07-11-2012, 03:20 PM
NTPC Limited
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BASIC CYCLES OF A THERMAL POWER PLANT:
Coal to steam cycle:
Coal from the coal wagons is unloaded in the coal handling plant. This coal is transported up to the raw coal bunkers, with the help of belt conveyers. Coal is transported to bowl mills by the coal feeders. The coal is pulverised in the bowl mill, where it is ground to a powder form. This crushed coal is taken away to the furnace through coal pipes, with the help of hot and cold air mixtures from the primary air fan (PA). The PA fan takes atmospheric air, a part of which is sent to the air pre heaters, for heating, while a part goes directly to the mill, for temperature control. Atmospheric air from forced draft ( FD) fan, is heated in the air heaters and sent to the furnace as combustion air.
Water from the boiler feed pump passes through the economiser and reaches the boiler drum. Water from the drum passes through the down comers and goes to the water ring header. Water from the bottom ring header is divided to all the four sides of the furnace. Due to heat and the density difference, the water rises up in the water wall tubes. Water is partly converted into steam, as it rises up in the furnace. This steam and water mixture is again taken to the boiler drum, where the steam is separated from water. Water follows the same path, while the steam is sent to the super heaters for superheating. The super heaters are located inside the furnace, and the steam is superheated (540 degrees) and finally goes to the turbines.
Flue gases from the furnace are extracted from the induced draft (ID) fan, which maintains a balanced draft in the furnace with FD fan.
Steam to mechanical power cycle:
From the boiler, a steam pipe conveys steam to the turbine through a stop valve (which can be used to shut off steam in an emergency) and through control valves, that automatically regulate the supply of steam to the turbine. Stop valves and control valves are located in the steam chest and a governor, driven from the main turbine shaft, operates the control valves to regulate the amount of steam used( this depends upon the speed of the turbine and the amount of electricity required from the generator).
Steam from the control valves enters the high pressure cylinder of the turbine, where it passes through a ring of stationary blades, fixed to the cylindrical walls. These act as nozzles and direct the steam into a second ring of moving blades mounted on a disc, secured to a turbine shaft. This second ring turns the shaft as a result of the force of the steam. The stationary and moving blades together constitute a ‘stage’ of the turbine. The steam passes through each stage in turn until it reaches the end of the high pressure cylinder and in its passage; some of its heat energy is changed into mechanical energy. The steam leaving the high pressure cylinder goes back to the boiler for reheating and returns by a further pipe to the intermediate pressure cylinder. Here it passes through another series of stationary and moving blades.
Mechanical power to electricity:
The turbine shaft is mechanically coupled to the generator rotor shaft through thrust bearings. The steam rotates the turbine at 3000 rpm thus the rotor of the generator also rotates at 3000 rpm. This speed is necessary to generate electricity at a frequency of 50 Hz with a two pole turbo- generator.
The rotor carries the field winding over it. This field winding is excited by a DC excitation system. The supply to the excitation system is tapped from the unit auxiliary transformer. The flux generated by this field current cuts the armature coil. The armature coil is star- star connected and is induced with three phase emf. The emf is tapped with the help of slip rings and brushes. This emf is carried over to the generator transformer through a bus duct. The bus duct is voltage transformer grounded.
The generator transformer has delta connection in the primary side and star connection in the secondary side. The generator bus supplies electric power per phase to the three-phase transformer or bank of three single-phase transformers. These transformers transmit electric power to the switchyard for further transmission.