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NERC Lessons learned

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://www.nerc.com/files/LL-Loss-Station-Observability.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.

 

RE: NERC Lessons learned

If the link coming in is on copper cable then I can see how a piece of equipment with a remote ground and a local ground at the substation could end up in trouble if the substation potential rose due to GPR. Unintentional grounding connections might force high voltages to occur across components which weren't designed to withstand them. I would argue that these days equipment should be able to withstand the GPR - the problem is hardly a new one, and manufacturers have been designing for substation environments for years - or it should communicate over fibre and make the problem go away.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: NERC Lessons learned

The paper is about how Ground Potential Rise affects communications equipment in a substation.  This should only be a problem if the equipment is connected to the outside world via metallic wire and not fiber.  I don't believe that removing the ground from the dc system would help very much, since the equipment will still be grounded via the case and a GPR of many thousand volts will still cause the equipment to fail.  There are many solutions available for this type of issue. RLH/Fiber Optic Link has some good information on their site. I have also used SNC products in the past.
http://www.fiberopticlink.com/Industry_Solutions/High_Voltage.html
http://www.sncmfg.com/telecom/high_voltage_protection.html

RE: NERC Lessons learned

(OP)
Thanks. I've decided it's best to push back at every available opportunity. NERC is pushing reliability, but provides unreliable information. Worse yet, FERC may see it and direct NERC to draft a standard on the subject. If you care to join in, the email addresses are in the linked document.

RE: NERC Lessons learned

Control or visibility could be compromised if coil and  transducer pickup & dropout voltages are not selected carefully with regard to 1/2 the battery voltage and there is enough cable capacitance to energize them. But why would they advocate hunting for grounds on an intermittent basis instead wanting proper design from the beginning?

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

stevenal,

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

(OP)
David,

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

The NERC employee I talked to explained several instances at his previous utility where SCADA equipment had been damaged during a system ground fault while there was also a battery ground present.  His explanation of common mode voltage coupling onto the battery system was a bit hard to follow over the phone, but I am looking forward to his recommended reading.

RE: NERC Lessons learned

I think NERC is referring to floating DC systems that feed comm equipment.  It isn't uncommon to have one rail of the DC inadvertantly get grounded out.  Usually this would mean it is in contact with the main ground grid.  Therefore it makes sense that a GPR on the AC system could have adverse effects on the comm equipment connected to the DC system.  The NERC document makes sense to me.  I would be very hesitant to work at a station that didn't have a ground detector relay on the DC system though.   

RE: NERC Lessons learned

(OP)
So how is a floating DC system with an inadvertent ground different from one that is intentionally grounded? Don't see how equipment failure can depend on intent.

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

marks,

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

(OP)
Missed a word above. I meant to say "not truly floating."

RE: NERC Lessons learned

"...and the communication links are isolated from remote potentials"

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

(OP)
I don't need to hope; we use fiber for our SCADA communication. But even if you use copper, fiber isolation units can and should be used.  

RE: NERC Lessons learned

When I called NERC, I specifically clarified that this lessons learned is in reference to floating substation batteries inside substations, not to grounded communication batteries and not to external communication circuits.  

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

(OP)
Update:
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.

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