09-01-2014, 01:20 PM
REPORT ON BASIC OPERATION OF VINDHYACHAL SUPER THERMAL POWER STATION
OPERATION OF VINDHYACHAL SUPER THERMAL.doc (Size: 953 KB / Downloads: 121)
INTRODUCTION
NTPC (National thermal power corporation) is India’s largest power generation company. It was set up in the year 1975 to accelerate power development in India. It has an installed capacity of 37,104 MW and planned to grow up to 75000 MW till year 2017.There are 15 coals based and 7 gas based plant in India located across the country. NTPC is the sixth largest thermal power generator in the world and the second most efficient utility in terms of capacity utilization based on data of 1998.
Coal to Electricity
The means and steps involved in the production of electricity in a coal-fired power station are described below.
The coal, brought to the station by train or other means, travels from the coal handling plant by conveyer belt to the coal bunkers, from where it is fed to the pulverizing mills which grinds it as fine as face powder. The finely powdered coal mixed with pre-heated air is then blown into the boiler by fan called Primary Air Fan, with additional amount of air called secondary air supplied by Forced Draft Fan. As the coal has been grounded so finely the resultant ash is also a fine powder. Some of this ash binds together to form lumps which fall into the ash pits at the bottom of the furnace. The water quenched ash from the bottom of the furnace is conveyed to pits for subsequent disposal or sale. Most of ash, still in fine particles form is carried out of the boiler to the precipitators as dust, where it is trapped by electrodes charged with high voltage electricity. The dust is th en conveyed by water to disposal areas or to bunkers for sale while the cleaned flue gases pass on through ID Fan to be discharged up the chimney.
Steps Involved In Power Generation In VSTPS
The conversion of coal into electricity consists of various intermediate stages. This are described below –
1.Coal handling Plant-
CHP is the first stage of any Thermal Power Plant .The coal from the mines is brought to the plant by use of Coal wagons. . Merry-Go-Round rail system of transport is adopted for transportation of coal from the mines to plant site. NTPC has its own MGR system which results in an annual profit of 45 crores. Coals are unloaded by track hoppers. As the coal from mines contain particles of different sizes, so water is sprinkled on the coal so as to stop flow of coal with air and to stop the loss. Coal in the track hopper is then send to crusher house for crushing it to size no less than 20mm. Crushed coal is sent to bunkers through conveyer belt system. Afterwards according to requirement coals are taken into mills through gravimetric feeder to pulverize it to a size of 200 mesh. Primary air mixed with pulverized coal is fed to the centre of boiler burning zone. Pre heated secondary air enters boiler, surrounds the pulverized coal and helps in combustion.
Electrostatic Precipitator (ESP):
It is a device which captures the dust particles from the flue gas thereby reducing the chimney emission. Precipitators function by electrostatically charging the dust particles in the gas stream. The charged particles are then attracted to and deposited on plates or other collection devices. When enough dust has accumulated, the collectors are shaken to dislodge the dust, causing it to fall with the force of gravity to hoppers below. The dust is then removed by a conveyor system for disposal or recycling.
Steam cycle:-
The demineralised water, which is used to produce steam, passes through various intermediate stages. The DM water passes through a series of water heaters namely LP heaters, Gland Steam Cooler(GSC),HP heaters and afterwards passes through Deaerator, Boiler feed pump, economizer and then goes to boiler drum. In LP heater and HP heater ,heat is transferred from steam (coming from turbine) to water which increases the temperature of the water. The direct mixing of steam with water is not allowed. In Deaerator, steam comes in direct contact of water. Deaerator is used to remove the air of feed water to prevent corrosion of boiler tubes and turbine blades. The presence of air also reduces the efficiency of cycle. The boiler feed pump is used to pump water to boiler drum through economizer. Boiler Economizer are feed-water heaters in which the heat from waste gases is recovered to raise the temperature of feed-water supplied to the boiler. In the midst of HPH and Economizer, a three element feed flow regulating system has been incorporated which regulates the steam flow from the boiler, the feed water flow to the boiler and the water level in the boiler drum.
Steam Turbine:
The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam. High pressure steam flows through the turbine blades and turns the turbine shaft. Steam turbines extract heat from steam and convert it into mechanical work by expanding the steam from high pressure to low pressure. Steam turbine shaft is connected to a synchronous generator for producing electricity.
Circulating Water System:
Bulk requirement of water is used in thermal plants for the purpose of cooling the steam in condensers. The requirement of water for this purpose is of the order of 1.5-to2.0 cusecs/MW of installation. In VSTPS closed loop cooling system with cooling towers is used for this purpose. The heat which the water extracts from the steam in the condenser is removed by pumping the water out to the cooling towers. The cooling towers are simply concrete shells acting as huge chimneys creating a draught (natural/mechanically assisted by fans) of air. The water is sprayed out at the top of towers and as it falls into the pond beneath it is cooled by the upward draught of air. The cold water in the pond is then circulated by pumps to the condensers. Inevitably, however, some of the water is drawn upwards as vapours by the draught and it is this which forms the familiar white clouds which emerge from the towers seen sometimes.
Seal and Lube Oil system:
Seals are employed to prevent leakage of hydrogen from the stator at the point of rotor exit. A continuous film between the rotor collar and the seal liner is maintained by means of oil at a pressure which is about 0.5 atmosphere above the casing hydrogen gas pressure. The thrust pad is held against the collar of rotor by means of thrust oil pressure, which is regulated in relation to the hydrogen pressure and provide a positive maintenance of the oil film thickness.
Hydrogen Cooling System:
Hydrogen is used as a cooling medium in large capacity generators for its high heat carrying capacity and low density. Proper arrangements are made for filling, purging as it forms explosive mixture if mixed in certain quantities with oxygen. In order to prevent escaping of hydrogen shaft sealing system and generator casing is done.
Stator Cooling System:
The stator winding is cooled by distillate, which is fed from one end of the machine by Teflon tube and flows through the upper bar and returns back through the lower bar of another slot. The stator winding is cooled in the system by circulating demineralized water through hollow conductors. DM water of specific resistance is selected for cooling purpose. The system is designed to maintain a constant rate of cooling water flow to stator winding at a nominal inlet water temperature of 40⁰C
Generator transformers:
The generator is connected to the Generating Transformer by means of isolated bus ducts. This transformer is used to step up the generating voltage of 15.75 KV to bus voltage 4ooKV.This Transformer has elaborate cooling system consisting of number of oil pumps and cooling fans apart from various accessories. The connection is delta on LV side and star with neutral grounded on HV side. There are water pipes to fight in case of fire condition.
Station Transformer-
This transformer is a step down transformer which transforms 132 KV to 6.6 KV voltage. This transformer is used to intake power from 132 KV bus for running the auxiliary equipments of an unit when the unit is being tripped. The station transformer is a tertiary winding transformer with each winding star connected with neutral grounded. The specification of this transformer is 80 MVA,132/11.5/6.6 KV.
Current Transformer (CT):
A current transformer is a type of instrument transformer designed to provide a current in its secondary winding proportional to the alternating current flowing in its primary. They are single phase oil immersed type. They are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages.In current transformers different ratios are achieved by secondary taps. Current transformers with five secondary’s (4 for protection and one for metering) are employed. . Current transformers provide satisfactory performance for burdens ranging from 25% to 100% of rated burden over a range of 10% to 100% of rated current in case of metering CTs and up to accuracy limit factor/knee point voltage in case of relaying CTs