Hydro-electric Power Station : Part 2

Tuesday, August 16, 2011

1.3 Constituents of Hydro-electric Power Station
       Let us discuss the constituent and their functions in the operation of the hydroelectric power station.
1.3.1 Dam 
       The water reservoir in the form of a dam is the main part of the power station. It stores the water, provides the continuous supply of water and maintains the necessary water head. The dams are built up of stones and concrete. The design and type of the dam us selected according to the topography of the site and economical aspects.
1.3.2 Spillways
       There are certain times when the river flow exceeds the storage capacity of the dam, due to the heavy rainfall. The spillways are provided to discharge this surplus water and maintain safe water level in the dam.
1.3.3 Surge Tank
       This is an important projecting device in a hydro-electric power plant. It is built just before the valve house. It protects the penstocks from bursting due to sudden pressure changes.
       If the load on the turbine is thrown off suddenly then by the governing action, the turbine input gates get suddenly closed. Thus there is sudden stopping of water at the lower end of the penstock. This time the excess water at the lower end of the penstock, rushes back to the surge tank. The surge tank water level increases. Thus the penstock is protected from bursting due to high pressure. The surge tank absorbs this high pressure swing by increasing its water level.
       On the other hand, when the load on the turbine suddenly increases, the additional water required is drawn from the surge tank. This satisfies the increased water demand instantly.
        Thus the surge tank controls the pressure changes created due to rapid changes in the water flow in penstock and hence protects the penstock from water hammer effects which might burst the penstock.
1.3.4 Penstocks
       The penstocks are made up of steel or concrete and arranged in the form of conduits, supported by the anchor blocks. The penstocks are used to carry water to the turbine. For the low head (less than 30 m) power stations, the concrete penstocks are used. The steel penstocks are suitable for any head.
Fig. 3  Protecting devices of penstock
       There are certain protective devices attached to the penstocks. These devices are shown in the Fig.3.
       The automatic butterfly valve completely shuts off the water flow if the penstock bursts.
       The air valve maintains the air pressure inside the penstock equal to the outside atmospheric pressure.
       The anchor block supports the penstock and holds it in the proper position.
       The surge tank also protects the penstock from sudden pressure changes.
1.3.5 Water Turbines
       The main two types of water turbines are,
i) Impulse      and    ii) Reaction
       In an impulse turbine, the entire pressure of water is converted into a kinetic energy in a nozzle. Then the water jet is forced on the turbine which a large velocity which drives the wheel. The pelton wheel is an example of impulse turbine which is shown in the Fig. 4.
Fig. 4  Impulse turbine
       It contains elliptical buckets mounted on the periphery of a wheel. The force of water jet on the buckets, drives the wheel and the turbine. There is a needle or spare at the tip of the nozzle. The governor controls the needle which controls the force of the jet, according to the load demand. The impulse turbines are used for the high head power stations.
       In the reaction turbines, the water enters the runner, partly with pressure and partly with velocity head. There are two type of reaction turbines.
i) Francis          and     ii) Kalpan
       The Fig. 5 shows the basic principle of reaction turbine. The reaction turbine consists of an outer ring of stationary guided blades and an inner ring of rotating blades. The guided blades control the flow of water to the turbine. Water flows radially inwards and changes to a downward direction when it passes through the rotating blades. While passing over the rotating blades, the pressure and velocity of water are decreased. This causes reaction force to exist which drives the turbine. For large variation of head, Kalpan is used as its efficiency does not vary with change in load. For fairly constant head, a Francis or propeller turbine is used.
       The reaction turbines are used for the low head power stations.
Fig. 5   Reaction turbine
1.4 Advantages
1. If the proper site is selected, the continuous water supply is available.
2. Requires no fuel as water is used.
3. No burning of fuel hence neat and clean site as no smoke or ash is produced.
4. It does not pollute the atmosphere.
5. The operating cost is very low as free water supply is available.
6. The turbines in this plants can be switched on and off in a very short period of time.
7. It is relatively simple in construction, self contained in operation and requires less maintenance.
8. It is robust and has very long life.
9. It gives high efficiency over a considerable range of load. This improves the overall system efficiency.
10. It provides the additional benefits like irrigation, food control, afforestation etc.
11. Being simple in design and operation, highly skilled workers are not necessary for the daily operation. Thus man power requirements is low.
1.5 Disadvantaes
1. Due to the construction of dam, very high capital cost.
2. The low rate of return.
3. Uncertainity of availability of water due to unpredictable rainfall.
4. As its location is in hilly areas and mountains, the long transmission lines are necessary for the transmission of generated electrical energy. This requires high cost.
5. The large power stations disturb the ecology of the area by the way of disforestation, destroying vegetation and uprooting people.
6. Highly skilled and experienced persons are necessary at the time of construction.

See Hydro-electric Power Station Part 1

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