Classification According to Available Head of Water in Hydro Power Plant
- Plants are classified as follows according to available head of water : (i) Low head plants, (ii) Medium head plants, (iii) High head plants.
(i) Low Head Plants : Low head plants have water head below 30 m. A low head power plant stores water by the construction of a dam or a barrage across a river and the power plant is installed near the base of the dam on the downstream side. Generally, one half of the barrage has regulating gates so that excess water is allowed to flow in the river from these gates to the river. Depending upon the quantity of water available, Kaplan turbine (in number and size) can be installed.
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| Figure A |
This type of construction have disadvantage that as the river discharge increases, it causes a rise in the level of downstream. This reducing head is available for turbine water. It results in reduction in quantity of water flowing to turbines. This in turn causes the reduction in output of generating station. This problem can be solved by causing the surplus water to pass over the wier and create a standing wave, so that forming an area in which the water level is lower than the downstream level. Sometimes the power house is constructed at the side of the river (Refer Figure A i) for creating a proper head race. But this requires excavation of rock and earth, increasing the cost of the plant. In such plants, Francis, Propeller or Kaplan type of turbines are used and they need no surge tanks.
(ii) Medium Head Plants : Such type of plants can be installed by making use of a localized head or a fall in the stream. In such cases a open channel or conduit may be carried along the side of the valley upto the power house, Figure C or the forebay provided at the beginning of penstock serves as a reservoir for such plants. In such plants, water is carried from main reservoir works in open channel to the forebay. From that to the power house through penstock. The forebay surves as a surge tank.
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| Figure B |
The prime movers used in such type of plants are Francis, Propeller and Kaplan type and the head of water lies between 30 m to 300 m.
Generally, these type of plants are installed by building a dam between the two rows of hills, through which water flows in hilly areas at a high level. This facilitates that large quantity of water at high head to be stored in the catchment area of the dam.
A pressure tunnel is taken off from the reservoir and water is brought to the valve house upto the start of penstocks. Penstocks are huge steel pipes Installed to carry large quantity of water from valve house to power house. The valve house contains main sluice valves and Isolating valves, which operate when penstock bursts by cutting-off supply of water through the penstock.
Surge Tank
- It is built in between the beginning of penstocks and valve house. It is open at top. When a certain heavy load is thrown off of the generating system, for controlling flow of water sudden closing of fixed gates of the water turbines is required. It causes water hammer on the fixed gates. It is absorbed by the surge tank by increase in its level of water, avoiding damage to fixed gates.
- At every point of deviation of the pipelines, either in horizontal or in vertical plane anchorages are constructed with expansion joints.
- The power house is installed close to the river so that tall race water can be discharged in the river. These plants are having high working head of water more than 300 m and they use Pelton wheel type turbines for their operation.
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| Figure C |