What is a shunt or self excited excitation system on an AC alternator?
An shunt or self excited excitation system on an AC alternator works by taking the power for the AVR directly from the main stator winding. This method of excitation is typically standard on many AC alternators and while functional, the alternatives such as an auxiliary winding or PMG system provide superior performance at relatively limited cost, compared to the cost of the whole unit.
In a standard shunt or self-excited AC alternator, because the power for the AVR is taken from the main generator terminals, when loads are applied to the AC Alternator, the terminal voltage dips and the power source of the AVR is reduced. This leads to slower voltage response, larger transient voltage dips and the possibility that the voltage will not recover to its preset level.
A Diagram from Stamford showing shunt / self excited alternator.
Why is it useful? How does it compare to other methods?
When cost is the main factor, a shunt / self excited alternator is probably the best option, however when performance is an issue, because increased voltage stability and lower transient voltage dips are required or large disturbing loads are being applied then an auxiliary winding or PMG system should be selected.