GENERATOR – MOTOR ISSUES BASIC INFORMATION



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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