Failure of IGBT in VFD

We have One VFD of Rockwell Powerflex-700 installed for controlling 32 motors. VFD is installed in a remote control room which is around 150 mtrs. away from motors. These VFD's are in regenerating mode ie they back feed to the main DC Bus. All these 32 motors are operated through an isolating contractor near the motor ie while the VFD is running, contractor is switched off to work on any one position out of 32 positions.

We have observed many failures of IGBT in these VFD's. Pl. reply with probable reasons of failures.

Srinivas Rao