Power Plant Engineering

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
Surge
tank
Access
gallery
P
re
s
su
re
s
h
a
ft
Underground
Power house
Access 
gallary
Valve
gallery
Tailrace
tunnel
River
Dam
Natural
tapography
Surge tank
Access
shaft
Pressure
tunnel
Pressure
shaft
Underground
Power House
Tailrace
tunnel
Surge Tank

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362
POWER PLANT ENGINEERING
of both surge tanks can be calculated independently according to the usual surge theory. If the prime
mover is reaction type, the oscillations in tailrace and headrace interfere with each other and, therefore,
the larger areas of surge tank is required than the volume required for surge tank used with impulsive
type prime mover.
The Santa Giustina power house in Italy is an example of this type of arrangement. Koyna power
project also comes under this class except there is no surge tank to the downstream side as the tailrace
tunnel is not very long.
4. Station arrangement without surge tank. If the pressure tunnel and tailrace tunnel are short
in length, the upstream and downstream surge tanks can be eliminated from the system. The main points
to be considered in this design are the maximum water hammer effect and danger of cavitations in the
turbine. The latter can be eliminated by selecting the proper turbine axis level above the draft tube axis
level.
A vertical or steep shafts (10 : 1) are provided to access the upstream or Swedish type power
house whereas horizontal or mild sloping tunnels prove more favourable. for downstream stations or
Alpine type layout.
Generally fresh air as supplied to the power house through the main access tunnel and warmed
up air is exhausted along the same tunnel through a separate duct. Sometimes separate air shafts are used
to exhaust the warmed up air.
The transformers and high voltage switching equipment were previously located outdoors al-
most without exception. In recent practice, transformers are also arranged underground in the trans-
former room excavated in the vicinity of power house. If the decision of the management is in the favour
of underground power house against air-raids, no significant gain could be achieved by locating the
transformer outdoor as it is one of the most sensitive and essential components of the power project. An
underground location of the transformer is further supported by the fact that the location of the trans-
former near the power house involves hardly any excess cost.
Draft tube
Closure
shaft
Power House
Transformer
Tailrace tunnel with expansion chamber
Access
tunnel
Cab
le s
haf
ts
Intake
Dam
P
res
s
ur
e s
h
af
t