Alternator High Voltage on all phases, 400V, 3 Phase Alternator, Off Load, How to fix it?
High alternator voltage on all three phases, Off Load
In generators that have separate circuits to power the AVR (fitted with auxiliary windings or PMG systems) the AVR can be powered, but it could lose the sensing circuit, either because of a break in the sensing cable, or an AVR fault. This loss of sensing will make it appear to the AVR that the voltage is low and it will therefore boost the excitation system to try to increase the voltage, causing the voltage on all three phases to rise.
To fix this, check the resistance of the sensing circuit according to the alternator manual (or at least check it isn't an open circuit) and swap the AVR to see if it fixes the problem.