Generator field ground relay tripping

[TWD82]

A coworker in our generation engineering department recently experienced this problem so I thought I’d check on here for some ideas for him, I’ll try to be brief, let me know if you need more information.

We had an 11.5kv, 9MW hydro unit out for 2 weeks of scheduled maintenance. During this yearly maintenance the plant operators also swap the leads to the field winding (why is this done?). They brought the unit up to speed and when the field was flashed the field ground relay (a GE PJG) tripped. When they were troubleshooting, they pulled the trip on this relay so that the field would flash, then put the trip back in and the unit got up to 4 MW before the relay tripped again. Throughout several tries the unit kept tripping at the transfer to exciter power and again at 4 MW of load.

The next day they were not even able to get load on the unit, the trip on the relay during the field flash was not intermittent like the day before, so they didn’t want to leave the test switch opened to try to bring it up to load. During troubleshooting they also put the leads to the field back to the way they were before the PM and the unit would not trip, it switched to exciter power and was brought up to load just fine.

Has anybody came across this problem, tripping the field ground relay only when the polarity is switched? Right now the unit is running just fine with the field polarity back to its pre-maintenance state.

 

[GTstartup]

The leads were swapped because the slip rings wear unevenly.

Swapping them evens it out over time. The field ground detector is normally connected to the negative slip ring. When swapping the polarity, the ground brush should also be swapped. I have not experienced misoperation if you don;t do this, but this is the first thing I would try.

 

[leon63]

Hello all,

I was one of the engineers who was troubleshooting the problem last week. I received more background information from one of the technicians last Friday.

Just before all the problems started, two nearby 50 mw generators lost their AGC setpoints. These setpoints were accidentally set to zero, and the generators started to "motor". As a result a large voltage sag occurred, the problem generator attempted to maintain the bus voltage. In the process, the exciter delivered full power to the field for a time, and then the generator tripped offline. After a few attempts to restart the unit, the field ground fault relay (type 64) would trip as soon as the field flash would occur.

After we reversed the field leads at the generator, the ground fault would not occur and the unit ran fine.

We meggered the field at 500 v and found no problems. We meggered the brush holders, slip rings, and field power leads at 1kv and found no problems.

Could the field core have become overly magnetized when this unit ran at max power, and then cause problems?

Also, if there is an insulation problem at either end of the field windings, swapping the leads would subject parts of the winding to different potentials. Could this be an issue?

I would appreciate any help!

Thanks,
Dan