4160 volt portable xfmer and generator grounding

Status
Not open for further replies.
S

stardust

Member
Our main (4160) volt feeders were shut down to reroute them to our facility, when work was complete (splicing new in the middle of old wire approx 11 years old) feeder failed megger test. Contractor brought in a cat 480v 3000a generator and a transformer to step up to 4160v. Connected it to our main at switchgear room. they ran a ground wire (I believe from Generator, worked 27 hours it is a little fuzzy) to our switchgear grounding bar which is bonded to our outside grounding array. When fired up we had strange things happening in one building. t-8's exploding, smoked electronics, etc. told them to shut it down and get generator checked out. left at 9:00am after 27 hrs. called back in by supervisor 5 hrs later, said generator tech had been there and everything checked out fine. wanted us to turn things on. I noticed on way to switchgear room that their grounding lead had been taken off our bar and connected to a ground rod driven in the ground. As we tried to energize our buildings three different 4160v breakers tripped. Left them off and tested them found bad lightning arrestors on two of them and cables meggering bad to the building that blew bulbs etc.
Was this generator improperly grounded at first?
If so could that have caused our problems at the three buildings?
If it didn't cause our problems, why would these show up all of a sudden when we energized?
forgot to mention this is an ungrounded delta feed.
Thanks for any help
 
Last edited:
stardust said:
Our main (4160) volt feeders were shut down to reroute them to our facility, when work was complete (splicing new in the middle of old wire approx 11 years old) feeder failed megger test. Contractor brought in a cat 480v 3000a generator and a transformer to step up to 4160v. Connected it to our main at switchgear room. they ran a ground wire (I believe from Generator, worked 27 hours it is a little fuzzy) to our switchgear grounding bar which is bonded to our outside grounding array. When fired up we had strange things happening in one building. t-8's exploding, smoked electronics, etc. told them to shut it down and get generator checked out. left at 9:00am after 27 hrs. called back in by supervisor 5 hrs later, said generator tech had been there and everything checked out fine. wanted us to turn things on. I noticed on way to switchgear room that their grounding lead had been taken off our bar and connected to a ground rod driven in the ground. As we tried to energize our buildings three different 4160v breakers tripped. Left them off and tested them found bad lightning arrestors on two of them and cables meggering bad to the building that blew bulbs etc.
Was this generator improperly grounded at first?
If so could that have caused our problems at the three buildings?
If it didn't cause our problems, why would these show up all of a sudden when we energized?
forgot to mention this is an ungrounded delta feed.
Thanks for any help
Click to expand...

One thing that might have caused all sorts of problems was that they made a ground connection on the 4160 side of their transformer. Just operating their generator itself in a grounded or ungrounded configuration, with or without a connection to your ground array, should not have had any effect on the HV side of the transformer.

scattershot ideas and WAGs:
1. Are you operating your 4160 gear as an ungrounded delta with ground detection?
2. Can you confirm that the ground detectors were working before the feeders were first shut down, and that no fault to ground was seen?
3. Was all of the equipment that failed running from 120-208-240 supplied by your own transformer stepping down from 4160?
4. Is it possible that in the course of the work, the neutral connection from the transformer(s) was disturbed. This could have caused 240 to appear on 120 volt branch circuits, among other things.
5. If your internal neutral wires were connected to your ground array (since no ground/neutral from POCO) and your low voltage neutral was also connected to grounding/bonding wires rather than directly to the transformer secondary, I think it may be possible that your entire ground system was driven away from ground by a fault or error, and the resistance of your ground electrode system was high enough that the ground/neutral voltage went to a high voltage without OCPDs being tripped.
6. Your observation of cables meggering bad to the building with the equipment blowing does seem to fit in with there being a fault to the raceway which could have changed the ground potential throughout the entire building. Current through a high resistance fault in the cables might not have either dropped the 4120 significantly or caused overcurrent devices to blow. But the second time you energized the cables, the fault went to lower resistance and tripped the breakers.
7. Did you megger the existing old wiring before re-energizing? There may have been a problem with moisture that built up during the time (27 hours?) that they were not energized which caused problems when suddenly hit with high voltage?
 
1. Are you operating your 4160 gear as an ungrounded delta with ground detection

Ungrounded, no ground detection that I know of.

2. Can you confirm that the ground detectors were working before the feeders were first shut down, and that no fault to ground was seen?
No

3. Was all of the equipment that failed running from 120-208-240 supplied by your own transformer stepping down from 4160?
Yes
4. Is it possible that in the course of the work, the neutral connection from the transformer(s) was disturbed. This could have caused 240 to appear on 120 volt branch circuits, among other things.
Nothing disturbed at xfmer's, one feed was meggered and upon startup shutdown with fault found to be a bad LA

5. If your internal neutral wires were connected to your ground array (since no ground/neutral from POCO) and your low voltage neutral was also connected to grounding/bonding wires rather than directly to the transformer secondary, I think it may be possible that your entire ground system was driven away from ground by a fault or error, and the resistance of your ground electrode system was high enough that the ground/neutral voltage went to a high voltage without OCPDs being tripped.

this ground array only attached to casing of 4160v switchgear and breakers, not to neutral, shielding on 4160 v conductors attached to this.

6. Your observation of cables meggering bad to the building with the equipment blowing does seem to fit in with there being a fault to the raceway which could have changed the ground potential throughout the entire building. Current through a high resistance fault in the cables might not have either dropped the 4120 significantly or caused over current devices to blow. But the second time you energized the cables, the fault went to lower resistance and tripped the breakers.

Thanks

7. Did you megger the existing old wiring before re-energizing? There may have been a problem with moisture that built up during the time (27 hours?) that they were not energized which caused problems when suddenly hit with high voltage?

the old wire with the new splice in the middle tested bad and was replaced (energized Thursday night, this all started Sunday at 6am, the bulbs popping started about 6:30 am Monday) the building with the bulbs popping etc. was not tested before re energizing at 6:30 Monday.

Thanks for your help.
 
Last edited:
Status
Not open for further replies.
Top