Pump storage is used in some hydropower plants to replenish
the water in the forebay each day and then deliver electric power during the
peak demand hours. During the pumping mode, the SG acts as a motor.
To start such a large power motor, even on no load, a pony
motor was traditionally used. It is also practical to use the back-to-back
generator and motor starting when a unit works as a generator and, during its
acceleration to speed, it supplies a motor unit and the two advance
synchronously to the rated synchronous speed.
A single generator may supply one after the other all
pump-storage units. An induction-motor-mode starting with self-synchronization
is also feasible, at reduced voltage for large power units. As the friction
torque may be reduced, by refined high-pressure oil bearings, to less than 1%
of rated torque, the power required to start the motor, with the waterless
turbine — pump, was notably reduced.
It is thus feasible to use a lower relative power rating
static power converter to start the motor and disconnect the converter after
the motor has been self-synchronized. An SG designed to be used also as a motor
for water pump storage undergoes about one start a day as a motor and one
synchronization a day as a generator.
Self-synchronization for motoring and automatic
synchronization for generation imply notable transients in currents and torque,
and the machine has to be designed to withstand such demanding conditions.
Typically, the direction of motion is changed for motoring
as required by the turbine pump. High wicket gate water leakage may cause the
“waterless” unit to start rotating still in the generator direction.
To avoid the transients that the static power converter
would incur in initiating the starting as a motor, the machine has to be
stalled before beginning to accelerate it on no load, in the motoring
direction.
Asynchronous self-starting is used for low and medium power
units, albeit using a step-down autotransformer or a variable reactor to reduce
starting currents. For units above 20 MW, pony motor, generator motor back to
back, or static power converter starting should be used.
The development of multiple-level voltage source converters
with IGBTs (up to 4.5 MW/unit) [22] and with GTOs or MCTs (up to 10 MW/unit and
above) might change the picture.
For the low power range (up to 3 MW), it is possible to use
a bidirectional voltage source IGBT converter, designed at a generator motor
rating, that will not only allow startup for the motoring, but will also
provide for generating and motoring at variable speed (to exploit optimally the
turbine pump).
It is known that the optimal speed ratio between pumping and
turbining is about 1.24. For high power, the multilevel voltage source GTO and
MCT converters may replace the controlled — rectifier — current source
converters that are used today to accelerate pump-storage units (above 20 MW or
so) and then self-synchronize them to the power grid.
The multilevel PWM back-to-back IGBT, GTO, MCT converters
provide quality starting while inflicting lower harmonics both on the power
line and on the motor side. They are smaller in size and provide controllable
(unity) power factor on the line side.
As there is a step-up transformer between the static power
converter and the machine, the starting cannot be initiated from zero frequency
but instead, from 1 to 2 Hz. This will cause transients that explain the
necessary oversizing of the static power converter for the scope.
It goes without saying that to start the motor as a
synchronous motor, a static exciter is required, because full excitation is
needed from 1 to 2 Hz speed. The static converters might be controlled for
asynchronous starting (up to a certain speed), and thus, their kilovoltampere
rating has to be increased.
Note that the “ideal” solution for pump storage is the
doubly fed induction generator motor that will
be studied in Variable Speed Generators.
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