High Voltage Failure

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Lars

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Arctic
In my job as an Electrical Technician I have come upon a serious problem; which I hope this forum might be able to help me with.

A heat tracing hot wire had a simple ground failure, which cut out a 20A breaker along with a 100A main breaker in a 480V installation, probably because of a very high short circuit currant (more than 3500A).

Under re-closing both the 20A and 100A breakers a short-circuit happened again, and this time not only are the two breakers cut out, but the whole high-voltage circuit (4,160V) is cut out on a HV earth failure.

This makes no sense to me - how can a high-voltage circuit see a low-voltage ground failure?

It is an ordinary d-y transformer and the high-voltage circuit is been checked out.
The relay is a Westinghouse type and calibrated every 2 years.

What can the reason be for the above failure?

The location is under Artic conditions and the grounding/bounding system, on witch the 480V neutral point is mounted, is connected to a copper plate placed in seawater.

I will be patiently waiting in anticipation of hearing from you.


Best regards,
 
Re: High Voltage Failure

I had a similar thing happen- a technician was taking measurements on the back of a circuit monitor on a 277/480V switchboard. The voltage was even going through a PT before hitting the circuit monitor. When the flash occurred, it burned the technicians hands, melted the sensing wiring inside the switchboard and opened a 13.2KV vacuum breaker over 300' away. If I hadn't seen it with my own eyes, I would not believe it. What has me scatching my head is the Arctic location where you do not have a large utility supplying a large fault current- you mentiond >3500A.

The engineer mentioned that there must be a coordination issue.
I think you have a coordination issue.
 
Re: High Voltage Failure

Under re-closing both the 20A and 100A breakers a short-circuit happened again, and this time not only are the two breakers cut out, but the whole high-voltage circuit (4,160V) is cut out on a HV earth failure.
Click to expand...
Please note that OSHA rules require that the cause of the trip be investigated and corrected before a tripped breaker is reclosed.
The low voltage fault current is seen by the transformer windings and its primary OCPD device. If the current is high enough on the secondary the primary device will act to clear the fault. Under normal conditions, proper OCPD coordination will have the secondary devices trip before the primary. Also note that on the secondary, you cannot selectively coordinate most small(less than 400A) breakers for high fault conditions
Don
 
Re: High Voltage Failure

I think I'm going to wonder about this one for a while. When you say "earth failure", I assume you mean ground fault. A ground fault on the LV side should not cause a ground fault on the HV side. Maybe and overcurrent trip, but not a ground fault.

Is is possible that there are multiple N-G bonds on the HV side? That might allow some LV short circuit current to flow through the HV ground fault sensor.

What is the time delay on the 4160 ground fault sensor? If it is very short (a few milliseconds), it might have tripped from a transient caused by the short?

Steve
 
Re: High Voltage Failure

Steve,
I think I'm going to wonder about this one for a while. When you say "earth failure", I assume you mean ground fault. A ground fault on the LV side should not cause a ground fault on the HV side. Maybe and overcurrent trip, but not a ground fault.
Click to expand...
Good point, I read right by where it said the HV trip was an earth fault.
Don
 
Re: High Voltage Failure

Well first off, what don said about resetting a breakers into a fault is an OSHA violation 1910.342.

I agree with your eng that you have a coordination problem on the LV side.

The 4160V breaker should not have tripped on GF (51N I assume), what do you mean that the HV side has been "checked out"?
 
Re: High Voltage Failure

Some 4160V. gear may have some very sensitive
ground fault relaying.

Maybe even with instantaneous tripping such as 50GS, 50/51GS 50/51N.
Unless you have done a very thorough coordination study & implemented the settings, things like this can happen sometimes.
 
Re: High Voltage Failure

kiloamp7,
Some 4160V. gear may have some very sensitive ground fault relaying.
Click to expand...
No matter how sensitive the ground fault relay is on the primary, it should never see a ground fault on the secondary. They are electrically separate systems.
Don

I should clarify my statement a bit. The primary protective relays should not trip on a ground fault as the result of a secondary ground fault. If the current on the secondary ground fault is high enough and if that fault is not cleared by the secondary OCPD, the reflected current on the primary could cause the primary relays to open based on high current.

[ January 13, 2006, 06:31 AM: Message edited by: don_resqcapt19 ]
 
Re: High Voltage Failure

I thank all of you for your response to my little problem.

Yes, of course I meant a ground failure. In the country I normally work, other terms are used which can be confusing, that?s why the misunderstanding.

This is an American installation on foreign ground, manned with foreign nationals, that is the reason why a substitute to OSHA exists on this site - SB6 = Danish NEC.

SB6 622.1 allowed one re-closing after a cut-off, but in this situation the consequences were a little unfortunate.

The power plant runs by 3 ea. diesel machines, totally deliver 9 MW.

The above and the fact that the original failure was near the 4.16/0.48 transformer, probably caused the relative high level of short-circuit currant.

After the HV failure, the HV line was checked by a suitable devise, - 7.5 KV in 15 min. each phase < 3 micro A - before re-closing.

Steve66:
What exact does you mean when you mentioned the multiple N-G bound?
The artic conditions cause some specially grounding problems - permafrost and rocky ground, I?m not sure, but I think that the HV grounding, LV Neutral and the LV grounding all are connected on the main-ground wire, can that be a part of the problem?

Again, I thank all of you for your answers, I?m trying to find out of exact types and setting of the different relays, than I?ll return with the data.
 
Re: High Voltage Failure

Lars - did the 4.16 kV fault come from a relay fed by a zero-sequence (ground fault) CT that encircles all of the shielded 5 kV conductors? If so, maybe the cable shields are not properly terminated around the CT.

If this is the case, some fault current may have flowed in the cable shields during the fault. Usually the shields are grounded at both ends of the cables and present a low impedance path from a fault location back towards the electrical room where all system grounds, 5 kV, 480V, and low voltage, are tied together.

In this scenario, there was no fault on the 4.16 kV system, low voltage fault current flowing in the grounded shield wires fooled the 4.16 kV relay into tripping.

To solve the problem, make sure any current flowing in a cable shield can not seen by the doughnut CT. If the cable shield is terminated above the CT, run the shield grounding wire back down through the CT to cancel the current.

Good Luck.
 
Re: High Voltage Failure

Lars:


I was just thinking of any way the low voltage ground fault current could flow through the high voltage ground fault sensor. If you have the same neutral and ground wire connected (bonded) at more than one point, that creates a lot of funny paths for fault current to flow on. It can also be a safety issue when some of that fault current flows through things it shouldn't.

Bob Wilson's above post mentiones another possible way this could happen.

At any rate, if the problem is LV ground fault current flowing through the HV ground fault sensor, fixing the ground fault should eliminate the problem. Normally, one tries to coordinate the settings and delays of the main ground fault sensor so the branch circuit trips before taking out the main service.

Steve
 
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