Sudden load current surges (e.g. due to motor starting) on a generator cause a
corresponding change in its output voltage.
This is due to an internal voltage drop in the generator windings and the effect is usually called voltage dip.
Similarly, load shedding will produce an over voltage at the bus-bars.
An unregulated or non-compounded generator excitation system would not be realistic on board ship due to the varying voltage caused by the fluctuating load demand.
Automatic voltage regulation (AVR) equipment is necessary to rapidly correct such voltage changes.
An AVR will control the generator’s voltage to 2.5% (or better) of its set value over the full load range.
This is its steady-state voltage regulation. Transient voltage dip is usually limited to 15% for a specified sudden load change with recovery back to rated voltage within 1.5 seconds.
In special cases where unusually large surges are expected (e.g. from heavy-duty cargo cranes) the generator / AVR performance limits may be extended.
The AVR senses the generator output voltage and acts to alter the field current to maintain the voltage at its set value.
A manual trimmer regulator may be fitted on the generator control panel to set the voltage level e.g. 440 V. More usually, the voltage trimmer potentiometer is
on the control card of the AVR so is not accessible to an operator.
The control circuit for a modern AVR consists of transformers, rectifiers, zener diodes, transistors and thyristors. These are mounted on one of more circuit cards fitted either within the switchboard or local to the generator.
Precaution must be taken when testing the insulation of generator cables and wiring connected to an AVR unit?
Electronic components such as transistors, integrated circuit chips, thyristors, etc. are likely to be damaged during a high voltage megger test.
To test the generator and its cables to earth and protect the electronic parts, either:
- Short-circuit all outgoing cable terminals during the IR test
- Remove electronic card(s)
- Disconnect all cables at both ends and test separately