Hello,

I'm looking for some suggestions on resolving an ground alarm that came in this week. We have a GE 7FA and I received a 125v dc battery ground alarm. This came in at the same time as an error alarm on our hydrogen purity monitor. I was able to resolve that alarm but the ground alarm is still not clearing. We had lightning in the area that night but I am not positive it was caused by a strike. I know in the past we had the same issue after a strike but those areas I have already checked and no dice. Any ideas??

 

MKV 125VDC grounds can be methodically located by isolating sections of the 125VDC distribution system one at a time, with the unit on turning gear or at standstill, while monitoring the "Tools" Diagnostic Counters <R>, <S> or <T>, TCQA power supply data, P125 & N125 values. These should be "balanced" at ~ +67VDC and -67VDC respectively when no grounds are active. The process of isolating sections starts in the 125 VDC MCC panelboard & DSW panel. This is done by turning off circuit breakers one at a time unitl the positive and negative balance.

The breakers provide control power different systems such as the switch yard controls, 480 & 4160 switchgear, fire protection relays, 52G (generator breaker), main, aux & isolation transformers and of course the MKV and EX2K.The MKV and EX2K generally share a breaker. Besides the MKV/EX2K ckt bkr. all the others power circuits that are "Energize to trip", so turning power off will not cause a trip, but this must be verified BEFORE cycling power to each system as your site could be different than the ones I have been on. The MKV/EX2K breaker does NOT need to be turned off for obvious reasons.

If the 125VDC system balances when a breaker is turned off then that provides a direction for continued circuit isolation. If all breakers except the MKV/EX2K bkr are cycled and the 125VDC balance still does not return than the MKV 125VDC IO can start to be isolated (it is very unlikely that the EX2K would have a ground as there is very little digital IO and none of that is exposed to the weather).

The MKV digital IO cards DTBA - DTBD have a 125VDC connector that can be removed one at time until the ground clears. DO your homework when doing this as you could easily take a unit of turning gear, purge the generator etc. When the IO card is with the ground on it is located the BJ jumpers can be removed one at time (this is only good for POSTIVE grounds) or each individual lead can be lifted until the circuit is identified.

This method has worked many many times. There was one time when all the leads on culprit IO card were lifted and the ground still did not clear, even though it cleared when the card power was removed!! It turned out to be a solder ball stuck behind the card.

Good Luck.

Greg Ponto
Lead Combustion Turbine Specialist
Elwood Operations
(815) 423-9883 x2232
Email: greg_ponto @ dom. com