Page 298 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Choice of site for hydro electric power stations
The following points should be taken into account while selecting the site for a hydro-electric power station.
i Availability of water: Since the primary requirement of a hydro-electric power station is the availability of
huge quantity of water, such plants should be built at a place (e.g., river, canal) where adequate water is
available at a good head.
ii Storage of water: There are wide variations in water supply from a river or canal during the year. This makes
it necessary to store water by constructing a dam in order to ensure the generation of power throughout the
year. The storage helps in equalising the flow of water so that any excess quantity of water at a certain period
of the year can be made available during times of very low flow in the river. This leads to the conclusion that
site selected for a hydro-electric plant should provide adequate facilities for erecting a dam and storage of
water.
iii Cost and type of land: The land for the construction of the plant should be available at a reasonable price.
Further, the bearing capacity of the ground should be adequate to withstand the weight of heavy equipment to
be installed.
iv Transportation facilities: The site selected for a hydro-electric plant should be accessible by rail and road
so that necessary equipment and machinery could be easily transported. It is clear from the above mentioned
factors that ideal choice of site for such a plant is near a river in hilly areas where dam can be conveniently
built and large reservoirs can be obtained.
Constituents of hydro- electric power plant
The constituents of a hydro-electric plant are
1 Hydraulic structures
2 Water turbines
3 Eectrical equipment.
1 Hydraulic structures
Hydraulic structures in a hydro-electric power station include dam, spillways, headwork, surge tank, penstock and
accessory works.
Dam: A dam is a barrier which stores water and creates water head. Dams are built of concrete or stone masonry,
earth or rock fill. The type and arrangement depends upon the topography of the site. A masonry dam may be built
in a narrow canyon. An earth dam may be best suited for a wide valley. The type of dam also depends upon the
foundation conditions, local materials and transportation available, occurrence of earthquakes and other hazards.
At most of sites, more than one type of dam may be suitable and the one which is most economical is chosen.
Spillways: There are times when the river flow exceeds the storage capacity of the reservoir. Such a situation
arises during heavy rainfall in the catchment area. In order to discharge the surplus water from the storage
reservoir into the river on the down-stream side of the dam, spillways are used. Spillways are constructed of
concrete piers on the top of the dam. Gates are provided between these piers and surplus water is discharged
over the crest of the dam by opening these gates.
Head works: The head works consists of the diversion structures at the head of an intake. They generally
include booms and racks for diverting floating debris, sluices for by-passing debris and sediments and valves for
controlling the flow of water to the turbine. The flow of water into and through head works should be as smooth
as possible to avoid head loss and cavitation. For this purpose, it is necessary to avoid sharp corners and abrupt
contractions or enlargements.
Surge tank: Open conduits leading water to the turbine require no protection. However, when closed conduits
are used, protection becomes necessary to limit the abnormal pressure in the conduit. For this reason, closed
conduits are always provided with a surge tank. A surge tank is a small reservoir or tank (open at the top) in which
water level rises or falls to reduce the pressure swings in the conduit. A surge tank is located near the beginning
of the conduit. When the turbine is running at a steady load, there are no surges in the flow of water through
the conduit i.e., the quantity of water flowing in the conduit is just sufficient to meet the turbine requirements.
However, when the load on the turbine decreases, the governor closes the gates of turbine, reducing water supply
to the turbine. The excess water at the lower end of the conduit rushes back to the surge tank and increases its
water level. Thus the conduit is prevented from bursting. On the other hand, when load on the turbine increases,
additional water is drawn from the surge tank to meet the increased load requirement. Hence, a surge tank
overcomes the abnormal pressure in the conduit when load on the turbine falls and acts as a reservoir during
increase of load on the turbine.

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CITS : Power - Electrician & Wireman - Lesson 106-116