Grounding of Concentric Neutral (or tape shield) Cable on a Delta MV System

[yesterlectric]

Some time ago, I worked with someone in some work in a substation. In addition to MV breakers, shutting the system off involved operating a knife switch. One had to open it, then close another set of blades. This was to clear the residual voltage from the metal cable casing. When this happened, you could hear and see a little bit of spark.

More recently, I have worked with some wye connected transformers, where this was not a requirement. The casing was grounded to the frame, so it is not live when the power is removed.

I never understood the difference in the systems and why we had to have it in the substation.

I currently am involved with a project with two transformers (one to step up and the other to step down) that have a 12.47 kV delta on the HV side. I am wondering if this scenario requires something like the substation I mentioned. Does one ground the concentric neutral or tape shield of the MV wire to earth in this case, or must it be isolated and then grounded when the circuit is opened? If the letter, how is this commonly done with a transformer that has integral load break switch?

 

[GoldDigger]

There will probably be a difference in practice between a concentric neutral and a tape shield, since the former is sized to be current carrying while the later may not be.

 

[yesterlectric]

There will probably be a difference in practice between a concentric neutral and a tape shield, since the former is sized to be current carrying while the later may not be.

Maybe so, but I think it has to be more than that. Tape shield is for applications where there is no imbalance. This makes a difference in terms of a safe shut down for servicing. Obviously, the safer is the system I described with a permenantly grounded case. I am wondering if the reason for the other system has to do withthe fact that it may have been a Delta connected system.

 

[big john]

...One had to open it, then close another set of blades. This was to clear the residual voltage from the metal cable casing....

If I had to guess what you were actually seeing was the isolated conductors being connected to a grounding-switch for safety. There won't be potential to ground on the shield, but there will be potential between the conductor and the shield because of the cable capacitance. Any time a line is taken out of service it needs to be grounded-down to bleed this off.

...More recently, I have worked with some wye connected transformers, where this was not a requirement....

Because in this case the windings are bonded to the earth. Once they are isolated from the system they immediately assume ground potential. Depending on the work being done, grounds may still have to be applied to the transformer for worker safety in the event of unintentional backfeed, but it doesn't have anything to do with discharging stored energy.

That said, an ungrounded delta system can absolutely hold a charge, and I have also seen those spark like you describe when they are being grounded-down.

...I currently am involved with a project with two transformers (one to step up and the other to step down) that have a 12.47 kV delta on the HV side. I am wondering if this scenario requires something like the substation I mentioned....

If you guys intend to work the high side of that transformer then personal protective grounds must be applied once the circuit is de-energized.

...If the letter, how is this commonly done with a transformer that has integral load break switch?

I assume you've got load-break elbows on the transformer for your cables? You pull them with a stick and park them on grounding bushings:

If this is unfamiliar territory, you should definitely get some assistance from an experienced high-voltage contractor. This is not the type of work where you should be learning as you go.

 

[yesterlectric]

If this is unfamiliar territory, you should definitely get some assistance from an experienced high-voltage contractor. This is not the type of work where you should be learning as you go.

Yes, I don't so much do that work. Trying to get familiar from a distance through observation.

 

[yesterlectric]

Thanks

 

[meternerd]

12,470 phase to phase is usually a Wye voltage used at 12,470/7200 for distribution. If you connect the three phases but not the neutral, it's really not Delta. It's likely still referenced to ground somewhere and should "bleed off" any voltage when the solid blade switches are opened. Ungrounded 12,470 Delta must be kinda rare in a substation. I've never seen it, anyway.

 

[meternerd]

As for what to do about the shields, if it is truly an ungrounded Delta or even an ungrounded Wye, that's a good question. I have no good answer...waiting for the pros.

 

[yesterlectric]

This isn't really a substation. My mention of a substation was just a different experience. We have a 480 wye connected to a 500kva transformer with a 12470 v Delta high side. From there it runs around 800 feet to another delta high side transformer with a 208v Wye secondary.

 

[GoldDigger]

This isn't really a substation. My mention of a substation was just a different experience. We have a 480 wye connected to a 500kva transformer with a 12470 v Delta high side. From there it runs around 800 feet to another delta high side transformer with a 208v Wye secondary.

Two possibilities:
1. You operate the delta feeder ungrounded. In that case the various comments about ungrounded systems apply. And a first fault will not cause any current in a grounded shield or concentric neutral.
2. You operate the delta feeder with a corner ground. In that case a first fault can cause high current in the EGC and/or grounded conductor.
The concentric neutral can handle this current up to the time that the OCPD opens, so you can solidly ground the concentric neutrals at one or both ends.
But since the foil tape cannot carry as much current without damage, you should run a separate EGC and only solidly ground the foil at one end. The other end could have a high impedance ground.

Note: Analysis is not from experience, just from basic electrical theory, so take it for what it is worth or less.