Medium Voltage Wye-Delta Grounding

I have a deisel power plant about 40 meg. One bank of gens is 11Kv wye with an earthing resistor connected to the neutral of each generator. another bank of gens is 416v steppped up thru trannies to 11Kv delta, ungrounded. these are all connected to the same bus.
We have some small earth leakage out in the distribution and also intermittant ground fault from birds, rain and whatever. My concern is that the ground fault that is returning to the wye gens is not a problem in itself, but since some of the current originates from the Delta source, and is trying to get home via the other two phases, it is doing so by passing thru the alternators at the wye source. This in itself might not be a problem except that it is known that groundfaults on an ungrounded system (consider the Delta source gens) have a tendency to build up capacitance which will release at 7 or 8 times the normal voltage. We are experiencing frequent fires in the termination boxes of the WYE source alterators. I think that it is caused by the capacitance discharging and then a path has been created for the rest of the burnout. Has anyone else tied wye and delta together like this or am I a guinea pig? does my theory hold water?