Hi Dear all
Our Plant has separate generation and our main loads are four synchronous motors 73000 HP, when we have suffered electrical disturbances on our HV bus always one or two thyristors of the AVR's thyristors bridge has resulted burned.
For along time we though that the cause of burned thyristors was due overvoltage on electrical system.
At present time we bealive that the cause is associated to a very high current, when voltage drops on motor terminals the AVR's response to recover voltage on motor terminal is too agressive, producing damage on thyristors.
Motors are brushless type, and the AVR power supply is taken from the auxiliary services bus.
I've been reading literature about thristors and most comments are that thyristor do not burned by current they resulted damage by voltage.
Does someone has experimented similar case or has an idea about this failure?

Attached two pictures and drawing

Regards

 

You want strangers on a hobby electronics forum to blindly diagnose a high power industrial fault for you on control electronics for a motor sized 54 MEGAwatts?

Your on-site electricians should have fault finding procedures in a manual for equipment that big and expensive. Likewise the manufacturer of the motors or thyristor gear should be very helpful to you if you contact them.