Power Plant Engineering

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.

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.

3. Intermediate station arrangement. Fig. 11.18 shows the economic site of the power house at
   an intermediate section of the entire power conduit. The specific characteristic of this arrangement is
   long headrace tunnel and a long tailrace tunnel. Upstream and downstream surge tanks are necessary as
   shown in figure to deal with the pressure oscillations in both headrace and tailrace. If the prime mover is
   impulse type, there is no interference between tailrace and headrace level and, therefore, the dimensions

Fig. 11.16

Fig. 11.17

Fig. 11.18

Penstock

Access shaft

Tailrace channel

Dam

Rugged Tarrain

Pressure tunnel Surgetank

Access

gallery

Pre

ssu

re (^) s
haf
t
Underground
Power house
Access
gallary
Valve
gallery Ta i l r a c etunnel
River
Dam
Naturaltapography
Surge tank
Accessshaft
Pressuretunnel
Pressureshaft
UndergroundPower House
Ta i l r a c etunnel
Surge Tank