Power Plant Engineering

Types of underground power stations. There are mainly five different types of underground
power stations as per hydraulic characteristics.
1. Free level tailrace tunnel without a
downstream surge tank. In this arrangement, the
long and steep tailrace tunnel is built to cope with
the discharge without putting the tunnel under
pressure, both under steady and unsteady flow
conditions. This type is more suitable with Pelton-
wheel because it does not interfere with the flow
in tailrace tunnel. The arrangement is shown in
Fig. 11.16.
The underground Innertkirchen power
house in Switzerland is the example of such Con-
struction. The head is 672.3 meters, length of pres-
sure tunnel is 10 kilometers and tailrace tunnel is
1294 metre with a slope of 4 : 1.
2. Downstream station arrangement or
Alpine type. The arrangement is shown in Fig.
11.17. In this arrangement, the water is carried
through a long horizontal pressure
tunnel to the point of emergence
to the surface, from where a step
pressure shaft continues dawn to
the power house as shown in fig-
ure. A surge tank is provided at the
junction of pressure tunnel and
pressure shaft as in the case of ex-
posed penstock and surface power
station. The valve chamber after
the surge tank is also provided un-
derground and the valves are also provided before the prime-mover.
These valves may be located either in the main cavern or a separate valves gallery is excavated
for this purpose. Access to both surge tank and power house is provided through horizontal tunnels as
shown in Fig. 11.17. The tailrace tunnel is considerably short in length. The arrangement is generally
preferred in mountain regions. The downstream surge tank is also used if a tailrace tunnel is long and
considerable surges are likely to occur in the tailwater.