OTHER COMPONENTS OF HYDROPOWER PLANTS 523
rev min^1 , their sizes being about 3 m high and 2.5–5 m in diameter. Hori-
zontal shaft units are generally suitable for low-speed plants and are
heavy, large and costly, whereas vertical shaft units used with high-speed
plants are rather small and less expensive. For speeds larger than 1000
rev min^1 additional stresses will be induced, warranting special materials
and designs. Special structural problems often arise in providing proper
support for generators. The generators must also be adequately ventilated
to keep them from overheating, and this is achieved by air and/or water
cooling.
For high-head (up to 300 m) storage schemes, reversible Francis-type
pump–turbines (Raabe, 1985) have been developed to operate at a relat-
ively high efficiency as either a pump or a turbine. The same electrical unit
works as a generator or motor by reversing the poles. A plant equipped
with reversible facilities may reduce the costs of a pumped storage scheme
through the elimination of the additional pumping equipment and
pumping house. There may be abnormal wear on all components of such
units due to their frequent operation in starting and stopping modes
several times a day. The hydraulic pump–turbine characteristics, dual rota-
tion and method of starting the units in pumping mode are some of the
additional factors which will affect the generator design.
Exceptionally large units – e.g. the turbines at the Three Gorges
Project with runner diameter 10 m and generator diameter 18.5 m
and weight 1800 tons – require ingenuity in their design to avoid vibration
problems.
The transformers connecting the power source (generators) and the
receiving circuit (transmission lines) step up the voltage for transmission,
thus reducing the power loss and permitting the use of smaller conductors
(cables) in the transmission line. The transformers are usually located in
outside switchyards adjacent to the power house, as a necessary precau-
tion to avoid high voltage and other hazards.
12.9.5 Power houseThe power house structure can be divided in two sections, a substructure
supporting the hydraulic and electrical equipment and a superstructure
housing the equipment. The substructure is usually a concrete block with
all the necessary waterways formed within it. The scroll case and draft
tube are usually cast integrally (especially in large low-head plants) with
the substructure with steel linings.
The superstructure usually houses the generating units and exciters,
the switch board and operating room. Vertical-axis units (whose turbines
are placed just below the floor level) generally require less floor space than
those mounted on horizontal axes. The cost of the superstructure can be