AC Generator Rotating Rectifier Diodes

hi all

would you please help me on this if you have any idea!

its 180kva ac brushless three phase generator 380v 50z, the cover was not fitted after we fitted new regulator and the operator run it and it was raining, so the main rotor winding got grounded and the rotating rectifier diodes were overheated and shorted.

when i checked the main rotor winding resistance it was 1.6 ohms, i checked the insulation to ground and it was 0.3 M ohms. so i managed to fix this by keeping space heater for one night inside the generator, in the next morning i found the insulation resistance is 105 M ohms, so i changed the diodes with new ones and run the generator. its working fine but only for 30 minutes and then the diodes overheated and shorted again.

i changed the diodes again and the same problem was happened,

what i am wondering is that the insulating resistance of the main rotor coil is high and the coil resistance is 1.6 ohms, so what caused the diodes to overheat!

is it because the rotor resistance is not right and because of the first problem the winding got shorted and i could not see this? maybe 1.6 ohms is not the right resistance of the rotor coil?

or it is something else which i could not see?

your help is highly appreciated

thanks