is sent to secondary crusher through forward conveyors where it is crushed from 150mm to 200mm as required at the mills. Then the coal is sent to boilers with the help of primary fans. The coal is burnt in the boiler. Boiler includes the pipes carrying water through them; heat produced from the combustion of coal is used to convert water in pipes into steam. This steam generated is used to run the turbine. When turbine rotates, the shaft of generator, which is mechanically coupled to the shaft of turbine, gets rotated so, three phase electric supply is produced.
The basic requirements are:-
♣ Fuel (coal)
♣ Steam turbine
♣ Ash handling system
♣ Unit auxiliaries
BRIEF HISTORY OF
Due to high rate of increasing population day by day, widening gap between power demand and its availability was one the basic reason for envisaging the G.N.D.T.P. for the state of Punjab. The other factors favoring the installation of the thermal power station were low initial cost and comparatively less gestation period as compared to hydro electric generating stations. The foundation stone of G.N.D.T.P. at bathinda was laid on 19th November 1969, the auspicious occasion of 500th birth anniversary of great Guru Nanak Dev Ji.
The historic town of bathinda was selected for this first and prestigious thermal project of the state due to its good railway connections for fast transportations of coal, availability of canal water and proximity to load center.
The total installed capacity of the power station 440MW with four units of 110MW each. The first unit of the plant was commissioned in September, 1974. Subsequently second, third and fourth units started generation in September 1975, March 1978, and January 1979 respectively. The power available from this plant gives spin to the wheels of industry and agricultural pumping sets.
R&M WORKS AT GNDTP, BATHINDA
R&M of GNDTP unit 1&2 has already been completed pending PG Test. R&M works of unit 3&4 is underway to improve performance, enhance capacity and extend operating life of the units. The present status of R&M works of GNDTP units is as under:
Unit I&II: - Against approved project Report of Rs. 229 Crores, Order was placed on M/S NASL, New Delhi for major R&M works on Turnkey basis at a total of Rs.183 Crores.
Unit II: R&M works completed in October, 2005 (Pending attending to some deficiencies by the firm). Average PLF achieved post R&M works is 87%.
Unit I: - R&M works completed and taken for normal operation in May, 2007(Pending attending to some deficiencies by the firm). Average PLF achieved post R&M during May’07 and June ’07 is 95.65%.
Unit III & IV: - Order for executing R&M works on Turnkey basis already placed on M/S BHEL at a total cost of Rs. 465.36 Crores. 10% advance payment has been made to M/S BHEL on 22/12/2006 and design and drawing work is in progress. As per Schedule, work is to be completed in a phased manner upto July 2009. Apart from enhancing the operating life and performance level of the units, it is also planned to upgrade the capacity from 110 MW to 120 MW each resulting in total capacity addition of 20 MW.
The selection of site for Thermal Power Plant is more difficult compared to Hydro Power Plant, as it involves number of factors to be considered for its economic justification. The following consideration should be examined in detail before selection of the site for the Plant. The location for plant should be made with full consideration not only of the trends in the development and location but also the availability and location of the cheapest source of primary energy:-
Availability of fuel
Ash disposal facilities
Nature of land
Availability of labour
Public society problems
Development of Backward Area
G.N.D.T.P. won an award of Rs. 3.16 crores from Govt. of India for better performance in 1983-84.
It achieved a rare distinction of scoring hart Rick by winning meritorious productivity awards of Govt. of India, Ministry of Energy for year 1987, 1988 and 1989 due to its better performance.
It again won meritorious productivity awards during the year 1992-1993 and 1993-94 and has become entitled for the year 1996-1997 for better performance.
It also won awards for reduction in fuel oil consumption under Govt. of India incentive scheme years from 1992-1993 (awards money for 1992, 1993 and 1994 already released for 1995, 1996 and 1997 under the consideration of Govt. of India).
Dept. of Mech. Engg. B.M.S.C.E. , MUKTSAR G.N.D.T.P. had achieved a generation of 2724240 LU’s (at a PLf of 70%) and registering an oil consumption as low as 1.76ml/kwh during the year 1993-94 has broken all previous records of performance since the inception of plant.
Guru Nanak Dev Thermal Plant, Bathinda, in addition to indirect contribution in various facts of state economy, is also responsible for:-
Narrowing the gap between power demand and power availability of the state.
Providing employment potentials to thousands of workers.
Covering the backward surrounding area into fully developed Industrial Township.
Providing additional relief to agricultural pumping sets to meet the irrigation needs for enhancing the agriculture production.
Reliability and improvement in continuity of supply and system voltage.
Achieving cent percent rural electrification of the state.
PLANT SALIENT FEATURES
Power plant 238 acres
Ash disposal 845
Residential colony 285
Marshalling yard 256
Total area 1804
TOTAL COST: - Rs. 115 crores
STATION CAPACITY: - four units of 110MW.each
Maximum continuous rating (M.C.R.) 375 T/hr.
Superheater outlet pressure 139 kg/cm²
Reheater outlet pressure 33.8 kg/cm²
Final superheater/reheater temperature 540C
Feed water temperature 240C
Coal consumption per day per unit 1400 tones (Approximate)
Rated output 110 MW.
Rated speed 3000 r.p.m.
Number of cylinders three
Rated pressure 130 kg/cm²
Rated temperature 535C
Condenser vacuum 0.9 kg/cm²
(Unit- 1 & 2) 125000KVA
(Unit -3 & 4) 137000KVA
Generator voltage 11000 volts
Rated phase current
(unit –1 & 2) 6560 Amps.
(unit –3 & 4) 7220 Amps.
Generator cooling hydrogen
BOILER FEED PUMPS:-
Number per unit two of 100% duty each
Rated discharge 445 T/hr.
Discharge head 1960 MWC.
Speed 4500 r.p.m.
CIRCULATING WATER PUMPS:-
Numbers for two units five of 50% duty each
Type mixed flow
Rated discharge 8600 T/hr.
Discharge head 24 MWC.
Water cooled 18000 T/hr.
Cooling range 10C
Height 120/12 metres
COAL PULVERISING MILLS:-
Numbers three per unit
Rated output 27 T/hr.
Coal bunkers 16 per unit
RATING OF 6.6 KV AUXILLIARY MOTORS:-
Coal mill 630 KW
Vapour fan 320 KW
C.W. Fan 800/746 KW
Coal crusher 520 KW
Primary air fan 320 KW
Forced draught fan 320 KW
Boiler feed pump 3500 KW
Induced draught fan 900/1000 KW
Condensate pump 175 KW
Coal received from collieries in the rail wagon is mechanically unloaded by Wagon Tippler and carried by belt Conveyor System Boiler Raw Coal Bunkers after crushing in the coal crusher. The crushed coal when not required for Raw Coal Bunker is carried to the coal storage area through belt conveyor. The raw coal feeder regulates the quantity of coal from coal bunker to the coal mill, where the coal is pulverized to a fine powder. The pulverized coal is then sucked by the vapour fan and finally stored in pulverized coal bunkers. The pulverized coal is then pushed to boiler furnace with the help of hot air steam supplied by primary air fan. The coal being in pulverized state gets burnt immediately in the boiler furnace, which is comprised of water tube wall all around through which water circulates. The water gets converted into steam by heat released by the combustion of fuel in the furnace. The air required for the combustion if coal is supplied by forced draught fan. This air is however heated by the outgoing flue gases in the air heaters before entering the furnace.
The products of combustion in the furnace are the flue gases and the ash. About 20% of the ash falls in the bottom ash hopper of the boiler and is periodically removed mechanically. The remaining ash carried by the flue gases, is separated in the electrostatic precipitators and further disposed off in the ash damping area. The cleaner flue gases are let off to atmosphere through the chimney by induced draught fan.
The chemically treated water running through the water walls of boiler furnace gets evaporated at high temperature into steam by absorption of furnace heat. The steam is further heated in the super heater. The dry steam at high temperature is then led to the turbine comprising of three cylinders. The thermal energy of this steam is utilized in turbine for rotating its shaft at high speed. The steam discharged from high pressure (H.P.) turbine is returned to boiler reheater for heating it once again before passing it into the medium pressure (M.P.) turbine. The steam is then let to the coupled to turbine shaft is the rotor of the generator, which produces electricity. The power from the generator is pumped into power grid system through the generator transformer by stepping up the voltage.
The steam after doing the useful work in turbine is condensed to water in the condenser for recycling in the boiler. The water is pumped to deaerator from the condenser by the condensate extraction pumps after being heated in the low pressure heater (L.P.H) from the deaerator, a hot water storage tank. The boiler feed pump discharge feed water to boiler at the economizer by the hot flue gases leaving the boiler, before entering the boiler drum to which the water walls and super heater of boiler are connected.
The condenser is having a large number of brass tubes through which the cold water is circulated continuously for condensing the steam passing out sides the surface of the brass tubes, which has discharged down by circulating it through the cooling tower shell. The natural draught of cold air is created in the cooling tower, cools the water fall in the sump and is then recirculated by circulating water pumps to the condenser.
BOILER FEED PUMP:-
As the heart is to human body, so is the boiler feed pump to the steam power plant. It is used for recycling feed water into the boiler at a high pressure for reconversion into steam. Two nos. 100% duty, barrel design, horizontal, centrifugal multistage feed pumps with hydraulic coupling are provided for each unit. This is the largest auxiliary of the power plant driven by 3500 KW electric motor.
The capacity of each boiler at GURU NANAK DEV THERMAL PLANT is 375 tones/hr. The pump which supplies feed water to the boiler is named as boiler feed pump. This is the largest auxiliary in the unit with 100% capacity which takes suction of feed water from feed water tank and supplies to the boiler drum after preheating the same in HP-1, HP-2 and economizer. The delivery capacity of each boiler feed pump is 445 tones/hr. to meet better requirements corresponding to the various loads, to control steam temperature, boiler make up water etc. The detailed particulars checking of protections and inter locks, starting permission etc. are as below:-
Particulars of BFP and its main motor:-
BOILER FEED PUMP: - The 110 MW turboset is provided with two boiler feed pumps, each of 100% of total quantity. It is of barrel design and is of horizontal arrangement, driven by an electric motor through a hydraulic coupling.
Type 200 KHI
No. of stages 6
Delivery capacity 445 t/hr.
Feed water temperature 158C
Speed 4500 rpm
Pressure at suction 8.30 kg/cm²
Stuffing box mechanical seal
Lubrication of pump by oil under pressure
And motor bearing supplied by hydraulic coupling
Consumption of cooling water 230 L/min.
WATER TREATMENT PLANT:-
The water before it can be used in the boiler has to be chemically treated, since untreated water results in scale formation in the boiler tubes especially at high pressure and temperatures. The water is demineralised by Ion Exchange Process. The water treatment plant has production capacity of 1800 Tonnes per day for meeting the make-up water requirement of the power station.
Coal Mill pulverizes the raw coal into a fine powder before it is burnt in the boiler furnace. The pulverizing of coal is achieved with the impact of falling steel balls, weighing 52.5 tonnes, contained in the mill drum rotating at a slow speed of 17.5 r.p.m. The raw coal is dried, before pulverizing, with inert hot flue gases tapped from the boiler. Three coal mills each with a pulverizing capacity of 27 T/hr. are provided for one unit.
INDUCED DRAUGHT FAN:-
Two nos. axial flow Induced Draught Fans are provided for each unit to exhaust ash laden flue gases from boiler furnace through dust extraction equipment and to chimney. The fan is driven by an electric motor through a flexible coupling and is equipped with remote controlled regulating vanes to balance draught conditions in the furnace. The fan is designed to handle hot flue gases with a small percentage of abrasive particles in suspension.
The control room is the operational nerve center of the power plant. The performance of all the equipments of the plant is constantly monitored here with the help of sophisticated instrumentation and controllers. Any adverse deviation in the parameters of various systems is immediately indicated by visual and audio warning and suitable corrective action is taken, accordingly. The control room is air conditioned to maintain the desired temperature for proper functioning of the instruments.
Electricity generated at 11 KV by the turbo-set is stepped-up by unit transformers to 132/220 KV for further transmission through high tension lines to Maur, Muktsar, Malout, N.F.L., Sangrur and Ludhiana. Transmission of power to grid is controlled through 7 nos. 220 KV and 15 nos. 132 KV. Air Blast Circuit Breakers along with their associated protective systems.
The coal received from the collieries, in more than 100 rail wagons a day, is unloaded mechanically by two nos. wagon tipplers out of which one serves as a standby. Each loaded wagon is emptied by tippling it in the underground coal hopper from where the coal is carried by conveyor to the crusher house. Arrangements have been provided for weighing each rail wagon before and after tippling. Each tippler is capable of unloading 6-8 rail wagons of 55 tonnes capacity in an hour.
Coal unloaded by the wagon tippler is carried to crusher house through conveyors for crushing. Two nos. hammer type coal crushers are provided, which can crush coal to a size of 10 mm. The crushed coal is then supplied to Boiler Raw Coal Bunkers. The surplus coal is carried to coal storage area by series of conveyors. Crushing of coal is an essential requirement for its optimum pulverizing and safe storage.
Cooling Towers of the power plant are the land mark of the Bathinda City even for a far distance of 8-10 kilometers. One cooling tower is provided for each unit for cooling 18000 tones of water per hour by 10C. cooling towers are massive Ferro-concrete structure having hyperbolic profile creating natural draught of air responsible for achieving the cooling effect. Cooling tower is as high as 40 storey building.
It is a single drum, balanced draught, natural circulation, reheat type, vertical combustion chamber consists of seamless steel tubes on all its sides through which water circulates and is converted into steam with the combustion of fuel. The temperature inside the furnace where the fuel is burnt is of the order of 1500C. The entire boiler structure is of 42meter height.
The flue from the boiler, after removal of ash in the precipitators, are let off to atmosphere through boiler chimney, a tall ferro-concrete structure standing as high as the historic Qutab Minar. Four chimneys, one for each unit, are installed. The chimney is lined with fire bricks for protection of ferro-concrete against hot flue gases. A protective coating of acid resistant paint is applied outside on its top 10 meters.
CIRCULATING WATER PUMP:-
Two nos. of circulating water pumps provided for each unit, circulate water at the rate of 17200 T/hr. in a closed cycle comprising of Turbine Condenser and Cooling Tower. An additional Circulating Water Pump provided serves by for two units. The water requirement for bearing cooling of all the plant auxiliaries is also catered by these pumps.
COAL MILLING PLANT
Since G.N.D.T.P. units are primarily coal fired units so each boiler is provided with closed milling circuits to pulverize the raw coal which is received from coal conveying system after coal crushes before it is fired in the furnace. The necessity of pulverizing the coal is to be ensuring its maximum possible combustion in the furnace. The coal data for units are: -
UNITS 1 & 2
UNITS 3 & 4
Type of Coal
Net Calorific Value
Volatile Matter Incombustible
Inlet of Coal
Raw coal of maximum size 10 mm – 20 mm is pulverized in the milling circuit and the output from the mill is fine coal. Milling circuits of the following main constituents: -
Raw Coal Bunkers (R.C. Bunkers).
Raw Coal Chain Feeders.
Drum Mill or Coal Mill.
Pulverized Coal Bunkers (P.C. Bunkers).
RAW COAL BUNKER:-
Each of three raw coal bunkers is fabricated from the sheet metal and is well stiffened all around. The storage capacity of each raw coal bunker is about 500 tones. There are four outlet gates with each bunker. The gates are electrically operated from site. In case of failure of the electric motors the gate can be hand operated from site. At a time only one gate opening is suffices but should be changed so that there is no pilling within the bunker.
RAW COAL CHAIN FEEDER:-
The raw coal chain feeder transports coal from raw coal bunker to the inlet chute leading to the pulverized/coal mills. There is a double link chain of high tensile strength steel, which moves on wheels and sweeps the raw coal falling over the top of the raw coal chute of the mill. The height of the coal bed in the chain feeder can be adjusted manually by means of lever operated damper. The maximum and minimum heights of the coal bed are 200mm and 120mm respectively. The signaling equipment indicates the absence of coal flow in the feeder, which is annunciated in the unit control board (U.C.B.). The main shaft on the driving end is connected to the driving unit, consisting of variator, a gear box and a motor all mounted as a single unit. The chain wheel on the driving end shaft is provided with a shear pin, which will shear off and disconnect the driving mechanism if there is any overload on the feeder. The speed of the chain feeder is regulated automatically/remotely by actuating the control spindle of the variator through a servomotor. A pump for circulating the oil in the gear box of variator is an integral part of variator driven by a separator motor. Some of the technical data about the raw coal chain feeder is given here:-
Output of the chain feeder 10-45 tonnes/hr.
Speed variations 0.0503-0.151m/sec.
Main motor 7.5kW, 415V, 50Hz.
Oil pump motor 0.05kW, 220V
Operating motor of each gate 3HP, 415V and 50Hz.
Each mill consists of single compartment drum, bearings driving motor, coal inlet and discharge piping, ball change and lubricating equipment for mill bearings. Mill drum is fabricated from thick steel plates and is supported on to the anti-friction bearings. The mill is driven by an electric motor of capacity 630kW, 990 rpm, 6.6kV through a reduction gear, which reduces the speed to 17.5 rpm. The ball charge for the mill consists of the three different sizes of forged steel balls detailed as below. The capacity of each mill is 27 T/hr. in case of unit 1 & 2 and 28 T/hr.
40mm diameter 22500 kg
50mm diameter 20000kg
60mm diameter 10000kg
Total Ball Charge 52500kg
During operation only 60mm diameter balls are added is approx. 500 kg per week and the guiding factor is the amperage of the coal mill, normally it should be 66-ampere approx. at full load and when it falls below the above value ball charging of the mill is carried out. Lubricating system consists of the oil tank, gear pump, oil cooler and base frame to mount all these equipments. Gear pump is driven by an electric motor of rating 1 H.P., 415 V, 1440 rpm. Suction side of the gear pump is connected to the tube oil tank and the delivery side is connected to inlet of the oil cooler and after cooling oil goes to the bearings. The oil from the bearings is cooled to the required temperature in the cooler by the means of plant bearing cooler water.
The classifier is fabricated from the steel plates. It is an equipment that separates fine pulverized coal from the coarser pieces. The pulverized coal along with the carrying as well as drying medium (flue gas) strikes the impact plate in the classifier and the coarser pieces get separated due to the change in the direction of flow and go back to mill. The stream then passes to the outlet branch of the classifier through an adjustable telescopic tube. At the outlet adjustable vanes are provided to change the size of coal when required.
The centrifugal type cyclone separator consists of two cyclones made up of welded sheets. It is equipment in the milling plant, which serves for separating the pulverized coal from the vapours i.e. carrying medium. The pulverized coal gets stored in the pulverized coal bunkers and vapours go to suction of vapour fan. At the bottom of the cyclone separator a rotary valve (Turnikete) is provided to transport coal from cyclone separator to P.C. bunker on the worm conveyor as the case may be.
Pulverized coal bunker is welded from thick steel sheets and has a capacity of 4 hours coal consumption at maximum continuous rating of the boiler. The whole bunker is insulated externally. The carbon-dioxide blanketing system has been provided in the P.C. bunker to prevent fire hazards inside the bunker. The while storage bunker is divide into four parts namely A, B C & D. Further four coal feeders are taken out from each bunker leading to each corner of the furnace.
CRUSHING OF COAL:-
When coal reaches the plant, normal size of coal is about 500mm. After unloading the coal from the rake is fed to primary crusher, which reduces the size to 120mm. Then coal is fed to secondary crusher which reduces the size to 25mm and this coal goes to bunker with the help of conveyor belt from where coal finally goes to coal mill where coal is transferred in form of pulverized coal. The coal is heated with the help of hot primary air. We maintain the temperature of about 70C in coal mill. This temperature is maintained with the help of cold air and a hot air damper.
USE OF OIL:-
Before the coal reaches the furnace, we preheat the furnace in order to remove the moisture and raise the temperature of furnace, so that coal can catch fire easily without any delay. This preheating of furnace is done with the help of oil. With burning of oil, we maintain the temperature of furnace at 350C. we cut the oil supply after 350C because oil is very costly. Source of oil for G.N.D.T.P., Bathinda is Mathura Oil Refinery. Other use of oil is in bearing system for cooling. There are large number of bearings for plant. For example bearing system of turbine. These bearings get heated upto high temperature, which is dangerous. So we cool the bearing by circulating water in bearing.
COAL FEEDING AND COAL MILL:-
From the coal handling plant, coal comes in two belts namely 5A and 5B and then by belts 6A and 6B coal comes in bunkers. Bunker capacity is 300 tonnes. Number of outlets of bunker is three. First gate is opened for one hour and second and then third. If open the one gate for long time, then coal will stop going to mill. That is why we open the gate turn by turn.
Raw coal chain feeder is just below raw coal bunker. It is a sliding chain which feed the coal to mill. We can change the quantity of coal which is fed to mill in two ways.
By changing the speed of chain
By changing the depth of coal in chain
Speed of chain can be changed by adding a gear system to motor. We connect the gear system with motor with a pin called shear pin. The prevent the overloading of motor because when the coal quantity of coal on chain is greater than its capacity then the pin will break and prevent the pin from overloading. Speed of Raw Coal chain is 2” to 6”/sec.
These are mainly of two types:-
Ball Mills: - In Ball Mills there are steel balls which are revolving in horizontal cylindrical drum. These balls are free from any shaft and balls are touching with each other and with internal body of drum. These types of mills are at Bathinda Thermal Plant. On the other hand, bowl mills part of the mill contain drive system i.e. it contains 6.6 kV electric motor and gear system which translates the revolution about horizontal axis to revolve about vertical axis. The revolving vertical axis contains a bowl about the driving system. This bowl is fixed with driving and revolving with shaft. There are also three rollers which are suspended at some inclination, so that there is a gap of few mm between roller surface. These rollers are free to rotate about the axis.
Bowl Mills: - The coal is grinded and then fed into the mill at the center or near of revolving bowl. It passes between the grinding ring in revolving bowl and rolls as centrifugal force causes the material to travel towards the out perimeter of bowl. The springs, which load the rolls, impart the necessary force for grinding. The partially pulverized coal continue going up and down and over the edge of bowl.
COAL HANDLING PLANT
The G.N.D.T.P. units are primarily coal-fired units and the coal consumption at maximum continuous rating (M.C.R.) per unit is about 58 T/Hr. the coal used at G.N.D.T.P. is of bituminous and sub-bituminous type and this is received from some collieries of M.P. and Bihar. The designed composition of coal is as below:-
Type Bituminous Coal
Net calorific value 4300 kcal/kg
Moisture content in coal 10%
Ash content 30%
Volatile matter in combustibles 24%
Grind ability index 50 Hard Groove
The coal handling plant at G.N.D.T.P. has been supplied and erected by M/s Elecon Engineering Company Limited, Vallabh Vidya Nagar, Gugarat. Coal is transported from the coal mines to the plant site by Railways. Generally, the raw coal comes by railway wagons of either eight wheels weighing about 75 to 80 tones each or four wheels weighing about 35 to 40 tones each. The loaded wagon rake is brought by railways main line loco and left on one of the loaded wagon tracks in the power station marshalling yard. The main line loco escapes through the engine track. The station marshalling yard is provided with 8 tracks. The arrangement of the tracks in the marshalling yard is as follows:-
DESTINATION NO. OF TRACKS
Loaded wagons receiving tracks Four
Empty wagon standing tracks Three
Engine escape tracks One
UNLOADING OF COAL:-
In order to unload coal from the wagons, two Roadside Tipplers of Elecon make are provided. Each is capable of unloading 12 open type of wagons per hour. Normally one tippler will be in operation while the other will be standby. The loaded wagons are brought to the tippler side by the loco shunters. Then with the help of inhaul beetle one wagon is brought on the tippler table. The wagon is then tilted upside down and emptied in the hopper down below. The emptied wagon comes back to the tippler table and the outhaul beetle handles the empty wagons on the discharge side of the tippler. The tippler is equipped with the integral weighbridge machine. This machine consists of a set of weighing levers centrally disposed relative to tippler. The rail platform rests on the weighing girders and free from rest of the tippler when the wagon is being weighed. After weighing the loaded wagons is tipped and returned empty to the weighing girders and again weighed. Thus the difference of the gross weight and the tare weight gives the weight of the wagon contents. The tipplers are run by motors of 80 H.P. each through gears only.
The tippler is designed to work on the following cycle of operation:-
Tipping 90 seconds
Pause 5-12 seconds
Return 90 secondS
Weighing 30 seconds
Total 215-222 seconds
Allowing 85 seconds for wagon changing it will be seen that 12 eight-wheel wagons or 24 four-wheel wagons per hours can be tipped. However since the coal carrying capacity is 500 tones per hour load of 12 wagons comes to 8 to 9 per hour.
DUST TRAPPING SYSTEM:-
The tippler is also provided with the dust trapping systems by which the dust nuisance will be minimized. As the tippler rotates, a normally closed hopper valve opens automatically and the discharged material passes through it into the hopper with its dust-setting chamber, there is an air valve of large area, which opens, simultaneously with the hopper valve. The object of this air valve is to blow back through the hopper valve into the tipping chamber, which must occur if, the settling chamber were closed, it being remembered that a large wagon contains some 240 cubic feet of material and that this volume of dust air would be forced back at each tip if the hopper chamber were a “closed bottle”. The air valve and the hopper valve are shut immediately on reversal of the tippler and are kept shut at all times except during the actual discharge. The hopper valve is operated by a motor of 10 H.P., 415 Volts and the air valve is operated by electro-hydraulic thruster. Inlet valve consists of large number of plates sliding under the wagon tippler grating. Coal in the wagon tippler hopper forms the heap and as such obstructs the movement of sliding valve and damaging the plates. The inlet and outlet valves have therefore been bypassed.
The unloaded material falls into the wagon tippler hopper (common to both tipplers) having a capacity of 210 tones. The hopper has been provided with a grating of 300mm X 300mm size at the top so as to large size boulders getting into the coal stream. There is also a provision of unloading the wagons manually into the MANUALLY UNLOADED HOPPER of 110 tones capacity. Manually unloading will be restored to while unloading coal from sick wagons or closed wagons.
On belt conveyor no. 4A and 4B, there have been provided high intensity electromagnetic pulleys for separating out tramp iron particles/pieces from the main stream of coal conveying. D.C. supply for the magnet is taken on 415 volt, 3 phase, 50 cycles A.C. supply system.
In addition to above high intensity suspension type electromagnets have also been provided on belt conveyors 4A and 4B for separating out tramp iron pieces/particles.
If the receipt of coal on any day more than the requirement of the boilers, the balanced material will be stocked via conveyor 7Aand 7B and through telescopic chute fitted at the end of the conveyor. At the end of the chute one tele level switch is provided, which automatically lifts the telescopic chute to a predetermined height every time. The tele level switch is actuated by the coal pile. When the telescopic chute reaches maximum height during operation, which will be cut off by limit, switch and stop the conveying system. When the pile under the telescopic chute is cleared, the telescopic chute can be independently lower manually by push buttons.
There are five bulldozers to spread and compact the coal pile. Bulldozers of Bharat Earth Movers Limited Make are fitted with 250 H.P. diesel engines. Each bulldozer is able to spread the crushed coal at the rate of 250 tones/hr. over a load distance of 60m the coal can be stacked to a height of 6m the stockpile stores coal for about 45 days for four units with an annual load factor of 0.66.
Whenever coal is to be reclaimed the bulldozers are employed to push the coal in the reclaim hopper having a capacity of 110 tones. The coal from the reclaim hopper is fed either 9A or 9B belt conveyor through vibratory feeders 8A and 8B.
The crusher house accommodates the discharge ends of the conveyor 4A, 4B receiving ends of conveyor 5A, 5B and conveyor 7A and 7B, two crushers, vibrating feeders and necessary chute work. There are two crushers each driven by 700H.P. electric motor, 3 phase, 50 cycles and 6.6 kV supply. The maximum size of the crushed coal is 10mm. The capacity of each crusher is 500 tones/hr. one crusher works at a time and the other is standby. From the crusher the coal can be fed either to the conveyors 5A, 5B or 7A, 7B by adjusting the flap provided for this purpose. There is built in arrangement of bypassing the crusher by which the coal can be fed directly to the conveyors bypassing crusher.
CONVEYOR BELT AND CRUSHER HOUSE
The apparatus including its associated auxiliaries employed for switching, controlling and protecting the electrical circuits and equipments is known as switchgear.
A tumbler switch, which is an ordinary fuse, is the simplest form of switchgear and is generally used to control and protect the domestic and commercial appliances and equipments. For high rating circuits, a high rupturing capacity (H.R.C.) fuse in conduction with switch may serve the purpose. However, such switchgear cannot be applied on power system operating at high voltages, i.e. more than 11 KV because of the following reasons: -
When fuse blows, it takes sometime to replace it and consequently there is interruption of power supply.
On high voltage system, a fuse cannot successfully interrupt large fault currents.
When fault occurs, fault takes sometime to blow. During this time the costly equipments e.g. generators, transformers etc. may be damaged.
Therefore in order to protect lines, generators, transformers and other electrical equipments from damage, an automatic protective device or switchgear are required. Automatic protective switchgear mainly consists of the relays and circuit breakers. A circuit breaker is switchgear, which can be open or close the circuit after an operation. Therefore, a circuit breaker is rather preferred even in the instance when a fuse is adequate.
It makes and breaks the circuit under full load or no load condition but cannot be operated under fault conditions. It is generally operated manually.
It is only operated under no load conditions. Its main purpose is to isolate a portion of the circuit from the other. Isolators are generally place on the both sides of a circuit breaker from the other in order to make repairs and maintenance on the circuit breaker without any danger. There are two types of isolators: -
TYPES OF ISOLATER:-
Single pole Isolator
Double pole Isolator
A fuse is short piece of metal, insert in series with the circuit, which melt excessive current flows through it and thus breaks the circuit. The material used for the fuse element should possess the following properties: -
Low melting point.
Free from oxidation.
The common materials used for the fuse element are copper, tin-lead alloy (tin 63% and lead 37%, silver, aluminum etc.) A fuse is connected in series with the circuit to be protected and carries the load current without overheating under normal conditions. However when abnormal condition occurs, an excessive current flows through it. This raises the temperature, which melt the fuse element and open the circuit. This protects the machine or apparatus from the damage, which can be used by excessive currents.
Circuit breaker is on/off switch operating in an electric circuit in normal as well as abnormal operating conditions. While making or breaking contact there is a transition stage of arcing between contacts which is governed by electric discharge between the contacts at instant of separation, thus current continuous in the circuit till discharge appears.
The study of this phenomenon is very important for design and operational characteristics of C.B.
Functions of Switchgear: -
Under different conditions CB is subjected to varying stresses as current varies from few amp due to no load current of T/F up to many K amp heaviest short circuit current and varying circuit impedance. CB not only interrupts, but also closes the circuit. If breaker closes during short circuit it causes some trouble because then the voltage break down that bridges the contact gap produces high current arc which melt the contact before closer such situations is not desirable as breaker may not reopen. Often automatic re-closing is required because usually fault is of temporary nature. About 20% of the short circuit current persists however immediately after re-closing the breaker has to re-interrupt the short circuit current. This is main function specially if there are extremely high currents, which would result in the contact wear and tear.
The main function, which CB has in addition to satisfy the rated breaking capacity and rated making and breaking times are: -
Short Circuit interruption.
Interruption of small induction currents.
Interruption of Shot Line Faults.
Operating Principle Of Circuit Breaker: -
A circuit breaker is a device which: -
Makes or Breaks a circuit either manually or by remote contact under normal (full load) conditions.
Breaks a circuit manually or by remote control under abnormal conditions.
Breaks a circuit automatically under abnormal conditions i.e. fault conditions.
Thus circuit breaker is just a switch, which can be operated under normal, and abnormal conditions both manually and automatically.
To perform the above operation a circuit breaker essentially consists of fixed and moving contacts, called electrodes. When fault occurs on the power system, the trip coil of the circuit breaker is energized, which pulls apart the moving contacts from the fixed contacts as shown in fig thus opens the circuit.
When the moving contacts are separated from the fixed contacts, an arc is struck between them. The production of arc not only delays the current interruption process but also generates enormous heat which may cause damage to the equipments of the power system or the breaker itself. Therefore every effort is made to extinguish the arc produced in the circuit breaker as quickly as possible.
Circuit Breaker Ratings: -
A circuit breaker is required to be operated under all conditions. However this major duty of the circuit breaker is to operate the circuit under short circuit condition. Under short circuit conditions, a circuit breaker is required to perform three major duties: -
It must be capable of operating circuit on the occurrence of the fault.
It must be capable of closing the circuit on fault.
It must be capable of carrying a fault current safely for a short time, while another circuit breaker (in series) is clearing the fault.
According to the duties to be performed by a circuit breaker, there are three types of ratings: -
Breaking Capacity: -
The r.m.s. value of current that a circuit breaker is capable of breaking at a recovery voltage under specified conditions (i.e. recovery voltage and rate of recovery voltage) is known as breaking capacity of a circuit breaker. Breaking Capacity in MVA= 3 rated voltage rated breaking current 10-6.
Making Capacity: -
The peak or maximum value of the current (including d.c. component) during the first cycle of the current wave after the circuit is closed by the breaker (under dead short circuit) is called making capacity.Making Capacity= 3 1.8 Symmetrical breaking capacity.
Short Time Capacity: -
The period for which the circuit breaker is able to carry the fault current while remaining closed is called short time capacity of the circuit breaker.
Normal Current Rating: -
The r.m.s. value of the current which a circuit breaker is capable of carrying continuously at its rated frequency under specified conditions without overheating the arc or contacts is called normal current rating.
Oil Circuit Breaker: -
In this circuit breaker; the current carrying contacts are immersed in transformer oil. When the contacts are separated, arc is struck between them. The heat of the arc dissociates the oil and gases viz. Hydrogen etc are evolved. The hydrogen gas bubble surrounds the arc and cool it downs which help in de-ionization of the medium between the contacts and extinguishes the arc. Moreover, gases setup turbulence in the oil and force it into the arc space when the current is zero which further helps in extinguishing the arc.
It absorbs the energy of the arc by decomposing the oil into gases.
The gases evolved provide good cooling effect.
The surrounding oil enclose the proximity to the arc provides cooling effect.
It has ability to flow in the arc space after the current is zero.
It acts as insulator between the live contacts and earthed tank.
It is easily inflammable.
It may form an explosive mixture with air.
It requires more maintenance.
Minimum Oil Circuit Breaker: -
In bulk oil circuit breaker, transformer oil is not only used extinguish the arc but also serves as insulation between the live and earthed parts. A heavy quantity of oil, depending upon system voltage, is used in the bulk oil circuit breakers (about 5000 liters in 220KV system). This not only increases the expenses but also increases the fire risk. Therefore minimum oil circuit breakers are designed in which only 10 % of the oil is used to extinguish the arc. The container of minimum oil circuit breakers is supported on porcelain insulators. To provide the required insulation between the live and earthed parts. Thus the minimum oil circuit breakers also require less space for installation.
MOCB’s have following merits and demerits: -
It requires lesser quantity of oil i.e. only for arc extinction.
It requires smaller space for installation.
Risk of fire is considerably reduced.
Due to smaller quantity of oil the degree of carbonization is increased therefore oil needs replacement after each operation.
Proper design is required to remove the gases from the contacts space in time
BRIEF DESCRIPTION OF 6.6 KV/415V
SUPPLIED TO G.N.D.T.P. BATHINDA
In 6.6/415 KV switchgear we have two unit transformer and one station T/f that after stepping down the voltage, fed it to two 6.6 KV unit buses and to station bus. Various feeders are connected to 6.6 KV buses and in order to avoid complete shutdown, supply is maintained by drawing supply from station bus, to 3B & 4B bus. Supply from 3A bus is stepped down to 415 V by 1000 KVA SWGR T/F – 1 & fed to 415 V bus. Same in 4B bus. In case of tripping standby bus used. Various feeders are connected to these buses. We generate electricity at 11 KV & step-down to 7 KV by UAT. Rating of UAT T/F is 15 MVA & station T/f is 225 MVA, 11/7 KV.
CIRCUIT BREAKER USED IN INDOOR SWITCHGEAR: -
Mainly two types of CB’s are used in switchgear according to the requirement: -
1. 6.6KV MOCB’s
2. 415 V ACB’s
Minimum Oil CB (6.6 KV): -
It is provided for each motor feeder of rating 6.6 KV and as incoming breaker for 6.6 KV bus. In these CB’s arc is quenched in arcing chamber with minimum quantity of oil.
SPECIFICATIONS OF MOCB OF MOTOR FEEDER: -
Rated Voltage 6.6 KV, 50 Hz, 3-pole
Rated current 1250 Amperes
Breaking Current 34.7 KA (Sym)-378 (Asym)
Breaking Capacity 395 MVA
Making Capacity 88 Peaks KA
Short Time Current Capacity 34.7 KA for 1 Sec
MOCB uses solid material for insulating purposes and use just minimum oil for arc quenching. The arc-interruption device is enclosed in a tank of insulating material, which is a line voltage in normal operation. Thus are also known as live tank breathers.
Various protections relay are used in conjunction with MOCB’s according to requirement of connected equipment are: -
Over current relay
Locked rotor relay
Unbalanced protection relay
Earth fault protection relay
Under voltage relay
415V ACB’s: -
In these CB’s air at atmospheric pressure is used for quenching the arc.
Specifications of ACB’s: -
Rated Voltage 660 V (AC)
Rated Current 1600 Amperes
Rated Making Capacity 95 KA (PEAK)
Rated Breaking Capacity 45 KA (rms)
Max. Switching Frequency/hour 15 make/break open
Opening Time 20msec
Total Opening Time Included
Arcing Time 30.35 msec
Closing Time 500 msec
This CB is Provided With Three Main Protection Trips: -
1. Thermal Delayed Over Current Trip: -
This consists of three bimetal strips, each headed by a current T/F, which is slid on to the appropriate phase conductor. Tuning the calibrated knob vary the setting. A temp, compensating strip is also used which makes the tripping time largely independent of ambient temperature.
2. Instantaneous Over Current Trip: -
This is fitted in contact assemblies. The U shaped magnet cores with associated armature are mounted on the conductor and energized by breaker current.
3. Under Voltage Trip: -
It opens the breaker instantly if the auxiliary as main voltage drop to 50% of the rated coil voltage.
VARIOUS REQUIREMENTS OF CB USED IN S/G
All the CB should be three-pole and there should be suitable for remote/local electrical operation and manual operation also.
The CB should be suitable for the operation on 220 V dc auxiliary control supply.
The CB is required to drive motors and also be suitable for incoming from LT T/F.
The closing Of CB should be direct motor drive type as stored energy type.
The CB should be provided with manual closing and tripping device also.
The CB should also provide with shunt tripping coil suitable for 220 V.
CB should have mechanical indication for ON/OFF position.
CB should be provided with the device, which does not allow closed breaker reached in as reached out
The CB’s should be suitable for locking, test & service position & it should also be suitable for electrical/mechanical operation in both testing service position.
Bus Bars (6.6 KV/415 V): -
This term is used for main bar on conductor carrying electric current through which many connections are made for connecting switches and the equipments like bus bar made of Aluminum because it has higher conductivity, corrosion resistant and lower cost as compared to copper
Switch Gear 6.6 KV
Circuit Breakers Minimum Oil Type
Rupturing Capacity 350 MVA
Current Rating 1250 Amperes
Switch Gear 415 V
Circuit Breakers Air Type
Rupturing Capacity 332 VA
Current Rating 0.8Ampere
D.C. SUPPLY SYSTEM
D.C Supply is the brain of the plant. Each unit has its own 220volts D.C system located in the electrical bay. 48 battery of requisite rating are also being provided. Each D.C system comprises of the following:-
1. Storage battery
2 Battery Charges
3. Distribution & sub-distribution boards
The batteries are of lead acid type. Battery cells have high discharge performance cell types each of 2volts. Battery charge are static type & capable of trickle charging & boost charging. Adequate standby provision is also made for the outage of the charger in the form of installation of standby charges.
The D.C distribution & sub-station boards are compartmentalized; draw out type, construction, housing switches & fuses for various feeders are as per the requirements for the plant.
The battery rooms are well ventilated & well lighted & there is adequate provision for expelling acid fumes & fumes h2 gas out from the battery room.
Dry cell batteries
2. Lead acid batteries
3. Alkaline cell batteries.
Lead acid cells are of further two types:-
Stationary lead acid battery.
Out of theses three types of the cells lead acid cell & alkaline cells are rechargeable whereas dry cells cannot be recharged. In case of lead acid ell both the electrodes are of the same material i.e. Lead in case of Alkaline cell electrodes are of two types:-
Electrodes, which is mostly used is potassium hydroxide. As the battery discharges, concentration of lead sulphate goes on increasing, at a specific gravity for 1.23 discharging stops & the battery will not provide any amount of D.C energy. The batteries which are used at G.N.D.T.P. Is having the rating of 600 Ah. If these having 10 rating, the battery supplies 60Amp. Of current rating 10 hours.
Batteries used in the G.N.D.T.P. for the main purpose of:-
Two sets of the batteries are used for each circuit. So that if one fails second comes in action.
BATTERY CHARGING SYSTEM AT G.N.D.T.P.
The various requirements for charging the batteries are listed bellow
connections of cells
A DC source capable of delivering current as specified. The voltage required will be two times the No. of cells in battery. The initial charging of the battery will takes approximate 55 to 90 hours.
The acid used filling battery is sulphuric gravity 1.190±0.005(at 27 c)
CONNECTIONS OF CELLS
The +ve terminal lug of the cell in one row is connected to the –ve lug of the end cell in the other row. The connections b/w the two rows may be made with the necessary length of CU of the size used b/w the switchboard & battery.
POLARITY OF THE CONNECTIONS
It’s very important that +ve terminal of the battery is connected to the +ve lead of the charging source.
To ascertain the polarity of the charging leads connected a lamp in the series & dip the ends in a glass of slightly saline water. Switch on the supply. Fine bubbles of the gas will be given off from the –ve lead. The lamp connected in the series eliminates the danger of accidental short circuit.
Batteries have to be charged occasionally to restore them to be in working condition. During the charging DC current is passed through the battery in the direction opposite to that when the battery is being used. Charging current is usually obtained from battery charger, which is the selenium or transformers designed to step up the voltage or down to suitable values.
BATTREY CHARGING EQUIPMENT
The charging of battery is done by a system known as trickle charging unit.
TRICKLE CHARGING UNIT
BRIEF DESCRIPTION OF THE BATTERY CHARGING
Float charger operates on constant voltage mode & maintains the DC output within +/-1% of the set value.
The boost charger operates on constant current mode till the o/p current reaches set value, beyond which it operates in constant current mode.
During charging or providing the equalizing charger to the battery the boost charger operates in constant current mode.
Under normal running condition the DC load is connected across the float charger & the batteries are also connected across the float charger through the DC contactor & gets the trickle charger from the float charger so that if in case suddenly the AC supply fails the batteries will supply the DC power to the continuous DC load & disturbance will not be created. The float charger floats on the DC bus that’s why it is called float charger. Basically the float charger is provided for the continuous Dc load & at the same time trickle chargers the batteries so that when the mains fail the load demand meet the battery immediately. Now if the battery gets discharged while supplying the load. To charge the battery again boost charge is provided which boostly charges up the battery to the desired level of the voltage.Both the boost & float charges are thyirosterised power supplies having automatic voltage current regulation features.
The working of the boost charger depends on the voltage of the cells of the battery i.e. when the voltage of the a single cell of the battery reaches 1.8 volts/cell the boost charger is automatically tuned ON 7 starts charging the battery cells & remains ON till the voltage reaches 2.7 volt/cell i.e. 35.1 volt of the total battery.
The float charger will determine the load voltage. When the AC supply is available both the float charger & the batteries are connected across the DC cont. load. The float charger simply converts the AC supply to DC supply & feed the continuous Dc load as well as to float the batteries at 2.16 volts/cell.
Suddenly if now AC fails the batteries will come across the Dc load current requirement. Now when the AC supplies resumes the boost charger is connected to the batteries to recharge them with the boost energy. Boost charger operates in the constant current mode.
COMPONENTSS OF THE FLOAT CHARGER
Contractor with thermal overload relay
Dc voltmeter & ammeter
The circuit Works on the AC phase control principle. The SCR is a semiconductor device with 3 terminals i.e. anode cathode gate. The main load current is carried by the anode, cathode, while the control current flows through the gate & cathode. Characteristics of the SCR are such that iot blocks the forward voltage when gate is not supplied with the anode current. While it goes into conduction when gate current reaches a specified level therefore charging the instant at which the gate current or the pulse is supplied can control the instant at which the SCR goes into conduction. Once the SCR is triggered it remains in conduction until anode current is reduced to zero or reverse voltage is applied anode.Thus changing the instant of firing of the SCR. Potentiometer RV 1 is used for adjusting the output voltage setting can control the output voltage of the rectifier bridge.
The DC output load current is sensed by the Dc shunt. The signal proportional to this load is fed to the controller. In the event of the load current exceeds the rated full load current, the output voltage starts dropping, thus limiting the load current. This inherent is provided in the float charger apart from the back-up.
SPECIFACTIONS OF FLOAT CHARGER
It is thyristor controlled power supply transformer is double wound 3-phase, natural air cooled. Rectifier Bridge is 3-phase, full wave, half controlled. Each device is protected from the voltage surge with suitable suppressor network & from overload with HRC s/c fuses. During overload the charger output is drops below set level, their allowing batteries to take current surge.Float charger is provided with I/P switch, HRC fuse, contactor with thermal overload relay ,DC voltmeter & auto manual switch.
MAIN POWER SUPPLY CIRCUIT
Main incoming power supply is connected to the main t/f through the rotary switch, fuses & AC contactors.Three indicating lamps are provided to indicate the AC main supply available.ON & OFF switch is provided for switching ON & OFF the contactor.Indicating lamps indicate the contactor OMN condition.The contactor would trip of during phase sequences fail or AC overload as the coil of the contactor is connected in the series with NC contacts of the phase fail relay & ac overload relay.The transformer is connected in star-delta configuration.The secondary of the main transformer is connected in delta & its 3 outgoing lines are connected to the diode(SCR half controlled rectifier bridge that consist of three diode D1,D2,D3,D4 & 3 SCR’S.SCR1, SCR2, SCR3.Diode is freewheeling diode.Surge suppressor RC1 TO RC7, which consist of the registers & capacitors n/w, protect the SCR’s diode against transit voltage.The output of the bridge is filtered using L-C circuit.The filtered output is connected to the load circuit or battery through a rotary switch.A shunt SH1 is used for current limit control which is also used for output current on ammeter.A DC voltmeter indicates the DC output voltage.Indicating lamp indicates DC on condition. Blocking diodes are used to prevent reverse current following from the battery to the charger.When the charger voltage goes below the battery voltage or the charger is off.The DC voltmeter reads voltage across float, boost or battery tap through a switch.
The o/p of the charger is controlled through the electronic controlled. The control circuit. Has plug in type cards with connections for external connections. The output is controlled using the phase control of the SCR & feedback.
MAIN FUNCTIONAL CIRCUITS
UJT firing for SCR control
DC under voltage/over voltage sensing
AC under voltage/over voltage sensing
Power supply in the form of power supply car. It provides Regulated power of +/-12 volts.Unregulated power of 24 volts used for IC”s & relays respectively.
In the boost charger main T/F is used to step down voltage. The primary has suitable I/P taps for +/-10% of the normal voltage. Secondary of the T/F is directly fed to 3 phase full wave full controlled bridge rectifier stack & the DC O/P is used for charging the batteries.
During the boost charging the DC contractor is de-energized & inhibits the excess voltage of the boost charger to reach the DC load terminals. If now during the boost charging of the battery again the AC supply fails. Then the DC contractor will operate & the full battery voltage is provided across the load.
In case of failure of the float charger the continuity if the DC supply is maintained through the blocking diodes connected through the tap cells of the battery.
To cure this problem of failure of the float charger the boost charger is also used to supply the DC load. But this is an extraordinary situation when the boost charger is operated in the constant voltage mode. And the DC contractor will not be DE energized during this time. The boost voltage is set to 2.16 volts/cell & would remain constant within +/-1% of the set voltage.
There are special designed features which are incorporated in the boost charger enables it to be used as a float charger so that the delay cause during rectifying the float charger does not effect the DC supply system.
The charger is so called because if floats on the DC bus. The charger is fed from 3 phase Ac supply & gives the DC stabilized O/P at rated full load current. The variation in DC O/P voltage is limited to +/-1% for 0-100% load variation & simultaneously AC voltage variation of +/-10% & frequency variation of +/-5% from 50 HZ.
Relay is a device that detects the fault mostly in the high voltage circuit. & initiates the operation of the CB to isolate the defective section from the rest of the circuit.Whenever fault occurs on the power system, the relay detects that fault & closes the trip coil circuit. This results in the opening of the CB which disconnects the faulty circuit. Thus the relay ensures the ensures the safely of the circuit. equipment from damage which the fault may cause.
PURPOSE OF PROTECTIVE RELAY AND RELAYING
The capital investment involved in a power system for the generation, transmission & distribution if electrical power is so great that the proper precautions must be taken to ensure that the equipment not only operates as nearly as possible to peak efficiency but also that it is protected from accidents. The normal path of the electric current is from the power source through copper conductors in the generators, transformers & transmission lines to the load & it is confined to this path by insulation. The insulation however may be broken down either by effect of temp. & age or by a physical accident, so that the current then follows an abnormal path generally known as short-circuit or fault. Whenever this occurs the destructive capabilities of the enormous energy the power system may be causes expensive damage to the equipment, severe drop in the voltage & loss of revenue due to interruption of service. Such faults may be made infrequent by good design of the power apparatus & lines the provision of protective devices, such as surge diverters & ground fault neutralizers, but a certain number will occur inevitably due to lightening & unforeseen accidental conditions.The purpose of protective relays & relaying systems is to operate correct C.B so as to disconnect only the faulty equipment from the system as quickly as possible.
All the relays have following three essential fundamental elements:
Sometime it is also called measuring element. It is element which is the responsible to the change in magnitude or phase of the quantity.
It is the element which compares the action of the actuating quantity of the relay with the pre-designed relay setting. The relay only picks up if the actuating quantity is more than the relay setting.
When the relay picks up it accomplishes a sudden change in controlled quantity such as closing of the trip coil circuit.
According to the conc. & principle of operation relays are of following three types.
The operation of these relays depends on the heading effect of the electric current.
E.M. ATTRACTION RELAYS:
These are the electromagnetic relays. The operation of these relays depends on the movement of armature under the influence of attractive forces due to magnetic field up by current flowing through the relay coil.
The operation of these relays depends on the electromagnetic induction phenomena. By induction, eddy currents are induced in the AL in the disc, free to rotate, which exerts torque on it.
VARIOUS TYPES OF RELAYS USED FOR PROTECTION ARE:
OVER CURRENT RELAY: -
In this protection trip coil is energized when current in the circuit is 10 times the normal current. This protection is applied b/w 3 phases.
The time operation of this relay is 0.1 sec. It is more effective where impedance b/w the sources &relay is small as compared with the impedance of section be operated.
LOCKED ROTOR RELAY:-
This protection is applied b/w 2 phases. In this protection the trip coil is energized when the current usually during starting the current is 5 to 6 times the normal current.
It protects against –ve phase sequence current. The relay normally used is an IDMT relay.
EARTH FAULT PROTECTION:-
The s1 terminals of 3 C.B’s are connected to 3 phases R,Y,B & second terminals S2 are connected together. This form neutral thus connections are star connected. Ideally there is no current in neutral. But if by any reason the circuit. Start to flow in the neutral the earth fault occurs & trip coil is energized thus tripping the C.B’s.
UNDER VOLTAGE RELAY:-
In this protection the coil is energized when the voltage drops to 70% of the normal value. It’s app. Is reverse time under voltage protection of a.c circuit capacitor, rectifier & m/c such as induction motor?
Dust extractions from industrial gases become a necessity for environmental reasons. Most of the plants in India use coal as fuel for generating steam. The exhaust gases contain large amount of smoke and dust, which are being emitted into atmosphere. This poses a real threat to the mankind as a health hazards. Hence it has become necessary to free the exhaust gases from smoke and dust.
Need For Installation Of New Electrostatic Precipitator at GNDTP Units:
The electrostatic precipitators installed at GNDTP units are designed to give an emission level of 789 mg/NM3 for a coal having an ash content of not more than 30%. However on actual testing it has been found that emission level from ESP’s was about 3.0 mg/M3. The high level of emission is due to the fact that coals burnt in the boiler have much higher ash content than what boilers are designed for. The pollution control board of Punjab Govt. has specified an emission level of 380 mg/M3 from chimney. In order to achieve this new emission level additional ESP’s have been installed at GNDTP Bathinda.
Working Principle: -
The Electrostatic precipitator utilizes electrostatic forces to separate the dust particle form the gas to be cleaned. The gas is conducted to a chamber containing “Curtains” of vertical steel plates. These curtains divide the chamber into a number of parallel gas passages. The frames are linked to each other to form a rigid framework.The entire framework is held in place by four supports insulators, which insulates it electrically from all parts, which are grounded. A high voltage DC is applied between the framework and the ground thereby creating a strong electrical field between the wires in the framework and the steel curtains. The electrical field becomes strongest near the surface of the wire, so strong that an electrical discharges. “The Corona” discharge is developed along the wires. The gas is ionized in the corona discharge and large quantities of positive and negative ions are formed. The positive wires are immediately attracted towards the negative wires by strength of the field induced. The negative ions however have to travel the entire space between the electrodes to reach the positive curtains. On routes towards the steel curtains the ions collide with each other and get charged and also this charge is transferred to the particles in the gas. The particles thereby become electrically charged and also begin to travel in the same direction as the ions towards the steel curtains. The electrical force on each particle becomes much greater than gravitational force. The speed of migration towards the steel curtains is therefore much greater than the speed of sedimentation in free fall.
General Description: -
There various parts of the precipitators are divided into two groups: -
Mechanical system comprising of casing, hoppers, gas distribution system, collecting and emitting systems, rapping mechanism, stairway and galleries.
Electrical system comprising of transformer rectifier units with Electronic Controller, Auxiliary Control Panels, Safety Interlocks and Field Equipment Devices.
Precipitator Casing: -
The precipitator casing is an all welded pre-fabricated wall and roof panels. The casing is provided with inspection doors for entry into the chamber at each field. The doors are of heavy construction with machined surface to ensure a gas tight seal.
The roof carries the precipitator’s internals, insulator housings, transformers etc. The casing rests on roller supports which allows for free thermal expansion of the casing during operating conditions. Galleries and stairway are provided on the sides of the casing in easy access to rapping motors, inspection doors, transformers etc. walkways are provided inside EP between fields for inspection and maintenance. The dust is collected in large quantities on the curtains, the collected electrodes. Due to periodic rapping, the dust falls into the hopper.
The hoppers are sized to hold the ash for 8 hrs. Collection. Buffer plates provided in each hopper to avoid gas leakage. Inspection door is provided on the one side of hoper wall. Thermostatically controlled heating elements are arranged at the bottom portion of the hopper to ensure free flow of ash.
Gas Distribution System: -
The good performance of the precipitators depends on the event distribution of gas over the entire cross-section of the field. As the gas expands ten-fold while entering the precipitator, guide vanes, splitters and screens are provided in the inlet funnel to distribute the flue gas evenly over the entire cross section of the EP.
Collecting Electrode system: -
The collecting plates are made of 1.6 mm cold rolled mild steel plate and shaped in piece by roll forming. The collecting plates and shaped in one piece by roll forming. The collecting electrode has unique profile with a special configuration on its longitudinal edges. This profile is designed to give rigidity and to contain the dust in quiescent zone free from re-entertainment; collecting plates are provided with hooks at their top edge for suspension. The hooks engage in slot of the supporting angle. All the collecting plates in arrow are held in position by a shock bar at the bottom. The shock bars are spaced by guides.
Figure: - Typical collection plates
Emitting Electrode system: -
The most essential part of precipitators is emitting electrode system. Four insulators support this, the frames for holding the emitting electrodes are located centrally between collecting electrodes curtains. The entire discharge frames are welded to form a rigid box like structure. The emitting electrodes are kept between the frames.
Fig: - Rigid frame discharge electrode design
Rapping System: -
Rapping mechanism is provided for collecting and emitting electrodes. Geared motors drive the rapping mechanism. The rapping system employs tumbling hammers, which are mounted on a horizontal shaft. As the shaft rotates slowly the hammers which are mounted on a horizontal shaft. As the shaft rotates slowly the hammers tumble on the shock bar/shock, which transmits blow to the electrodes. One complete revolution of the rapping shaft will clean the entire field. The rapper programmer decided the frequency of rapping. The tumbling hammers disposition and the periodicity of the rapping are selected in such a way that less than 2% of the collecting area is rapped at one time. This avoids re-entertainment of dust and puffing at the stock outlet.
The rapping shaft of emitting electrodes system is electrical isolated from the geared motor driven by a shaft insulator. The space around the shaft insulator is continuously heated to avoid condensation.
Following Are the Modules for the Outgoing Feeders: -
Hopper heater for each field
Support insulator heaters.
Shaft insulator heaters.
Collecting electrode-rapping motor for each field.
Emitting electrode rapping motor for each filed.
High Voltage Transformer Rectifier (HVR) with Electronic Controlled (EC): -
The rectifier supplies the power for as particle charging and collection. The basic function of the EC is to feed the precipitator with maximum power input under constant current regulation should there be any flash between collecting and emitting electrodes, the EC will sense the flash and quickly react by bringing the input period voltage to zero and blocking it for a specific period. After the ionized gases are cleaned and the dielectric strength restored, the control will quickly bring back the power to a present value and raise it to the original non-sparking level. Thus the EC ensure the electrical disturbance within precipitator. Regulated AC power from EC is fed to the primary of the transformer, which is stepped up and rectified to give a full wave power output. The transformer is mounted on roof of the precipitator while the EC is located in an air conditional room.
Auxiliary Control Panel (ACP): -
The ACP houses the power and circuits required for energizing rapping motor and heating elements of the precipitator. ACP controls each gas path. The complete ACP is of modular type with individual module for each feeder. Each module houses the power and control circuit with meters. Push buttons, witches and indicating lamps are mounted on the door of the compartments.
Maximizing The Performance OF ESP: -
The performance of the ESP is influenced by a number of factors many of which may be controllable. It should be the aim of every operator to maximize the performance by judiciously adjusting the controllable variables.
Cleaning Of Electrodes: -
The performance of the ESP depends on the amount of electrical power absorbed by the system. The highest collection efficiency is achieved when maximum possible electric power for a given set of operating conditions is utilized on the fields. Too thick a dust layer on the collecting plates will lead to drop in the effective voltage, which consequently reduces the collection efficiency. It also leads to unstable to unstable operating conditions. Therefore the rapping system of collecting and emitting electrodes should be kept in perfectly working condition. All the rapping motors have been programmed to achieve the optimum efficiency.
Ash Hopper Evacuation: -
Improper/incomplete hopper evacuation is a major cause for the precipitator malfunction. If the hopper are not emptied regularly, the dust will build up to the high tension emitting system causing shot circuiting. Also the dust can push the internals up causing misalignment of the electrodes. Though the hoppers have been designed for a storage capacity of 8 hours, under MCR conditions, this provision should be used in case of emergency. Normally, the hopper should not be regarded as storage as storage as storage space for the collected ash.
Oil combustion: -
The combustion of oil used during start up or for stabilization of the flames can have an important impact on precipitator operation. Un burnt oil, if passed into ESP can deposit on the emitting and collecting electrodes and deteriorates the electrical condition i.e. reduce the precipitators operating voltage due to high electrical resistivity and consequently the ESP’s performance is affected adversely. The precipitator performance remains poor until the oil vaporizes and the ash layer gets rapped off, which usually takes along time.
TECHNICAL DATA OF ELECTROSTATIC PRECIPITATOR
Total No. of collecting plates
Nominal height of collecting plate
Nominal Length of collecting plate
Gas flow rate
Number of precipitator
Number of gas path per boiler
No. of fields in series in each gas pass
No. of electrodes in each field
RAPPERS FOR COLLECTING ELECTRODES
1. No. & type of
One drop hammer per row
of collecting electrodes
surface area 90 m2
Varying from 12 raps/hr at the inlet field to 1 rap/hr at
Geared electric motor
Controlled by synch.
At the bottom of
RAPPERS FOR EMMITING ELECTRODES
1.No. and type of rappers
2. Frequency of Rap
Approx. one drop hammers/two
rows of electrodes
Geared Electric Motor controlled by
On the side of emitting frame
2.No of Hoppers
8 hour storage
RAPPING OF EMMITING ELECTODE
0.33hp/2.5 rpm at 3-
phase 415 V 50 Hz
On the top EP
RAPPING OF COLLECTING ELECTODE
Geared Motor, 33
hp/2.5 rpm at 3
Phase 415 V 50 Hz.
On the top EP
1. Rectifier Rating
70 KV (peak)
800 MA (Mean)
Silicon Diode Full Wave,
Mounted on the top of
RECTIFIER CONTROL PANEL
1. Type of Control
In the Control Room
The steam generating unit is designed to meet the nominal requirements of 110MW turbo generator set. The unit is designed for a maximum continuous rating of 375 tones/hr. at a pressure of 139kg/cm2 and a steam temperature of 5400C. the reheated steam flows at MCR 32H tones/hr. at the feed water temp at MCR is 2400C. The unit is a balance draught dry bottom; single drum natural circulation, vertical water tube type, construction with skin casing and a single reheat system. The furnace is arranged for dry ash discharge and is fitted with burners located at the four corners. Each corner burner comprises coal, vapour oil and secondary air compartments. The unit is provided with three ball mills and arranged to operate with intermediate cool powder bunker. The steam super heater consists of 4 stages Viz. Ceiling, convection, platen and final superheated. The ceiling super heated forms the roof of the furnace and horizontal pass and finishes as the rear wall of the second pass. The convection super heated is made up of horizontal banks located in the second pass. While the platens are located at the furnace exit, the portion above the furnace nose encloses the final superheated reheater are in two stages, first stage is the triflux heat exchangers located in the second pass, which absorbs heat from superheated steam as well as from the flue gases. The second stage is exit reheater located in the horizontal pass as pendant tubular loops.
(a) The flue gas for drying the cool in the mills is tapped off after the triflux heat exchangers. The damper located in the hot flue gases pipe leading to mill controls the quantity. Control the circulating vapour of the mill entry effect temperature control.
Immediately after the triflux heat exchanger, the air heaters and economizers are located. The air heater is in 2 stages.
(b) The hot air for combustion from air heater stage 2 is led into the common wind box located on the sided of the furnace. 4 cool air mixed pipes from pulverized coal bounders are connected to 4 cool burners’ nozzle at the corners. There will be totally 16 coal nozzles. 4 located in each corner. Oil guns will be located in the secondary air nozzle for coal burning. The turn down ratio of the guns will be so selected that it will be possible to use them also for pulverized fuels flame stabilization while operating under load below the control point.
(c) Take into consideration the high % age of ash and the relatively poor quality of coal due regards has been paid to wide pitching the tubes and to the gas velocity across the heating surface areas. In order to insure reliable and continuous operation sample sot blowing equipment is provided. There are short retractable steam root blowers provide at the top of furnace fully retractable rotary type blowers are located for cleaning of the secondary super heater and final heater partly retractable steam blowers are arranged for the horizontal reheater and super heaters in the second pass. The steam root blowers are electrically operated.
(d) Root blowing nozzles using blow down from boilers drum are provide for the cleaning of areas around the burners nozzles zone for dislodging of slag boulder if any in the bottom ash hopper in the furnace.
(e) Two FD fans are provided per boiler. The FD fans are of the axial type driven by constant speed motor. The regulation of quantity and pressure is done by inlet vane control. The flue gases are sucked through the mechanical and electrostatic precipitators by I.D. fans and delivered into the chimney. Two I.D. fans are provided for each boiler and they are of the axial type driven by constant speed motors. Inlet vane control effects the capacity change with reference to load. Both the I.D. and FD fans have been dimensioned taking into account the minimum margins of 15% on volume and 32% on pressure.
Maximum continuous rating 375tones/hr.
Super heater outlet pressure 139kg /cm2
Reheater outlet pressure 33.8 kg/cm2
Final super heater temperature 540 deg.c
Feed water temperature 240deg.c
Efficiency 86% (stage-1)
Coal consumption per day 1500 tones
BOTTOM ASH SYSTEM
The ash deposited at the bottom of the furnace is collected in a water impounded hopper where a continuous flow of water is maintained to limit the temperature of ash inside the hopper. The bottom ash cleaning is done in every cycle of 8 hours. The bottom ash system is local manually operated. On opening of feed gate ash is allowed to discharge into a double roll linder grinder where it is grounded to smaller size, which can be transported through the pipe line below the linker grinder there is a venturi which sucks
the ground ash the vaccum created at the venturi throat by the flow of high pressure water tapped. Dawn stream of the discharge of the ash water pumps. The pressure recovered at the end of venturi is adequate to convey the slurry to disposal area.
The design of the power cycle based on the modern concept, where a unit consists of a steam generator with its independent firing system tied to the steam generation. The steam generator is designed for maximum continuous rating of 375-tonnes/hr. and steam Pressure of 139-kg/cm2 at temperature of 540C respectively. The steam generator is designed to supply to a single reheat type condensing steam turbine with a 8 non regulated extraction points of steam for heading the condensate and feed water. The steam cycle can be classified into the following three divisions: -
Main steam (b) Reheat steam (c) extraction steam
Saturated steam from the steam generator drum is led to the super heater bank to heat if up to 540C saturated steam from the drum is led to the ceiling super hearter (between SHH1 and SHH2) from ceiling super steam goes to convection super heater (between SHH2 and SHH3) the first regulated infection for at temperature takes place after convection super heater (between SHH9 and SHH10). Before entry to final super heater the steam is again at temperature by regulated injection. The steam is coming out from the final super heater normally at a pressure of 139 kg/cm2
Temperature takes place after convection super heater (between SHH9 and SHH10). Before entry to final super heater the steam is again at temperature by regulated injection. The steam is coming out from the final super heater normally at a pressure of 139 kg/cm2 at a temperature of 540oC. This steam is feed to the control valve. In each of the two live steam lines there is one turbine side main steam stop valve and one high pressure quick closing valve along with two control valves.
(b) EXTRACTION STEAM
Steam for heating of the condenser and the feed steam is extracted from 8 non regulated extraction points from the turbine. Heating is carried out in five stages of L.P. heaters, one deareating heater and in two H.P. heaters extraction 1,2,3, is taken from L.P. turbine. Extraction 4, 5, 6 and 7 are taken from M.P. turbine. Extraction 8 is obtained from C.R.H. line first and second stage of heating is done by two sets of twin low-pressure heaters mounted directly in the L.P. casing of the turbine. Extraction 3,4 and 5 are connected to
the deaereating heater placed above feed water storage tank 7th and 8th extraction steam is fed to the vertical H.P. heaters respectively.
(C) REHEAT STEAM
Exit steam from the H.P. turbine is taken back to the reheater section of the steam generating unit. Reheating is done in two stages both by flue gas and by super heated steam. The steam to be reheated is first pass through the triple-heated exchanger, where super heated steam is used as the heating media. The steam is finally reheated in final reheaters (RHH3) RHH4 and RHH5) suspended in the horizontal pass of the furnace. Reheat steam at a normal pressure of 36.4 kg/cm2 at a temperature of 540C respectively is fed to the M.P. cylinder by two hot reheat steam pipes through strainers and combined stop and interceptor valves. In each of the cold reheat steam lines from H.P. cylinder a non-return valve is operated by oil pressure is provided.
The crushers crush the coal upto the dia of 20 mm. This coal comes to the raw coal bunker through conveyer belts. This coal is fed into the ball mill through chain feeder, operated by motor. In drum the steel balls are used to make it pulverized. At one end coal enters the mill and from the other end pulverized coal is sucked by vapour fan. The pulverized coal is used for burning in the furnace. On its way the p.c. (pulverized coal) bunker, it goes from classifier and cyclone separator function of classifier is to use the coal for burning coal which could not pass the classifier, is collected on the gravity damper. When the weight of this coal is enough, gravity damper is opened because of the weight of the coal and this coal goes back to mill through run back piping and is further pulverized. In cyclone separator much finer particles of coal is stored in P.C. bunker because of centrifugal force. This coal is fed to the P.C. bunker through the warm feeder. Through warm feeder, we can collect the pulverized coal in any of the P.C. bunker. Warm feeder runs with the help of motor and gearbox.
COAL PULVERIZING MILL
Coal mill pulverizes the raw coal into a fine powder before it is burnt in the boiler furnace. The pulverizing of coal is achieved with the impact of falling steel balls, weighing 52.5 tones contained in the mill drum rotating at a slow speed of 17.5 rpm. The raw coal is dried before pulverizing with inert hot flue gases tapped from the boiler. Three coal mills each having a pulverizing capacity of 27 tones per hours are provided for one unit.
Numbers Three per unit
Type Drum ball
Rated output 30-32 tonnes/hr.
EM - 2 CELL
(RELAY & TURBINE)
The turbine is the prime mover for the generator in the power plant Different types of steam turbines used in thermal power plants, but the ones. Which are used at G.N.D.T. P. are categorized as follows
S.No. Type of Turbine Turbine at G.N.D.T.P.
1. Horizontal/vertical Horizontal
2. Single/multi cylinder Multi cylinder
3. Condensing/non condensing condensing
4. Reheat/ non-reheat Reheat
5. Regenerative/non regenerative Regenerative
6. With by pas/without by pass with by pass (stage-1)
Without by pass (stage-2)
BASIC WORKING OF TURBINE
First of all the turbine is run on gear motor with the help of exciter. At that time steam is kept on recirculating with the help of by pass valve. When the pressure of steam is increased to on optimum level and turbine acquires a particular rpm then steam is introduced in the H.P. (high-pressure) cylinder first. The temperature of steam at entrance is 540C and pressure is about 139 Kg/cm2. After doing its work on the H.P. Turbine, the steam is taken out for reheating rated temperature of steam at reheater inlet is 360C. The temperature of steam is increased upto 535C in the boiler shell and steam is again introduced in M.P (Medium pressure) turbine. After M.P.turbine, the steam is passed on to L.P. (Low-pressure) turbine. This process helps the turbine to reach the speed of 3000 rpm. After L.P. turbine, the steam is condensed in condenser, build below the turbine unit. The condenser contains a number of brass tubes through which cooling out from L.P. turbine it comes in contact with colder brass tubes then steam get transformed into water. This water get collected in HOT WELL just below the condenser. From here the hot water is again pumped with the help of condensate pumps. The cooling water is used to condense steam gets heated up and is cooled by falling from cooling tower. This completes the processing of steam through turbine and condenser.
Rotor of Turbine
All the rotors are manually by means of rigid coupling, including the rotors of the generator. The speed of whole system of rotor lies in the following ranges of the speed at the operating conditions: -
1900 to 2000 rpm Best noticed on the M.P. and L.P. rotors and generators.
2350 rpm Best noticed on H.P. rotor.
1. Bearing of rotors
The axial load of the entire system of rotors is taken up by a double-sided axial bearing located in the bearing stand between the H.P. and M.P casing. These are two protections mounted near the axial bearing one hydro chemical and one electromagnetic, which fouls the turboset during the non-permissible movement of the rotor.
The rotors are placed on radial bearing which are machined to elliptical shape. Further scrapping operations or change top and side clearances and change in temperature of oil, influence the oil wedge and the position of the journal bearing to maintain the same condition as existed during the initial assembly.
In the lower half of bearing a hollow groove is provided in the babbit metal through which oil the supplied through a drilled hole through H.P. jacking oil pumps.
The high-pressure oil rotors are lifted in the bearing so that any scrapping of the bearing is prevented.
2. TURBINE CASING
The high-pressure part of turbine is consisted of two-concentric horizontal casing. Inner casing is connected in such a way to the other casing that it enables to expand in all direction. The nozzles are attached to the inner casing. The steam pipe is connected to the condensers and the condensers are supported by springs. The casings are inter connected by the system of guide keys through bearing pedestals in such a way that thermal expansion of casing does not destroy the various parts of turbine.
The displacement-bearing pedestal between M.P. and H.P parts is measured by the electromagnetic pick up. This valve is about of 15 mm to prevent deformation at the casing. It is very important that sliding part clean lubricated and free from hazard for connecting parts bolts elements. The heating of bolts before tightening up and before the locking presents. The flanges of M.P. and H.P. casings are designed to heat up by steam during the starting up of turbo boost by which the difference in temperature between the cylindrical position of the casing flanges and connected bolts is reduced to limited deformation. The thermo couples are used for measuring temperatures. The thermo couple is partially connected to the indicated apparatus. Cooling fluid is generally used for reducing the temperature of various parts.
3. Regulation and Safety Equipment for Turbine Governing
The quality of steam entering in the turbine is regulated by the four governing valves on the inlet to the M.P. part. The amount of opening at any instant of these valves is controlled by the pressure of secondary oil, which is indirectly depending on the primary oil pressure and directly depending upon the spring force in the transformer during the stand still and during starting of the turbo set. The pressure of primary oil is directly depending on the speed of the set through the speed-sensing element. Operating the speed changer or the normal speed changer can very the tension in the spring in the transformer.
Thus make it possible to vary the speed before synchronizing. In case break down of any equipment of the block the quick closing devices are provided in the regulation system of the turbo set. H.P. quick closing valves (H.P.Q.C) M.P. quick closing valves (M.P.Q.C) at return flap valves are operated by either directly by the tripling lever or through the relay magnet on the main relay which creates instantaneously loss of pressure of the quick closing oil by the change of flow of oil inside the relay.
Distribution is used for checking the function of H.P.Q.C and M.P.Q.C valves. The H.P. and M.P quick closing valves, the non return flaps and non return extraction valves during normal operating condition have only two positions one is fully opened another fully closed.
4. STEAM CYCLE
(a) Main steam (b) Reheat steam (c) extraction steam
(a) MAIN STEAM
Saturated steam from the steam generator drum is led to the super heater bank to heat if up to 540C saturated steam from the drum is led to the ceiling super hearter (between SHH1 and SHH2) from ceiling super steam goes to convection
Super heater (between SHH2 and SHH3) the first regulated infection for at temperature takes place after convection super heater (between SHH9 and SHH10). Before entry to final super heater the steam is again at temperature by regulated injection. The steam is coming out from the final super heater normally at a pressure of 139 kg/cm2 at a temperature of 540oC. This steam is feed to the control valve. In each of the two live steam lines there is one turbine side main steam stop valve and one high pressure quick closing valve along with two control valves.
Steam for heating of the condenser and the feed steam is extracted from 8 non regulated extraction points from the turbine. Heating is carried out in five stages of L.P. heaters, one deareating heater and in two H.P. heaters extraction 1, 2, 3, is taken from L.P. turbine. Extraction 4, 5, 6 and 7 are taken from M.P. turbine. Extraction 8 is obtained from C.R.H. line first and second stage of heating is done by two sets of twin low-pressure heaters mounted directly in the L.P. casing of the turbine. Extraction 3, 4 and 5 are connected to the deaereating heater placed above feed water storage tank 7th and 8th extraction steam is fed to the vertical H.P. heaters respectively.
5. Turbine Accessories and Auxiliaries
Steam jet air ejector
LP and HP heaters.
Chimney steam condenser.
Gland steam condenser.
Oil purifier or centrifuge.
Clean oil pump with clean oil tank
Dirty oil pump with clean oil tank.
Auxiliary oil pump with auxiliary oil tank
Starting oil pump.
Emergency oil pump.
(I) SURFACE CONDENSER
Two surface condensers are used for condensing the steam which has worked in the turbine. The coolant for condensing the steam is circulating water which is inside the condenser brass tubes and steam is outside.
Technical data of Condenser
Cooling Area 3300 msq.
Number of brass tubes 6000
Circulating water required 7500 tonnes/hr.
Vacuum in the condenser 0.90 kg/cm sq.
(II) STEAM JET AIR EJECTOR
Starting ejector is used for quick evacuation of the turbo set during starting whereas main steam jet air ejector is used to maintain Vacuum in the condenser. It works on the principle of ‘VENTURI’ with steam working media to eject air from the condenser.
(III) LP AND HP HEATERS
In regenerative system there is a steam of 5 LP heaters, one Deaereator, 2 HP heaters. All LP and HP heaters are of surface type i.e. condensate or feed water is inside the heater tubes in the heater shells. L.P. heaters are of single flow whereas HP heaters are of double flow type. Deaereator is contact type heater in which steam and condensate come in direct contact.
(IV, V) CHIMNEY STEAM AND GLAND STEAM CODENSER: - There are additional two heating stages provided in the regeneration system of the turbine for heating the condense flowing through it steam leaks off from the turbine glands is used for heating the condensate in these heaters.
(VI, VII, VIII, IX, X) VARIOUS OIL PUMPS
Centrifuge is an oil purifier used to remove moisture and other impurities from the turbine oil. Maximum allowable moisture content in the turbine oil is 0.2%. In case the oil level of the main oil tank is to be made up then either oil can transferred from clean oil tank to main oil tank with centrifuge or from dirty oil tank to main oil tank with centrifuge.
(XI) STARTING OIL PUMPS AND EMERGENCY OIL PUMPS
Starting oil pumps supply the necessary turbine oil during starting of the turbine and upto turbine speed of 2930 rpm till the main oil pump mounted on the turbine rotor at the HP extension takes manually in order to provide lubrication oil for the turbo set. Emergency oil pumps are meant to start on auto, when turbine trips and lubrication oil pressure falls in order to provide lubrication to the turbine and generator bearings.
MAIN TECHNICAL DATA ABOUT TURBINE
The Basic Parameters
Rated output measured at terminal of the generator. 110,000KW
Economical output. 95,000KW
Rated speed 3,000 RPM
Rated temp. Of steam just before the stop valve. 535C
Max temp. Of steam before the stop valve 545C
Rated pressure of steam before the MP casing 31.63 ata
Max. Pressure of steam before the MP casing 35 ata
Rated temp. Of steam before the MP casing 535C
Max. Temp. of steam before the MP casing 545C
System of turbine:
Governing valves 2 interceptor valves
HP cylinder 2 Row Curtis wheel +8 moving wheels.
Wt. of HP rotor is approx. 5,5000kg.
MP cylinder 12 moving wheels.
Wt. Of MP rotor is approx. 11,000kg
LP cylinder 4 Moving wheels of double flow design.
Wt. of MP rotor is approx. 24,000.
Direction of the turbine rotation is to the right when looking at the turbine from the front bearing pedestal.
Protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective section from the rest of the system.
We have seen that whenever fault occurs on the power system, the relay detects the fault and closes the trip coil circuit. This results in the opening of circuit breaker, which disconnects the faulty section. Thus a relay ensures the safety of the circuit equipments from damage which may be causes by the faulty current.
ESSENTIAL ELEMENTS OF A RELAY
All the relays have the following three essential fundamental elements as shown in block diagram see fig.
(a) Sensing element: - Sensing or measuring element is the element which responds to the change in magnitude or phase of the actuating quantity e.g. current in the over current relay.
(b) Comparing element: - It is the element which compares the action of the actuation quantity of the relay with pre-designed relay setting. The relays only pick up if the actuating quantity is more than the relay setting.
(c) Control Element: - When a relay picks up it accomplishes a sudden change in the controlled quantity such as closing of trip coil circuit.
1. TYPES OF RELAYS
There are many kinds of relays applied in the power system. The relays can be designed and constructed. To, operate in response to one or more electrical quantities such as voltage, current, phase angle etc. The relays are classified in different ways.
According to construction and principle of operation
(i) Thermal relays: - The heating effect of electric current is used for the operation of these relays.
(ii) Electromagnetic attraction relays: - The operation of these relays depends upon the movement of an armature under the influence of attractive forces due to the magnetic field set up by current flowing through the relay coil.
(iii) Induction Relays: - Electromagnetic induction phenomenon is used for the operation of these relays by induction, eddy currents are induced in the aluminum disc, free to rotate, which exerts torque on it.
2. ACCORDING TO APPLICATION
(i) Over current, over voltage or over power relays:-These relays operate when the current, voltage or power rises beyond a specific value.
(ii) Directional or reverse current relays: - These relays operate when the applied current assumes a specified phase displacement with respect to the applied voltage and the relay is compensated for fall in voltage.
(iii) Under current, under voltage or under power relays: - These relays operated when the current, voltage or power falls below a specific value.
(IV) Directional or reverse power relays: - These relays operate when the applied voltage and current assumes a specified phase. Displacement and no compensation is allowed for fall in voltage.
(V) Distance relays: - The operation of these relays depends upon the ratio of the voltage to the current.
(Vi) Differential relays: - The operation of these relays takes place at some specific phase difference or magnitude difference between two or more electrical quantities.
3. According to the time of operation:-
(i) Instantaneous relays:-In these relays, complete operation takes place instantaneously i.e., the operation is complete in a negligibly small interval of time from the incidence of the actuating quantity.
(ii) Definite time lag Relays: - In these relays operation takes place after definite time lag which is independent of the magnitude of actuating quantity.
(iii) Inverse time lag Relays:–In these relays the time of operation is inversely proportional to the magnitude of actuating quantity.
Inverse Definite Minimum Time Lag Relays: - In these relays, the time of operation is approximately inversely proportional to the actuating quantity, but is never less than a definite minimum time for which relay is set.
A relay in which heating effect of electric current is used for its operation is known as thermal relays. These relays may be actuated by a.c. or d.c.
The schematic diagram of an indirectly heated general purpose thermal relay is shown in fig. It has a bimetallic strip which is heated by heating element which gets supply from a current transformer. An insulated contact arm carrying a moving contact is pivoted and is held by a spring. The other contact of trip circuit is a fixed contact. The spring tension can be varied by changing the position of contact arm with the help of sector plate.
Under normal conditions, the current flowing through the heating element is proportional to the normal full load current of the circuit. The heat produced by the heating element, under this condition is not sufficient to bend the bimetallic strip. However, when fault occurs current flowing through the heating element increases which produces heat sufficient to bend the bimetallic strip. This releases the contact arm and because of the spring tension the relays contacts are closed which closes the trip coil circuit or the alarm circuit once the alarm circuit or the trip coil circuit is closed; it operates the alarm circuit or the circuit breaker to open the circuit respectively.
These over current tripping relays are use mostly for motor controls. The heating elements of such relays are designed to with stand short time overload up to 7 times the normal full load.
ELECTROMAGNETIC ATTRACTION RELAYS
Electromagnetic attraction type relays are operated by virtue of an armature being attracted towards the poles of an electromagnet. These relays may be actuated by D.C. or A.C. quantities.
The schematic diagram of an electromagnetic attraction type relay is shown in fig. It consists of a magnet which carries a relay coil having number of tapings. The armature is held by the spring attached it. The armature has spring loaded moving contact which bridges the trip coil circuit.
Under normal conditions, the current flowing through the relay coil is such that spring tension is more than the attractive force of the electromagnet. Therefore armature is held in the open position. However when fault occurs, current flowing through the relay coil increases. This increases the attractive force of the electromagnet. At the instant when attractive force of electromagnet is more than the spring tension, the armature is tilted down wards and moving contact bridges the fixed contacts. This closes the trip coil circuit.
The current setting can be adjusted by changing the number of turn of relay coil. The larger numbers of turns are introduced in the operating coil, the smaller is the value of actuating current. The time setting can adjust by changing the tension of spring by a screw. Terminal AC act as normally can also be used for the operation of another circuit.
The basic principle of operation of these relays is electromagnetic induction. These relays are only actuated by a.c. An induction relay essentially consists of a pivoted aluminum disc place in between two alternating fields of the same frequency but displaced from each other by some angle. A torque is produced in the disc by the interaction of two fields. Such relays may be over current reverse power or directional over current relays as discussed in the coming articles.
INDUCTION TYPE OVER CURENT RELAY
An induction type over current relay is shown in fig. 5 (a) it consists of an aluminum disc which is free to rotate to be placed in between the two electromagnets. The upper magnet has three limbs whereas lower magnet has two. The tapped winding is wound on the central limbs of the upper magnet. This winding is connected to the CT of the line to be protected. The tapings one connected to a plug setting bridge as shown in fig.5 (a) by changing the position of plug by which the number of active turns of the primary winding can be varried, thereby the desired current setting is obtained. The secondary is a closed winding and wound on the central limb of the upper magnet and both the limbs of the lower magnet. The winding is energized by the primary winding.
The controlling torque is provided by connected a spiral spring on the spindle of the disc. The spindle of the disc also carries a moving contact, when the disc rotated through a preset angle, the moving contact bridges the two fixed contact of the trip coil circuit as shown in fig.5 (b). The preset angle can be adjusted to any value between O and 360, by adjusting the angle, the travel of the moving contact can be adjusted and the relay can be set for any desired time setting.
When current flows through the primary winding, an e.m.f. is induced in the secondary winding by induction. Since secondary is closed, a current flows through it. The fluxes are produced by the currents flows through primary and secondary winding. These fluxes are separated in phase and space and produces a driving torque on the disc. This torque is opposed by the restraining torque provided by the spring. Under normal conditions, the restraining torque is more than the driving torque, therefore, the disc remains stationary.
However when a fault occurs, the current flowing through the primary exceeds the preset value. The driving torque becomes more than the restraining torque consequently the disc rotates and moving contact bridges the fixed contacts when the disc rotates through a pre-set angle.
Specification of over current relay
Model NO. CDG 31EG 1212 A5
Type Auxiliary voltage 220V d.c.
Manufacturer Jyoti Ltd. Baroda.
WATER TREATMENT PLANT
WATER TREATMENT PLANT
The basic requirement of Water Treatment in the thermal power station is to provide suitable water for the boiler i.e. the water, which is free from dissolved, suspended, and any other type of impurities. If the water is taken as such without any treatment then this will result in scale and sludge formation, caustic embrittlement and corrosion in the boiler and the pipes. Water Treatment at GNDTP, Bhatinda may be broadly divided into
A) External Water Treatment.
B) Internal Water Treatment.
External Water Treatment comprises of: -
b) Sedimentation & Clarification.
Bacteria and other living organisms results in the formation of algae on the surface of tanks, pipes and other equipments. Addition of oxidizing agents such as chlorine & bleaching powder destroy the bacteria or any other micro organisms. At present, chlorine dosing is done at Intake. Pump House and at both CW Pump House with Gas Chlorinator of vacuum type at the rate of 10 Kg/Hr.
SEDIMENTATION & CLARIFICATION:
The suspended impurities in water are removed by sedimentation and clarification. When the river or cannel water is allowed to stand for sometime in a big tank or reservoir most of the suspended material settles down. The process of clarification is done in clarifier and is accelerated by adding coagulant such as alum (aluminium and ferrous sulphate) or Sodium Aluminate. These results in the formation of precipitate of aluminium hydroxide, which tends to agglomerate colloidal,Organic and suspended impurities in water. The precipitates so formed settles at the bottom of clarifier. These are removed by operating desludging valve.
Al2(SO4)3 + 3Ca(HCO3)2 3CaSO4 + 2Al(OH)3 +CO2.
Impurities not removed in this process are removed by filteration. At present, we have 4 clarifiers with a clarifying capacity of 1200 MT/Hr each to reduce turbidity upto 20ppm.
It is the process of passing of liquid containing suspended matter through a suitable porous material (Filtering Medium) to effectively remove the suspended matter in the liquid. For the process of filtration there are 3 pressure filters in each D.M.Plant having a capacity of 27 M3/Hr each. Turbidity of filtered water from pressure filters is not greater than 2 ppm.
Filter media commonly employed are graded and washed sand of effective size of 0.35 mm to 0.5 mm resting on supporting underbed of crushed gravel and pebbles of four varying size with coarsest size at the bottom of the bed.
DEMINERALIZATION: (REMOVAL OF DISSOLVED IMPURITIES)
The development of modern high-pressure boilers has been accompanied by serious problems connected with the formation of scale, corrosion etc.
The principle scale forming and hardness producing substances found in natural water are the soluble salts of calcium and magnesium. The most common are bi-carbonates Ca(HCO3)2, Mg(HCO3)2.The sulphate CaSO4,MgSO4. The chlorides CaCl2, Mg/Cl2 and sometimes nitrates are also present. The processes which are used for water softening are:
Boiling (For removing temporary hardness only).
Lime Soda Treatment.
Base Exchanger Zeolite process.
The demineralization of water at GNDTP, BATHINDA is done by ion exchange process. There are two D.M. Plants each having capacity 900 MT/day design for 5% station make up. The ion exchange process is used in removing all the above scale forming constituents. This is the most modern and latest method. The ion exchange is the process in which there are irreversible interchanges of ions of like sign between a solution and an insoluble solid. In this process the cations in water are exchanged with H+ ions of cations resins and anions are exchanged with OHions of anion resins.
Resin consists of a giant organic molecule arranged in the form of porus framework, having replaceable H+ions attached to it in case of cations resin and replaceable OH-ions in case of anion resins.
The demineralization process consists of 3 units in series, one is called “Cation Exchange Unit” and the other is called “Anion Exchange Unit”. Mixed bed unit follows this.
Cation Exchange Unit:
This exchanger removes all the cations such as Sodium (Na+), Potassium (K+), Calcium(Ca++), Magnesium(Mg++)etc. When the water passes through cations resin the functional hydrogen ion are replaced by the cations with the formation of respective acid. The equation is represented as:
(a) R'H+ + Ca(HCO3)2 R'Ca+ + H2CO3
(b) R'H+ + CaSO4 R'Ca+ + H2SO4
(c) R'H+ + NaCl R'Na + HCl
When the resin gets exhausted, it needs regeneration (i.e. reconversion of resin into the operating form) for cation exchange units mineral acid such as hydrochloric acid is used for regeneration
RCa + 2HCl 2RH+ + CaCl2
The treatment water is passed through degasser unit, since it has large amount of CO2 and here the CO2 is removed. This reduces load on the anion exchanger. The treatment water from cation exchanger is sprayed from the top of the degasser tower. The degasser tower is packed with rasching, rings and air flowing arrangement is provided from bottom to top with air outlet at the top. Air which is being pushed from the bottom comes in contact with water droplets & finally, CO2 is removed in sufficient amount & is ejected out by air draft.
H2CO3 H2O + CO2
Anion Exchanger Units:
This unit removes all the anions such as Sulphates, Chlorides, Nitrates, SiO2 and residual CO2from degasser. When the water passes through anion exchange resin, all the anions are exchanged with functional OH- group of the resin.
R+OH' + HCl RCl + H2O
R+OH' + H2SIO4 RSiO3 + H2O
After sometimes when it gets exhausted, it needs regeneration. It is regenerated with 5% NaOH
R+Cl'+ NaOH ROH + NaCl
(Exhausted Resin) (Regenerated to Drain Resin)
Mixed bed contains both the cations and anion resins. Any cation or anion which has slipped from the cation exchanger and anion exchanger are removed here in the mix bed unit. After mixes bed treated water is quit suitable for use in boiler.
PH = 6.8 to 7.2
Conductivity = 1.0 micromhos/cm
SiO2 =< 0.02 ppm.
Others = NIL
Internal Water Treatment:
When after standard treatment it is necessary to further condition the boiler feed water because D.M. Water dissolves CO2 andO2 in the storage tanks and becomes slightly acidic and corrosive in character. This is treatment at various stages of feed water is called internal water treatment. Internal Water Treatment is required by chemical dosing to combat the following:
Corrosion is the gradual destruction of metal by chemical or electro chemical reaction of metal with the surrounding medium. Corrosion begins at the surface and gradually penetrates into the metal. It also changes the mechanical and physical properties of material.
Once in boiler the water is heated to saturation. The temperature thus evaporates at the point of contact with heated tube surface. The impurities are left in boiler water whose concentration thereby increases. The impurities to deposit on the tube surface a scale. Scaling may take place in boiler drum, water walls heater and feed water piping. It reduces the flow requiring an increase in pressure to maintain water delivery and more fuel consumption. Then this condition occurs tube failure due to overheating, blistering and rupturing may be expected.
When minute holes are created on metal surface by oxidation it is know as pitting. This type of corrosion is caused by dissolved oxygen in water. The residual oxygen is removed by treatment with hydrazine.
Foaming priming and carry over are closely associated terms production of stable foam over the surface of water is called foaming. Too high concentration of dissolved salts is the cause of foaming.
The tendency of caustic (Sodium Hydroxide) to concentrate in drum seals, under rivets or at rolled tube joints injuring the metal is called caustic embrittlement. Foaming, priming, carryover, caustic embrittlement can be controlled by maintain proper alkalinity, operating blow down and maintaining proper drum level i.e. 30 to – 60 mm.
CHEMICAL DOZING FOR INTERNAL TREATMENT OF WATER
To take care of scaling and corrosion following chemical dosing is done to neutralize effect of CO2, O2 Calcium Magnesium, Salts and silica etc.
This dozing is done to increase the pH of the feed water and remove any in the system
2C4H9ON + 2CO2 2C4H9CO3 + N2
The pH of the feed and steam cycle is maintained between 8.4 to 8.8 to minimize corrosion, it is dozed at the discharge of condensate extraction pump.
It is a powerful reducing agent which reacts with dissolved oxygen under boiler water condition to produce water and nitrogen only as follows.
N2H4 + O2 2H2O2 + N2
Hydrazine also reduces non-protective iron oxide to protective magnetite.
It is done with two aims:
Any hardness (salts of Ca and Mg) entering the boiler is likely to form scale in boiler. The addition of phosphate prevents this. The phosphate reacts with calcium and magnesium to form sludge, which can be removed by blow down. In this way, Ca and Mg scales are completely removed from the boiler drum.
T.S.P. maintains proper pH of the boiler water. T.S.P. on hydrolysis with boiler water liberates NaOH with the reaction.
Na3PO4 + H2O Na2HPO4 + NaOH
132/220KV Switch Yard
The electricity generated at 11 K.V. by the turbo generator sets is step up by power transformers up to 132 KV in case of stage-1 and 220 KV in case of stage-2. For further transmission of power from power station to grid is controlled through 7 no’s 220 KV and 15 no’s 132 KV air blast circuit breakers along with their associated protective system.
According to design substation are classified:-
In these substations the equipments are installed within the building of the substation and hence the name indoor substation. Such substations are usually designed for 11 KV but can be erected for 33 KV or 66 KV, if the surrounding atmosphere is containing impurities, which may damage the equipments.
In these substations, the equipments are installed open and hence the name outdoor substation. Such substation can be designed to handle low, high and extra high voltages. The outdoor substations may be further classified as:
Pole mounted substation
Foundation mounted substation
POLE MOUNTED SUBSTATION
Such substations are designed for monthly distribution transformers of capacity unto 300 KVA. Such substations are cheapest simple and smallest of other substations. All the equipment’s are of outdoor type and mounted on the supporting structure of HT distribution line. Tripple pole mechanically operated switch is used for switching on and off of H.T transmission line. To control L.T. side, iron clad low-tension switch of suitable capacity with fuses is installed. Lighting arrestors are installed over the HT line to protect the installation from the surges. Substations are earthed at two or more places. Generally transformers of capacity upto 125 KVA are mounted on double pole structure and for transformers of capacity above 125 KVA 4 pole structure with suitable platform is used such substations are situated in very thickly populated location.
(II) FOUNDATION MOUNTED SUBSTATION
These substations are built entirely in the open and in such substations all the equipments are assembled into one unit usually enclosed by a fence from the point of view of safety. Substations for primary and secondary transmission and for secondary distribution are of this type.
The major equipment’s installed at G.N.D.T.P. substations are
(1)TRANSFORMER: - It is a static device which transfers a.c. electrical power from one circuit to the other at same frequency. It is used to step up or step down the voltage. In all the substations except the generating station transformers are employed. These are of following types: -
(A) POWER TRANSFORMER:-These are provided for stepping up the voltage. For units 1&2 the power transformers step up the voltage from 11 KV to 132 KV and for units 3&4 the power transformers step up the voltage from 11 KV to 220 KV. All the four transformers have a rated capacity of 125 MVA.
Specification of power transformer
Sr. No. 6002737
Electrical specific No. 600250
Manufacturer Heavy electrical Ltd. (Bhopal)
Year of manufacturing 1972
Insulation level :
H.V. 650 KV peak
H.V. neutral 38 KV rms
L.V. 75 KV peak
Weight of core and winding 94000 kg
Weight of oil 37200 kg.
Total weight 188880 kg
Oil quantity 43890 liters
Oil circulation litre/min. 2 * 2724
Air circulation cubic/metre 10 * 385
(B) Inter bus linking (auto) transformer: - The auto transformers are used to balance the load between 132 kv bus bars and 220 Kv bus bars. These transformers have a capacity of 100 MVA each.
Specification of Inter bus linking transformer
Electrical specification No. 410216
Manufacturer Heavy Electrical Ltd. (Bhopal)
Year of manufacturing May, 1972
Weight of core and winding 75200 kg.
Total weight 151104 kg.
Rated capacity 100 MVA
(C) Unit auxiliary transformer: There is one unit auxiliary transformer provided on each unit to step down the voltage from 11 Kv to 6.6 Kv, which is required to run the major plant auxiliaries.
Manufacturer Heavy Electrical Ltd. (Baroda)
Year of manufacturing 1972
Rated Capacity 15 MVA
Weight of core of winding 11685 Kg.
Total weight 22188 kg.
Oil in main tank 12200 litre.
Oil in cooler including pipes 2300 litre.
Total oil quantity 15620 litre.
Oil in OLTC 1120 litre.
(D) Station Transformer: There are two station transformers one for each unit is provided to step down the voltage from 132 Kv to 6.6 Kv. These transformers have capacity of 22.5 MVA. They serve as a stand by source of supply to auxiliaries.
S. No. 23600
Total weight 70 tones
Manufacturer Made in Bombay Martin Burn Ltd.
Oil in main tank 14940 lt.
Oil in cooler including pipe work 4500 lt.
Oil in OLTC 1760 lt.
Total oil 21200 lt.
Circuit breaker is a device, which may be operated under different conditions i.e. no load, full load fault conditions. However the major duty of a circuit breaker is to operate the circuit breaker under fault conditions under short circuit conditions, a circuit breaker required to perform three major duties:-
It must be capable of opening the circuit under abnormal conditions.
It must be capable of classing the circuit on to a fault.
It must be capable of carrying a fault current safely for a short time while another circuit breaker is clearing the fault.
OPERATING PRINCIPLE OF A CIRCUIT BREAKER
A circuit breaker is a device which.
Makes or breaks a circuit either manually or by remote control under normal conditions.
Breaks a circuit automatically under abnormal conditions.
Makes a circuit manually or by remote control under abnormal conditions.
Thus, circuit breaker is just a switch which can be opened under normal abnormal conditions both manually and automatically.
To perform the above operations, a circuit breaker essentially consists of fixed and moving contacts, called electrodes, under normal operating conditions; these contacts remain closed until and unless these are not operated manually or by remote control. However when a fault occurs on the power system, the trip coil of the circuit breaker is energized, which pulls apart the moving contracts from the fixed contacts thus opens the circuit.
When the moving contacts are separated from the fixed contacts, an arc is struck between them. The production of are not only delays the current interruption proxies but is also generates enormous heat which causes damage to the equipment of the power system or to the breaker itself. Therefore every effort is made to extinguish the arc produced in the circuit breaker as quickly as possible.
CIRCUIT BREAKER RATINGS
According to the duties to be performed by a circuit breaker they have the following three ratings:-
Short time capacity.
(i)Breaking capacity: - when fault occurs the contacts are separated in the circuit breaker. The r.m.s. value of current that a circuit breaker is capable of breaking at a given recovery voltage and under specified conditions is known as breaking capacity of the circuit breaker.If the current is symmetrical, breaking capacity is referred to is symmetrical breaking capacity. Whereas if the current is asymmetrical it is referred as asymmetrical breaking capacity. Symmetrical breaking current is the r.m.s. value of the a.c. component of the short circuit current at the instant when contacts are separated. Whereas the asymmetrical breaking current is the r.m.s. value of the total current which compresses of a.c. and d.c. component of current at the instant when contacts are separated.The breaking capacity rating of a circuit breaker is generally expresses in terms of MVA.In case of 3-phase circuit breakers breaking capacity in MVA = 3* rated voltage * rated breaking current * 10-6.
(ii)Making capacity: - There is every possibility of closing the circuit breaker under deed short circuit conditions. The capacity of a circuit breaker to close the circuit under short circuit conditions depends upon its ability to with stand the effects of electromagnetic forces. These forces are proportional to the square of the peak value of the current on closing.
The peak value of the current during the first cycle of current wave after the circuit the first cycle of current wave after the circuit is closed by the circuit breaker is called making capacity. The making capacity is stated in terms of a peak value of current instead of and r.m.s. value.
To find out the making capacity the symmetrical breaking current is multiplied by 2 to convert the r.m.s value to peak value and then it is multiplied by 1.8 to include the doubling effect of maximum asymmetry. Making capacity = 2 * 1.8 * symmetrical breaking capacity = 2.55* symmetrical breaking capacity.
(iii)Short time capacity :- Sometimes fault occurs in the power system in such a way that one circuit breaker is clearing the fault at that time the other circuit breaker connected in series must carry the fault current safely for a short period. Moreover sometimes the fault on the power system is of very temporary nature and presets for a small period after which the fault is automatically cleared. In the interest of continuity of supply the circuit breaker should not be allowed to trip in such situations and should be capable of carrying the fault current safely for a short period.
At G.N.D.T.P switch yard air blast circuit breakers and SF6 circuit breakers are employed instead of oil Circuit breakers due to following reasons:-
There is no risk of fire hazard and explosion.
Due to less arc duration in it as compared to that in oil circuit breakers, burning of contact is less.
It requires less maintenance
They provide facility of high speed reclosure.
Axial-Blast Air Circuit Breakers
A schematic arrangement of an axial-blast air circuit breaker is as shown in figure. The arcing potions of the fixed and moving contacts are coated with silver tungsten alloy. The moving contacts are coated to a piston and shaft of the contact is guided by guide spring.
Opening the lower air valve closes the circuit breaker and under normal conditions the valve remains open. Whenever a fault occurs, the upper valve is opened and the lower valve is closed by the mechanism not shown in figure. Air enters the upper vessel at a high pressure, which separates the moving contacts from the fixed. An arc is struck between the contacts, which is extinguished by the axial blast of cold air and current is interrupted. Once the arc is extinguished, the upper valve is closed and the lower valve is opened to close the circuit.
SF6 CIRCUIT BREAKERS
Sulpher hexafluoride CB is shown in fig. In this the movable cylinder is coupled with the moving contacts, whereas the piston is fixed. When fault occurs, the moving contacts are separated from the fixed contact. Since the movable cylinder is attached with the moving contacts, it moves against the fixed piston. Thus the gas filled in the cylinder is compressed and released through the nozzle as shown in fig. The gas moves along the arc and reduces its diameter by axial convection and radial dissipation. At zero current, the diameter becomes too small and the arc gets extinguished. The gas is not exhausted to the atmosphere; it is rather again used for arc extinction.
They are smaller in size because of high dielectric strength of SF6 gas.
No danger of or explosion.
They require minimum maintenance.
Since same gas is recycled, a small quantity of SF6 gas is required for long run.
They give silent operation; they do not make any sound like A.C.B. during operation.
It requires less maintenance.
Rated voltage 132 Kv
Rated frequency 50 Hz
Rated gas pressure 18 kg/cm2
Replaced with SF6* 7 kg/cm2
Rated current 1600 Amp.
Rated making capacity 17850 MVA
Rated breaking capacity 7000 MVA
(3) Isolator: - These are knife switches which are operated only at no-load. Their main function is to isolate a portion of the circuit from the other. These are generally placed on both sides of a circuit breaker in order to do repair and maintenance on the circuit breaker without any danger. For maintenance first of all circuit breakers are opened then isolators are opened and properly earthed. Only then maintenance is done. Isolators do not have the arc control devices and therefore cannot be used to interrupt current at which an arc will be drawn across the contacts. The open arc that would be drawn in such a case is very dangerous in that it will not only damage the isolator and the equipment surrounding it, but will also as a rule will cause flash over between phases. In other words results in short circuit in the installation. That is why isolators are used only for disconnecting and connecting parts or units after first de-energizing them by opening their circuits with respective circuit breakers.
Manufacturer Hi-Velm industries Pvt. Ltd., Madras
Unit type THP
Auxiliary switch no. of pairs 12
Pressure 16 kg/cm2
Isolator kV 132 kV
Control voltage 220 V
Isolator current 800 Amp.
(4)Bus Bars: - The thick conductors run on the towers at the generating stations, grid stations or sub-stations operating at constant voltage required to connect a number of generators or feeders operating at the same voltage are called bus-bars.
The bus bars are arranged in different manner. The main aim of any particular arrangement of bus bars is to achieve adequate operating flexibility, sufficient reliability and minimum cost. At G.N.D.T.P. there are two no’s 132 kV Bus bars are made up of aluminum. The bus coupler number two connects the 132 kV bus bars and 220 kV bus bars with each other.
(5)Lightning Arrestor: - A lightning arrestor or surge diverter is a device which provides an easy conducting path or relatively low impedance path for the flow of current when the system voltage increases more than the designed value and regains its original properties of an insulator at normal voltage.These are the arc apparatus devices designed to protect insulators of power lines and electrical installation from lighting surges by diverting the surge to earth and instantly restoring the circuit insulation to its normal strength with respect to earth. These are connected between earth and line. Their purpose is to protect the t/f winding against over voltages.
Necessity of lightning arrestors
The ground wires and earth screens do not provide protection against the high voltage waves reaching at the terminals of costly equipment such as transformers. These high voltage waves may cause the following damages.
(i) The waves may cause flash over in the internal windings of transformer and spoils the winding insulation.
It may cause internal flash over between turns of the same winding of transformer.
It may cause external flash over between the terminals of electrical equipment, which may cause damage to insulator.
It may cause internal or external flash over causing building up of the oscillations in the electrical apparatus. Hence it is absolutely necessary to divert this high voltage wave to earth before it reaches at the terminals of the equipments. This is achieved by connecting a lighting arrestor between line and earth.
(6)Current Transformers: - The current transformers are basically step up transformers. The connections of an ammeter when used in conjunction with a current transformer for measurement of current are shown in fig.
The primary winding having one or a few turns of thick wire is connected in series with the line, whose current is to be measured. The secondary winding having large number of turns of fine wire carries the instrument directly connected across it. The working of current transformer is slightly different to that of on ordinary power transformer. In case
Of current transformer, the load impedance or burden on the secondary is very small, therefore it is considered to be short circuited. Hence current transformer works under short circuit conditions. Moreover the current in secondary winding is not governed by its load impedance rather it depends upon the current flowing through the primary.
(7)Potential Transformer:-The potential transformers are basically step down transformers. The connection of a voltmeter is used in conjunction with a potential transformer for measurement of high ac voltages. The voltage to be measured is applied across primary winding, which has a large no. of turns. The secondary side, which has much smaller number of turns, is coupled magnetically to the primary winding. The turn ratio is so adjusted that the secondary voltage is 110V, when full rated primary voltage is applied to primary. Potential transformers are used to operate voltmeter, the potential coils of wattmeter and relays from high voltage lines. The design of potential transformers in quite similar to that of a power transformer, but the loading of a potential transformer is very small in comparison to that of a power transformer. The loading of potential transformer sometimes is only a few volt amperes. These transformers are made shell type because this condition develops a higher degree of accuracy. For medium voltage i.e. unto 6.6 KV to 11 kV they may be either dry or oil immersed but for voltage more than 11 kV they are always oil immersed type. An out door type oil immersed voltage transformer having rating 66000/3 or 110/ 3. The working of potential transformer is essentially the same as that of a power transformer, the main point of difference is that the power loading of a potential transformer is very small and consequently the exciting current is almost of the same order as that of secondary current. Whereas in power transformers exciting current is very small fraction of secondary load current.
(8)Insulators: - They are used to prevent the flow of current from bare conductors to earth through line support; the conductors are secured to insulators. They provide insulation between the conductors and earthed steel towers. The insulators are usually placed on the cross arms which is clamped on line support. Thus the successful operation of transmission system depends to a great extent on the quality and maintenance of line insulators. Generally suspension and strain type insulators are employed at the substations.
(9)Wave traps: - These are used in carrier communication circuits and are mounted on lines. Wave trap or line trap contains a main coil, lightning arrestor and a tuning device. All are connected in parallel as shown in fig..
The main coil has an inductance of 0.2 MH to 2.0 MH. This inductance offers high impedance to the frequency (50 kHz to 500 kHz) carrier signals and blocks them here. It does not allow them to enter the power system equipment. However it offers very low impedance to the power frequency signal (i.e. power system voltage and current). Hence it acts as an insulator for high frequency carrier signals and a conductor for the low.
Since the main coil is connected in series with the line, it has to carry the line current even under fault conditions. There fore it is designed from the current rating point of view. The current rating of the main coil may vary from 400 A to 4000 A.
The lighting arrestor is used to protect the main coil from high voltage surge whereas the tuning device is used to block the signals of narrow band carrier frequency.
All these components of the wave trap are hanged in air through a string of insulators as shown in fig.
Sometimes these components of wave trap are immersed in oil and are enclosed in a drum type porcelain container having sufficient mechanical strength.
Spending my six months of training in Guru Nanak Dev Thermal Plant, Bathinda, I concluded that this is a very excellent industry of its own type. They have achieved milestones in the field of power generation. They guide well to every person in the industry i.e. trainees or any worker. I had an opportunity to work in various sections namely switch gear, Boiler section, Turbine section, De-mineralized water plant, E.S.P, EM-2 CELL etc. while attending various equipments and machines. I had got an endeverous knowledge about the handling of coal, various processes involved like unloading, belting, crushing and firing of coal. The other machines related to my field that I got familiar with boiler, turbine, compressors, condenser etc. I found that there existed a big gap between the working in an institute workshop and that in the industry. Above all the knowledge about the production of electricity from steam helped me a lot to discover and sort out my problems in my mind related to the steam turbine, their manufacture, their capacity, their angle of blades and their manufacturing. The training that I had undergone in this industry will definitely help me to apply theoretical knowledge to the practical situation with confidence.
1. A Textbook of electrical technology (A.C & D.C Machines) – B.L.THERAJA, A.K.THERAJA
2. A course in electrical power: - J.B.GUPTA
3. A course in electrical & electronic measurement and instrumentation: - A.K.SAWHNEY
4. A course in electrical power: - M.L.SONI, P.V.GUPTA
5. A textbook of power plant engineering: - Er. R.K.RAJPUT
8. Manual of GURU NANAK DEV THERMAL PLANT, BATHINDAype oil immersed voltage transformer having rating 66000/3 or 110/ 3. The working of potential transformer is essentially the same as that of a power transformer, the main point of difference is that the power loading of a potential transformer is very small and consequently the exciting current is almost of the same order as that of secondary current. Whereas in power transformers exciting current is very small fraction of secondary load current.
Spending my six months of training in Guru Nanak Dev Thermal Plant, Bathinda, I concluded that this is a very excellent industry of its own type. They have achieved milestones in the field of power generation. They guide well to every person in the industry i.e. trainees or any worker. I had an opportunity to work in various sections namely switch gear, Boiler section, Turbine section, De-mineralized water plant, E.S.P, EM-2 CELL etc. while attending various equipments and machines. I had got an endeverous knowledge about the handling of coal, various processes involved like unloading, belting, crushing and firing