Hello
I think IGBTs blows, because of overvoltage tips, which capacitors C52, C53 and C54 should absorb. The consequence is that the collector voltage exceeds the gate and closes over some of the diodes D36, D37 and D38, and then through a common diode D328 closes the circuit to the emitter of IGBT's. Attached is a scheme of excitation 3 IGBTs whose emitters are connected to the negative pole of the DC link. I marked with yellow marker above circuit. It is necessary to replace the capacitors C52 - C54, then one of the IGBT diode D36 - D38 (BYG 22D) and always diode D328. For diode D328 can not find catalog information. Her label is F AJ14 LT, I found on another regulator F AK23 LT. Please if anyone has another label diode D328 to let me know.

Best regard

if attachment doesn't work, here is link to image of the schematic: