360 POWER PLANT ENGINEERING
beauty of landscape. The overall cost is further reduced by the modern techniques in tunnel
work and better excavation process.- The shorter power conduit of underground power house reduces the head losses.
- The regular maintenance and repair costs are lower for underground stations as the mainte-
nance required for rock tunnels is less. - The power plant is free from landslides, avalanches, heavy snow and rainfall.
- The useful life of the structures excavated in rock is considerably longer than that of concrete
and reinforced concrete structures. - It is possible to improve the governing of the turbines with the construction of underground
power house. - The construction period is reduced mainly due to the possibility of full-scale construction
work in winter. - Underground power station is bomb-proof and may be preferred for military reasons: They
are perfectly protected against air-raids. The military considerations became more predomi-
nant with the increased shadow of the war and the building of underground power stations
underwent a rapid evolution after second world war.
Disadvantages - 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. - 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.