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Positive Ground on 125 VDC Station Batteries #220088
05/22/19 07:05 PM
05/22/19 07:05 PM
The Watt Doctor  Offline OP
Member
Joined: Dec 2001
Posts: 437
Mont Belvieu, TX
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.


The Watt Doctor
Altura Cogen
Channelview, TX
Work Gear for Electricians and the Trades
Re: Positive Ground on 125 VDC Station Batteries [Re: The Watt Doctor] #220089
05/22/19 09:53 PM
05/22/19 09:53 PM
G
gfretwell  Online Content

Member
Joined: Jul 2004
Posts: 9,368
Estero,Fl,usa
There are Hall effect devices that can detect current in DC circuits if that helps


Greg Fretwell
Re: Positive Ground on 125 VDC Station Batteries [Re: gfretwell] #220090
05/23/19 08:55 AM
05/23/19 08:55 AM
The Watt Doctor  Offline OP
Member
Joined: Dec 2001
Posts: 437
Mont Belvieu, TX
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.


The Watt Doctor
Altura Cogen
Channelview, TX
Re: Positive Ground on 125 VDC Station Batteries [Re: The Watt Doctor] #220091
05/27/19 07:57 PM
05/27/19 07:57 PM
J
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 806
Chicago, Il.
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

Re: Positive Ground on 125 VDC Station Batteries [Re: The Watt Doctor] #220092
05/27/19 08:08 PM
05/27/19 08:08 PM
J
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 806
Chicago, Il.
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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