Power Plant Engineering

Disadvantages
1. The construction cost of the underground power house is more compared with the over ground
power house :
(a) The excavation of the caverns required for housing the turbine generator units and auxiliary
equipments (machine hall of Koyna project is $00
′
× 120
′
× 60
′
in dimensions) is very expen-
sive.
(b) The costs of access tunnels are considerable.
(c) The separate gallery excavated for the inlet valves adds the extra cost.
(d) The construction of air ducts and bus galleries also adds in total construction costs.
(e) Special ventilation and air-conditioning equipment required for underground adds in the con-
structional costs.
(f) In some cases, the tailrace tunnel of an underground power house requires a more elaborate
solution than a tailrace tunnel designed for the surface arrangement. The advantage gained by
reducing the pressure conduit would be lost by extending the tailrace tunnel.
(g) The first cost is also increased by locating the transformer and high-voltage switchgear un-
derground. The above-mentioned constructions increase the capital cost of the plant.
2. The operational cost of the power plant increases due to following reasons :
(a) The lighting cost.
(b) The running cost of air-conditioned plant.
(c) The removal of water seeping may be more costly than for the surface arrangement.
Adequate lighting, proper ventilation, maintenance of uniform climatic conditions within the
power houses, provision of the necessary safety equipments against flooding, maintenance of proper
acoustical conditions, augmenting the feeling of safety by providing a sufficient number of well placed
exit; and finally artistic shaping and outfitting of machine hall increases the overall cost of the under-
ground power house compared with ground surface power house.

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HYDRO-ELECTRIC POWER PLANTS
361
The choice of the site for the power house either over ground or underground requires a consid-
erable economical analysis according to the available topography and no thumb rule can be applied for
its selection.