What's wrong with this picture?

[Bugman1400]

Anybody see any issues with this picture? Specifically, what is the recommended distance between the ground strap and the insulated bus bar? Check out Phase A!

 

[rlundsrud]

Hmmm, the B phase cables are marked with white tape and they used minimally expanding foam to seal the cable entrance.

 

[MD84]

This probably does not need to comply with NEC. Manufacturer would specify distance. Is this a collector substation for power generation?

That B phase ground stud should have its insulating cap installed.

34.5Kv? It probably was hipot tested at 80kV

 

[K8MHZ]

Anybody see any issues with this picture? Specifically, what is the recommended distance between the ground strap and the insulated bus bar? Check out Phase A!

Is that a sign of an arc to the bus, or is it something else? I see no evidence of arcing on the strap, but it does look like it could be just a bit farther away.

 

[MD84]

I didn't see that concentric neutral strap there. I thought you were asking about that ground bus. That strap is close. I would say at least 6". A good idea would be to check with the manufacturer or use the as built tolerances. If you can check the closest distance from any ungrounded bus to ground then just don't go any closer than that. I can see that being a partial discharge problem.

 

[Jraef]

... I would say at least 6". A good idea would be to check with the manufacturer or use the as built tolerances. ...

Bingo. Although there are design standards relating to the distance from LIVE conductors to ground, once it is insulated like that, it comes down to what the mfr has tested the equipment at when getting a BIL rating. If they had a ground strap touching the insulation and passed at 60kV BIL, then they passed and there is nothing you can say about it.

Now, it's highly unlikely that it would pass like that, and there is likely something mentioned in the installation instructions of the equipment, but there isn't an ANSI standard for what that distance needs to be, just that it needs to show a BIL level and pass at that level. In my experience that would not pass, but without testing it I can't tell you what distance would make it pass.

 

[Bugman1400]

This probably does not need to comply with NEC. Manufacturer would specify distance. Is this a collector substation for power generation?

That B phase ground stud should have its insulating cap installed.

34.5Kv? It probably was hipot tested at 80kV

Yes, this is a 24kV collector bus swgr to a PV farm.

 

[GoldDigger]

That B phase ground stud should have its insulating cap installed.

Why, if this is really the grounded conductor?

 

[Bugman1400]

Here is another picture of an adjacent cubicle with a similar issue with the ground strap but, on Phase B. The cause for the investigation is shown in the next picture which shows the obvious cable rupture.

So, since the bus was insulated, the direct contact from the ground strap did not cause an immediate fault. However, the partial discharge over time, eventually breaks down the insulation of both the bus and the cable. Has anybody seen similar issue with the way contractors install MV swgr and MV cables?

 

[MD84]

Why, if this is really the grounded conductor?

First I would not assume it is the grounded conductor. The generator owner may have simply chosen Red White Blue for its ungrounded phase identification.

Second the system is designed to have an insulated bus during normal operation. The system may not achieve its nameplate BIL in this configuration.

 

[MD84]

Here is another picture of an adjacent cubicle with a similar issue with the ground strap but, on Phase B. The cause for the investigation is shown in the next picture which shows the obvious cable rupture.

So, since the bus was insulated, the direct contact from the ground strap did not cause an immediate fault. However, the partial discharge over time, eventually breaks down the insulation of both the bus and the cable. Has anybody seen similar issue with the way contractors install MV swgr and MV cables?

I have not seen that exact problem likely because different methods were used.

Typically I have seen on that arrangement where each phase has a separate cable entry point. The cable entry for phase a is in front of bus a and so on. This way you don't have those issues. The concentric neutrals were all brought out away from the busses to be bonded.

The cable entries would be better placed further from the bus and closer to the door.

Was a factory grade PD test performed?

 

[don_resqcapt19]

The damage looks like corona damage.

 

[K8MHZ]

The damage looks like corona damage.

So that would make it obvious that something is too close.

In the second set of pics, it looks like the outer covering, which should go all the way to the shrink wrap, was damaged when it was installed.

This may be one of those things where the code minimum isn't sufficient, just legal. If it were me, the minimum distance would be 'as far as practicable'.

I have seen red white blue used in power plants for marking their phases with white not being the grounded conductor.

 

[Bugman1400]

The damage looks like corona damage.

What is the best way to mitigate or eliminate the corona damage? Should the MV cables be moved closer to the swgr doors and away from the bus as MD84 suggests? Also, what damage does corona cause....does it eventually cause a hole in the insulation material which causes a subsequent fault? Should the contractor have used a PD camera to detect the corona?

 

[MD84]

Corona is a form of PD. I would not call your situation Corona. Corona is a partial discharge in air. This partial discharge is taking place in the insulation. Eventually the partial discharge will tree it's way through the insulation and arc to the grounded strap.

I am not aware of a camera that will detect partial discharge. A corona camera will help visualize the corona in air. A factory grade PD test is the best way to determine if you have excessive PD. There are EMI detectors that can be used but they give little empirical data.

To reduce corona or PD you need to limit the voltage field imposed at the subject area. Usually you see corona balls or toroids used to mitigate corona. These are used on exposed conductors which utilize only air as insulation. The idea is to increase the surface area and mitigate pointy electrodes. This reduces the voltage field about an area.

First off I would contact the manufacturer. In my opinion to mitigate the PD I would suggest wrapping the problem area with additional insulation and weaken the voltage field by increasing the distance between the ungrounded bus and the ground straps. Consider this to be a bandaid and I would not trust it until gaining approval from the manufacturer and testing the repair.

You may not simply be able to move the cable entry points. If you are lucky you may have a cable loop and enough slack. If not you may need to add a splice.

 

[Bugman1400]

Corona is a form of PD. I would not call your situation Corona. Corona is a partial discharge in air. This partial discharge is taking place in the insulation. Eventually the partial discharge will tree it's way through the insulation and arc to the grounded strap.

I am not aware of a camera that will detect partial discharge. A corona camera will help visualize the corona in air. A factory grade PD test is the best way to determine if you have excessive PD. There are EMI detectors that can be used but they give little empirical data.

To reduce corona or PD you need to limit the voltage field imposed at the subject area. Usually you see corona balls or toroids used to mitigate corona. These are used on exposed conductors which utilize only air as insulation. The idea is to increase the surface area and mitigate pointy electrodes. This reduces the voltage field about an area.

First off I would contact the manufacturer. In my opinion to mitigate the PD I would suggest wrapping the problem area with additional insulation and weaken the voltage field by increasing the distance between the ungrounded bus and the ground straps. Consider this to be a bandaid and I would not trust it until gaining approval from the manufacturer and testing the repair.

You may not simply be able to move the cable entry points. If you are lucky you may have a cable loop and enough slack. If not you may need to add a splice.

Do you have any links to literature that explains the effects of PD or corona on MV swgr? I see corona rings all the time but, its typically on EHV equipment; not on MV equipment.

Thanks for the info!

 

[Tony S]

Have a read of this.