A rapid closing or opening of the nozzle or guide vanes (gates) is
undesirable, as serious waterhammer problems may result in the pen-
stocks. Sudden changes may be avoided in the case of a Pelton turbine if a
deflector is activated in front of the jet, thus diverting part of the flow
away from the turbine. Similarly, in the case of a reaction-type turbine a
relief valve may allow a part of the discharge to flow directly to the tail
race without entering the runner.
12.9.4 Generators and transformers
The generator is an electrical machine coupled to the turbine shaft (either
horizontally or vertically). The alternating current (a.c.) synchronous gen-
erator is widely used in hydroelectric power production practice. It has
two elements, a magnetic field consisting of an assembly of electromagnets
(poles) which rotates (hence rotor), within a stator (stationary unit) which
is a system of conductors (armature windings). The relative displacement
between the rotor and the stator induces an alternating electromotive
force. The a.c. supply in the UK is produced with a standard frequency of
50 Hz. The turbine is governed to operate at constant speed and the gener-
ator is designed with the appropriate number of poles to produce the
designed frequency at the selected speed (equation (12.4)).
Owing to the physical limitations imposed by the mechanical proper-
ties of the materials, the rotational speeds of a hydroelectric machine are
limited, e.g. for an output of 15 MW a limiting speed of 1500 rev min^1 ,
250 MW at 600 rev min^1 , 1000 MW at 120 rev min^1 etc. At the sites with
larger potential power, the turbine speed is generally chosen at 100–375
rev min^1 , thus reducing the number of sets for the same installed capacity.
The sizes of the generators vary depending on their rating and on
their shaft arrangements (either vertical or horizontal). A.c. generators are
rated in kilovolt-amperes (kVA). The apparent or nominal rating (output)
differs from the actual output, Pa, the difference Pmbeing used to magnet-
ize the rotor field. Thus the rated kVA is given by
kVA(P^2 aP^2 m)1/2 (12.28)
and the power factor is given by
PFcosPa/kVA. (12.29)
Exciters (direct-current generators) mounted on the generator shaft
energize the rotor field of the main generator.
A.c. generators with a rating of 300 kVA vary in weight from about
160 kN for an operating speed of 900 rev min^1 to 500 kN for a speed of 100
522 HYDROELECTRIC POWER DEVELOPMENT
