NERC Lessons learned
NERC Lessons learned
(OP)
"The ability of RTUs, transducers, and related communications systems to withstand ground potential rise often present during a fault can be compromised when dc supply circuits to these systems are not adequately isolated from ground. Electrical isolation of these components from ground must be preserved to maintain system operator visibility and control of transmission system elements during disturbances. Loads connected to the station battery should be tested regularly for grounds. If grounds are detected on station battery circuits, immediate action should be taken to locate and eliminate them."
http://ww w.nerc.com /files/LL- Loss-Stati on-Observa bility.pdf
Does this make sense to anyone? I'm aware of the problems that transfered potentials can cause if single point grounding is not used. I'm at a loss, though, as to how grounds on the DC supply would interact with a case ground during a ground fault and resulting AC GPR. Does NERC have this right? Our com people run a positive ground on their systems, isolated from our protection/control ungrounded systems.
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Does this make sense to anyone? I'm aware of the problems that transfered potentials can cause if single point grounding is not used. I'm at a loss, though, as to how grounds on the DC supply would interact with a case ground during a ground fault and resulting AC GPR. Does NERC have this right? Our com people run a positive ground on their systems, isolated from our protection/control ungrounded systems.






RE: NERC Lessons learned
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If we learn from our mistakes I'm getting a great education!
RE: NERC Lessons learned
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RE: NERC Lessons learned
RE: NERC Lessons learned
That being said regularly checking for grounds seems quite reasonable to ensure the first ground gets fixed before a second on occurs.
RE: NERC Lessons learned
I'm not sure how NERC is determining their priorities these days - some of this seems pretty random at times.
But operating with a positive ground for the comms circuit might be something you may want to re-visit. This will never be truly isolated from the dc power or anything else referenced to ground in the sub, no matter what you do.
Even the floating dc systems find ways to ground themselves.
David Castor
www.cvoes.com
RE: NERC Lessons learned
Where our commies want a positive ground, they install a DC to DC converter to provide isolation. Seems to keep everyone happy, at least until NERC chimed in.
RE: NERC Lessons learned
RE: NERC Lessons learned
RE: NERC Lessons learned
And if ground detection is available full time, it means the DC circuit is truly floating, but connected to ground through a resistance.
So a ground fault occurs and the station mat rises in potential in relation to the potential of remote earth. The case ground also rises, along with any grounds on the DC. Single point grounding is observed, and the communication links are isolated from remote potentials. Where does the the voltage stress occur? Our comm box is a bird on a wire, with no knowledge that anything is abnormal.
RE: NERC Lessons learned
The GPR will lift the potential of everything within the substation together. I don't think you will find many DC systems designed to withstand the GPR across their internal insulation, which it would have to do in order to protect remote equipment connected by copper cable.
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If we learn from our mistakes I'm getting a great education!
RE: NERC Lessons learned
RE: NERC Lessons learned
You hope! I remain deeply suspicious of metallic connections coming in from remote locations and connecting to sensitive equipment. Pilot wire relays are perhaps the exception because they are designed to handle the GPR.
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If we learn from our mistakes I'm getting a great education!
RE: NERC Lessons learned
RE: NERC Lessons learned
One other possible failure mode is that multiple solid grounds could allow excessive current to flow just as grounding both ends of a cable shield may cause too much current to flow unless a large grounding conductor is in parallel with the cable.
One of the references he recommended is "Noise Reduction Techniques in Electronic Systems" by Henry Ott, Second Edition 1988, but I haven't purchased a copy yet.
RE: NERC Lessons learned
I've traded emails with Mr. Shockley and bacon4life has it right. Seems that an RTU was damaged when an undetected inadvertent ground caused a transferred (not a word they used) potential. I'm assuming the ground occurred at a remote location from the RTU, and the close in fault caused an unequal GPR across the station. Also spoke of common mode noise, although I'm not sure why noise is an issue once the RTU is toast. (Even if the RTU was rugged enough to survive, the noise would only last until the fault cleared. And what the heck is a "common mode differential" voltage anyway? Sounds like an oxymoron to me.) In any case they are considering rewriting the document to add clarity.