Page 314 - Centrifugal Pumps 2E

Hydraulic Power Recovery Turbines 279

covery may be realized by using a hydraulic turbine to drive an electric
generator of either the synchronous or induction type. For the smaller
systems, the induction-type generator is attractive for economic consid-
erations.
A squirrel-cage induction machine becomes an excellent power gener-
ator when it is excited by AC power while the shaft is rotated above syn-
chronous speed. Frequency of the generated power is that of the excita-
tion; shaft speed determines only the amount of power consumed or
delivered. If the shaft is rotated much faster than synchronous speed, the
machine can burn out. But the system tends to be self-regulating because
the shaft becomes increasingly harder to rotate as speed increases above
synchronous.
When the induction machine is excited by AC power from a utility sys-
tem, power is fed back into the power grid as the speed reaches and sur-
passes rated synchronous speed. The power grid provides the excitation
voltage needed by the induction machine for both motor and generating
action.
When an induction generator must work without a source of AC power,
excitation can be supplied by residual magnetism and capacitors con-
nected phase to phase. A storage battery can be used to provide a current
pulse through one of the windings and thus leave sufficient flux to start
generation.
Generation occurs when the capacitor current exceeds the excitation
current of the windings. Generation stops when the shaft speed is low-
ered to the point where capacitive reactance exceeds that of the winding
or when the load absorbs too large a portion of the capacitor current.

Operation and Control Equipment

As the flow through the HPRT increases from the no-flow condition,
the fluid velocity through the runner gradually imparts to the runner not
only enough energy to overcome internal friction but also to permit some
net power output. This point usually occurs at about 40% of design flow
or capacity. As in any turbine driver, the machine will speed up until the
load imposed on the shaft coupling equals the power developed by the
turbine. The hydraulic turbine must operate to satisfy its own head-ca-
pacity-speed-horsepower relationship within the available head and im-
posed speed limits.
Consider a power recovery turbine operating as the only driver. If
more liquid is allowed to flow to the power recovery turbine than is
needed to produce the horsepower required, the turbine will speed up and
try to handle the liquid; at the same time the driven pump or compressor