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 over voltage on electrical system.

At present time we believe 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 aggressive, 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 thyristors 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?

Regards

 

Without a lot more information, I would look at false triggering as
well. Fired in the proper sequence, the circuit constants should limit
the current. Conduction at the wrong times can, in some circuits, put conducting devices across the line with the current limited only by what is available. But one would really have to know the particulars to do more than guess. dV/dT issues and temperature can cause an SCR to fire as well. I worked on a couple 9Mw induction iron melters and improper triggering produced the most spectacular results. Really made you sweat when you went to close the breaker again after you believe you've fixed it.

Regards
cww