64 S 1

[Guardiano]

Hi,

I just hope that someone could pop in and shed some light. My generator 60 MW 11 k V tripped on earth fault 64 S last night. At the time of fault, the load on the machine was 58 MW.The generator tripped a second time when the machine was brought to excitation and it tripped again on 64 S when the stator voltage reached 5 k V. The differential protection did not see anything. The stator was meggered this morning but the insulations tests were normal with polarisation index > 4. The generator is grounded through a 635 ohm earthing resistor. What should we test next, the integrity test on all the protection relays were sucessful.
Any help.

Guardiano

 

[rasevskii]

Guardiano:

Is the generator connected to a delta connected unit transformer with or without a CB in between? If you are getting a 64S trip as soon as you take up voltage without the unit CB being closed (the normal situation), then look for a bad VT (voltage transformer) or other device conn to the HV end of the stator (for example the VTs that supply the voltage sensing circuits of the AVR and relays).
You would have had to disconnect or otherwise isolate these VTs to be able to megger the stator. Therefore the stator would megger OK, but the fault may be external to it in the switchgear.

Also a bad surge suppressor or arrestor could be the fault.

For testing a VT, you can back energize it from the 110V side (using a relay test set) and measure the mag current. Use safety precautions as the HV side will become energized.

rasevskii

 

[Guardiano]

Thanks Rasevskii. We are also suspecting the VT, we measured the primary resistances of the VT on the three phases and the reading on one phase differed from the other two phases. We are taking step to measure the magnetising currents.

Guardiano