What voltage is your link?
What current is the inverter rated for
What voltage is the IGBT rated for
What current is the inverter rated for
What kind of packaging is the IGBT's - 6pack? (3phase), 4pack (single phase), induvidual
What IGBT's are failing? Is it a phase leg (ie potential shoot-though), or what
How are they failing? use a DVM to check for Collector-Emitter impedance (short or high impdance), check the anti-parallel diodes, check the gate capacitance and gate impedance.
If you can crack open the module and examine the die (by eye). Overcurrent should be big craters, over-voltage should be stress/fracters at the corners along the edge of the die =since this is a 50kW inverter shold be easy to see with teh naked eye
What is the topology used? what cind of control (voltage, current?)
How long is it running before failure (after a few minuits ~ control problem, after a few days/weeks ~ control/thermal problem)
hope this is of help