What is a generator PMG system?
An AC alternator with PMG has a separately-excited AVR which is independently powered from a separate permanent magnet generator (PMG). Made of two parts, the PMG rotor and the PMG stator. The PMG rotor is mounted on the main alternator rotor shaft and voltage is induced in the PMG stator as the PMG rotor moves the permanent magnets. The power from the PMG stator is then used to power the AVR.
Why use a PMG system rather than a standard self excited generator?
In a standard AC generator, the AVR is fed from the generator terminals. In systems which have high inductive loads that cause a large drop in the stator voltage before the AVR can compensate, the AVR suffers, because its input voltage is reduced. This can lead to situations where the AVR is not able to bring the voltage back to its preset value, because the terminal voltage and hence its power source has dropped too far.
The advantage from the PMG is therefore that the power to the AVR is kept near constant, allowing the AVR to quickly recover the voltage of the AC generator back to its nominal preset value. The AVR reacts quicker reducing the TVD associated with applying a load to an AC alternator.