Automatic Control of the Shaft Generators

When the order is issued for the main engine to operate at a constant speed corresponding to the correct frequency, and if the load on the distribution network is not in excess of the maximum permissible load on the shaft generator, and if no alarm is operative on the shaft generator, this combination of conditions is indicated on the control panel by the "shaft generator ready" lamp lighting up.

The generator then rotates at the correct speed although without excitation, i. e. it generates no voltage.

If the change-over switch on the control panel is set to the "shaft generator on" position the generator will be excited and will generate a voltage. When about 85% of the normal voltage is attained, control of excitation shaft generator will be taken over by the voltage regulator in the switchgear.

The diesel-driven generator or generators supplying the distribution network are then synchronized to the shaft generator by the synchronizing unit in the shaft generator unit. When the generators are running in synchronism, the shaft generator breaker will receive a closure signal from the automatic control equipment.

When the shaft generator is in service and no alarm is operative, a shut-down signal will be applied to all diesel-driven generators. These generators will be unloaded, and the circuit breakers will then trip the generators from the distribution network. The diesel engines will then be stopped.

A synchronizing alarm will be initiated if the shaft generator is not synchronized to the network within about one minute. Other alarms in the equipment indicate high voltage, high frequency, low voltage and low frequency.

All alarms can be delayed by an adjustable period ranging between 1 second and 60 seconds. If a shaft generator alarm should become operative the number of diesel-driven units necessary to replace the shaft generator will then be started immediately and will be synchronized to the network. When a sufficient number of diesel-driven units are in service, the shaft generator will be unloaded and its circuit breaker will be tripped.

If no diesel-driven unit is synchronized to the network after an adjustable time delay, the shaft generator breaker will trip and blackout will occur. The time delay for tripping the shaft generator circuit breaker can be set at any value between about 1 second and 60 seconds. The black-out will last only about 1 second, since the necessary number of diesel-driven units are already in service, although not synchronized to the network.

In automatic electrical power plants using shaft generator additional shaft generator control unit is installed for automatic change over of the power supply between the shaft generator and the diesel-driven generators.

The shaft generator unit provides the following functions:
  1. Excitation of the shaft generator.
  2. Synchronization to the distribution network and closure of the circuit breaker.
  3. Unloading and shutting-down of the diesel-driven generators operating in parallel
  4. In the event of an alarm on the shaft generator, changing over from this generator to the stand-by generators.
  5. Switching a diesel-driven generator onto the distribution network and unloading of the shaft generator when "Shaft generator off" is requested.
  6. Field suppression on the shaft generator after tripping.