National Thermal Power Corporation (NTPC)

National Thermal Power Corporation Limited was formed in 1975 to plan, promote and organize an integrated and efficient development of Central Sector Power Stations.The Singrauli Super Thermal Power Station was the first of the series of pithead power stations along with 400kV AC transmission line network. It is located on the banks of Govind Ballabh Pant Sagar (Rihand Reservoir), about 200km south of Varanasi in the Sonebhadra district of Uttar Pradesh. For coal transportation, a  railway system with rapid loading and unloading facility known as Merry-Go Round (MGR), continuously hauls coal from the Jayant block of Singrauli coalfields to the plant site. The rake consists of 30 wagons and will deliver 1800 MT of coal in each cycle. The average daily consumption of coal is 25,000 MT per day i.e. 8.0 million tonnes per annum considering average calorific value of 4000 kcal/kg and 7000 hrs of operation in an year for the ultimate capacity of the plant of 2000 MW having 5 units of 200 MW each and 2 units of 500 MW each.The 5´200MW generating units of Stage I are each equipped with coal-fired, regenerative, re-heat type steam generators with electrostatic precipitators, each generating 700 tonnes/hr of steam at 138 kg/cm pressure and 535°C temperature. The steam generator feeds steam to a condensing, horizontal, tandem compound 3-cylinder re-heat type turbo generator rotating at 3000 rpm and each generates 200 MW. Three phase generator transformer of 250 MVA capacity steps up the generation voltage from 15.75 KV to 400 KV. Cooling water from the Rihand Reservoir is drawn through an approach channel. It is then pumped into concrete intake duct by vertical pumps of 15000 m/hr capacity each. From the ducts, the water is circulated through condensers and is then discharged into a duct from where it flows into an open channel. This open channel carries the water for a distance of 6 km to affect sufficient cooling before it joins back into Rihand Reservoir.

 The 2´500MW generating units of Stage II are each equipped with coal-fired, regenerative, re-heat type steam generators with electrostatic precipitators, each generating 1725 tonnes/hr of steam at 178 kg/cm pressure and 540°C temperature. The steam generator feeds steam to a condensing, horizontal, tandem compound 3-cylinder re-heat type turbo generator rotating at 3000 rpm and each generates 500 MW. Turbine is a single shaft machine with separate high pressure (HP), intermediate pressure (IP) and low pressure (LP) parts. The HP part being a single flow cylinder and the IP and LP parts double flow cylinders. The individual rotor generator is connected by rigid coupling. The generator is three-phase, horizontal, 2-pole cylindrical rotor type with a rated output of 588 MVA and terminal voltage of 21 KV and full load current of 16,200 A. Three single phase generator transformers of 200 MVA capacity each steps up the generation voltage from 21 KV to 400 KV.

              The circulating water system for cooling the steam in condensers is an open cycle system utilizing the water from Rihand Reservoir through 2.9 km long intake channel and pumped through underground RCC duct and the return water is discharged to the reservoir through 6 km long discharge canal. The intake channel and the discharge canal are common for both stage I and II units. For supplying cooling water 6 nos. of vertical pumps each of 27,000 m/hr capacity have been provided.To reduce the air pollution 220 m high multi-flue stack are there for better dispersion of the gases emitted by the boilers. There are total four stacks is SSTPS ¾ one for units 1, 2 & 3, second for units 4 & 5 and one each for units 6 & 7. Electrostatic precipitators are provided between the boiler and stack in each unit to precipitate the ash content of the flue gases and help in the reduction of air pollution. The ash so collected is dumped in the ash disposal yard in the slurry form.

PRINCIPLE OF OPERATION

For each process in a vapour power cycle, it is possible to assume a hypothetical or ideal process which represents the basis intended operation and do not produce any extraneous effect like heat loss.

1. For steam boiler, this would be a reversible constant pressure heating process of water to form steam.
2. For turbine, the ideal process would be a reversible adiabatic expansion of steam.
3. For condenser, it would be a reversible a constant pressure heat rejection as the steam condenser till it becomes saturated liquid.
4. For pump, the ideal process would be the reversible adiabatic compression of liquid ending at the initial pressure.

When all the above four cycles are combined, the cycle achieved is called RANKINE CYCLE. Hence the working of a thermal power plant is based upon Rankine cycle with some modification.

2.1 PROCEDURE

2.1.1 COAL TO STEAM

                Coal from the coal wagons is unloaded in the coal handling plant. This coal is transported upto the raw coal bunkers with the help of belt conveyors. Coal is transported to bowl mills by coal feeders. The coal is pulverized in the bowl mill, where it is ground to a powder form. The mill consists of a round metallic table on which coal particles fall. This table is rotated with the help of a motor. There are three large steel rollers, which are spaced 120° apart. When there is no coal, these rollers do not rotate but when the coal is fed to the table it packs up between the roller and the table and this forces the roller to rotate. Coal is crushed by the crushing action between the rollers and the rotating table. This crushed coal is taken away to the furnace through coal pipes with the help of hot and cold air mixture from the primary air (P.A.) fan. The P.A. fan takes atmospheric air, a part of which is sent to the air preheaters for heating while a part goes directly to the mill for temperature control. Atmospheric air from forced draft (F.D.0 fan is heated in the air heaters and sent to the furnace as combustion air.

                Water from the boiler feed pump passes through economiser and reaches the boiler drum. Water from the drum passes through down comers and goes to bottom 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 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 steam is sent to the superheaters for superheating. The superheaters are located inside the furnace and the steam is superheated (540°C) and finally goes to the turbine.

               Flue gases from the furnace are extracted by the induced draft (I.D.) fan, which maintains a balanced draft in the furnace with F.D. fan. These flue gases emit their heat energy to various superheaters in the plant house and finally pass through the air preheaters and goes to the electrostatic precipitator where the ash particles are extracted. Electrostatic precipitators consist of metal plates, which are electrically charged. Ash particles are attracted to these plates, so that they do not pass through the chimney to pollute the atmosphere. Regular mechanical hammer blows cause the accumulation of ash to fall to the bottom of the precipitator where they are collected in a hopper for disposal. This ash is mixed with water to form slurry and is pumped to ash dyke. 

2.1.2 STEAM TO MECHANICAL POWER

               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 wall. These act as nozzles and direct the steam into a second ring of moving blades mounted on a disc secured to the turbine shaft. This second ring turns the shafts as a result of the force of the steam. The stationary and moving blades together constitute a ‘stage’ of the turbine and in practice many stages are necessary, so that the cylinder contains a number of rings of stationary blades with rings of moving blades arranged between them. 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.Finally, the steam is taken to the low pressure cylinders, each of which it enters at the center flowing outwards in opposite directions through the rows of turbine blades – an arrangement known as double flow – to the extremities of the cylinder. As the steam gives up its heat energy to drive the turbine, its temperature and pressure fall and it expands. Because of this expansion the blades are much larger and longer towards the low pressure end of the turbine.

             The turbine shaft usually rotates at 3,000 rpm. This speed is determined by the frequency of the electrical system used in the country. In India, it is the speed at which a 2- pole generator is driven to generate alternating current at 50 Hz.When as much energy as possible has been extracted from the steam it is exhausted directly to the condenser. This runs the length of the low pressure part of the turbine and may be beneath or on either side of it. The condenser consists of a large vessel containing some 20,000 tubes, each about 25 mm in diameter. Cold water from the water source i.e. the Rihand Reservoir is circulated through these tubes and as the steam from the turbine passes round them it is rapidly condensed into water condensate. Because water has a much smaller comparative volume than steam, a vacuum is created in the condenser. This allows the steam pressure to reduce down to pressure below that of the normal atmosphere and more energy can be utilized.  

From the condenser, the condensate is pumped through low pressure heaters by the extraction pump, after which its pressure is raised to boiler pressure by the boiler feed pump. It is further passed through feed heaters to the economiser and the boiler for reconversion into steam. The cooling water drawn from the reservoir is returned directly to the source after use.    

2.1.3 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. These transformers also supply the unit auxiliary transformers required for the working of various electric motors, pumps and other equipments installed in the unit.

2.1.4 TRANSMISSION

The electricity is usually produced in the stator windings of large modern generators and is fed through terminal connections to one side of a generator transformer that steps up the voltage to 400KV. From here conductors carry it to a series of three switches comprising of an isolator, a circuit breaker and another isolator.

The circuit breaker, which is a heavy- duty switch capable of operating in a fraction of second, is used to switch off the current flowing to the transmission lines. Once the current has been interrupted the isolators can be opened. These isolate the circuit breaker connected to its terminals. Here after the maintenance or repair work can be carried out safely. From the circuit breakers the current is taken to the busbar conductors, which run the length of the switching compound – and then to another circuit breaker with its associated isolators, before being fed to the Grid. Each generator in a power station has its own transformer, circuit breaker and associated isolators but the electricity generated is fed into a common set of busbars. Circuit breakers work like combined switches and fuses but they have certain special features and are very different from the domestic switch and fuse. When electrical current is switched off by separating two contacts, an arc is created between them. At the voltage use in homes, this arc is very small and lasts for a fraction of a second but at very high voltages used for transmission, the size and power of the arc is considerable and it must be quickly quenched to prevent damage. Three phase, four-wire system is used for large power transmission, as it is cheaper than the single-phase two-wire system that supplies the home. Also power is generated in a three-phase system.

                 The center of the power station is the control room. Here the engineers monitor the output of electricity, supervising and controlling the operation of generating plant and high voltage switchgear and directing power to the grid system as required. Instruments on the control panels show the output and the existing condition of the whole main plant and a miniature diagram indicates the precise state of the electrical system.