MEP-805B Overvoltage

I have a MEP-805B 30Kw genset that was working great, came back from site and as soon as your fire it up the voltage spikes to 480+ volts and shuts down the unit. When it shuts down and you reset, you hold the AC interrupter switch to open, the genset stays running, as soon as you close the AC interrupter or hit the start switch to excite the generator then voltage climbs past 480vlts and shuts down the unit. Any assistance or direction would be great. Regards