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Positive Ground on 125 VDC Station Batteries

Posted By: The Watt Doctor

Positive Ground on 125 VDC Station Batteries - 05/22/19 11:05 PM

It's been a long time since I've logged on to this forum. I spent a lot time here back when I was working in the Electrical Contracting world. I'm currently working in a 600 MW Cogen Facility. We have a recurring problem with our 125 VDC station battery system. A ground on the positive bus will mysteriously appear, stick around for a while, and then vanish. It's a nuance more than anything. The trouble is that affects the secondary trip circuits for our Oil Filled Circuit Breakers, Gas Breakers, Breaker Failure Relays, Sudden Pressure Relays on our GSU's (Generator Step-Up Transformers), 86GT Lockout Relays, etc, etc, etc. So, systematically turning off breakers until I find the affected circuit is NOT an option. What I'm looking for may not exist. I'm looking for reading material on how to work on DC systems in industrial plants in hopes that I can develop a strategy to resolve this problem. Any help is appreciated.
Posted By: gfretwell

Re: Positive Ground on 125 VDC Station Batteries - 05/23/19 01:53 AM

There are Hall effect devices that can detect current in DC circuits if that helps
Posted By: The Watt Doctor

Re: Positive Ground on 125 VDC Station Batteries - 05/23/19 12:55 PM

Thanks, gfretwell. We have Hall Effect devices on our large DC Motor Drives. However, that's not what I'm looking for in the particular case.
Posted By: JoeTestingEngr

Re: Positive Ground on 125 VDC Station Batteries - 05/27/19 11:57 PM

I have quite a bit of experience tracking down ground faults in traction power substations. I find troubleshooting them as some of my more enjoyable work. Our 125VDC is really 133.8VDC Float and I imagine yours is about the same. And our ground faults aren't usually really ground faults, but skin faults. That means they're elevated above ground by the voltage imposed by the 64 (164) Hot/Grounded Structure relays. So I will often see about 25VDC between battery + and Ground. Of course, I always isolate the charger from the batteries first and the problem almost always stays with the batteries. Then, I can usually lift the power from the 64 relays, one at a time, until I see the battery + to ground voltage drop. The problem is almost always in the DC SWGR Lineup. From there, it's a matter of asking our Power Controllers for permission to open individual power sections. Our breakers have under voltage relays so pulling control power will cause a trip. We just open the control power breakers or knife switches one at a time until the fault goes away. Then we know which cubicle to check. Then we rack the breaker out or to test to check further.

In our recent experience our DC Breaker charging motors have been developing leakage between the + connection and their cases.
Joe
Posted By: JoeTestingEngr

Re: Positive Ground on 125 VDC Station Batteries - 05/28/19 12:08 AM

OK, I just posted the above and it's 7:06 CDT on Memorial Day, 5/27. Why is it showing that I replied "Yesterday at 11:57 PM"???
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