Our Gas turbine generator got tripped on rotor earth fault while start up after On exciter breaker (Voltage up to 9 KV). Then we have stopped machine and tested rotor to earth and IR value is zero ohm on 100 V megger.

Next we disconnected Generator rotor connection from rotating rectifier wheel and found 3 Mohm and main exciter rotor (diode connections removed at rectifier wheel) value found 1 MOHM. But outlet of rectifier wheel terminal to earth IR valve is only 0.01 MOHM on 100 V megger.
After that we have cleaned all parts of rectifier wheel and IR value doesn't improve.

Pls suggest what we can do for further IR value improvement.

 

Vijayakumar... your measurement indicates the fault is on the exciter side of the rotating diode disk. The value measured, 10 kOhm, is not particularly dangerous even it was located in the main field. The danger comes if a second fault occurs.

However, I would suggest that you repeat the test with an Ohm-meter having a lower Ohm-range. In addition take a second set of readings with the Ohm-meter leads reversed.

It would also help if you could provide additional information regarding the rotor earth-fault protection method used; specifically the manufacturer and model!

Regards, Phil Corso

 

I would suggest try meggaring with higher voltage say up to 250 to 500,u may have to isolate the diodes though.

Another suspect could be the plates on which the diodes are mounted,this have to be insulated from the shaft,sometimes just a insulated washer on the fastening bolts are used to retain the ring plates.U may have to do a insulation test with the plates(on which the diodes are mounted)and the shaft of the exciter.