4160 Delta Secondary Grounded or Ungrounded

[infinity]

We have several 3Ø, 208-4160 WYE/Delta step-up transformers. How are these typically grounded, corner ground or are they typically installed as an ungrounded Delta?

 

[infinity]

Bump, have I finally stumped the engineers on this forum?

 

[Dale001289]

Bump, have I finally stumped the engineers on this forum?

I’ve never seen this before nor have I researched it - but why couldn’t you do either or? Personally I think you’d be better off with a corner grounded delta; at least you’d have reference point - plus we rarely see ungrounded deltas these days.


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[GoldDigger]

IMHO a lot depends on whether the voltage will be stepped back down again after a long run (motivation was VD) or whether the 4160 will be utilization voltage for equipment (and if so, what kind.)

If just a distribution run to step-down transformer, the risk of high voltage to ground caused by restriking arc fault may not be catastrophic. But no disadvantage to corner grounding either.
If run to utilization equipment, unless it is an industrial process where unplanned shutdown is problematic, I would avoid ungrounded operation.
Would the difference in max line to ground voltage be a safety issue for your environment?

 

[infinity]

The service voltage of 208Y/120 is stepped up to 4160 and then run vertically up through the building where it is stepped back down to 208Y/120. Right now the design calls for a ungrounded Delta, I was just wondering if ungrounded is the typical set up for these types of systems.

 

[Ingenieur]

weird set-up

why not take the service at 4160 wye?
how many floors?
how many 208 taps/xfmrs?

 

[infinity]

There are 4-4000 amp services in the building all 208Y/120. There are about 10-208 to 4160 step up and another 10-4160 to 208y/120 step down tranformers, all step ups are setup as ungrounded. The step ups are Wye/Delta and the step downs are Delta/Wye.

 

[electrofelon]

There are 4-4000 amp services in the building all 208Y/120. There are about 10-208 to 4160 step up and another 10-4160 to 208y/120 step down tranformers, all step ups are setup as ungrounded. The step ups are Wye/Delta and the step downs are Delta/Wye.

Wow

 

[electrofelon]

I think ungrounded would be fine. You already have ungrounded stuff there, keeps it all the same. Also it sounds like there will be people maintaining this stuff that know what they are doing. Finally, regarding the potential over voltage issue from restriking faults, I would think pretty much everything you would install nowadays would be 15KV class so you have plenty of headroom if something along those lines were to happen.

 

[packersparky]

We have several 3Ø, 208-4160 WYE/Delta step-up transformers. How are these typically grounded, corner ground or are they typically installed as an ungrounded Delta?

250.20(C) permits such systems to be grounded, but does not require them to be grounded.

(C) Alternating-Current Systems of over 1000 Volts.
Alternating-current systems supplying mobile or portable
equipment shall be grounded as specified in 250.188. Where
supplying other than mobile or portable equipment, such
systems shall be permitted to be grounded.

 

[jim dungar]

In years gone past, I used to recommend that delta secondaries be ungrounded when they feed a single circuit and corner-grounded when they feed multiple circuits.

What are you planning to do for a ground detection scheme? Will you have any protective relaying on the 4160V?

 

[infinity]

In years gone past, I used to recommend that delta secondaries be ungrounded when they feed a single circuit and corner-grounded when they feed multiple circuits.

What are you planning to do for a ground detection scheme? Will you have any protective relaying on the 4160V?

The 4160 secondary feeds a fused disconnect and as far as I can see that's it. Is ground detection required?

 

[don_resqcapt19]

The 4160 secondary feeds a fused disconnect and as far as I can see that's it. Is ground detection required?

250.21(B) only requires ground detection for systems operating at 1000 volts or less. I don't see anything in Part X of 250 requiring ground detection for the higher voltage systems.

 

[infinity]

250.21(B) only requires ground detection for systems operating at 1000 volts or less. I don't see anything in Part X of 250 requiring ground detection for the higher voltage systems.

That's what I thought as well, seems to be a design issue not a code issue

 

[jim dungar]

That's what I thought as well, seems to be a design issue not a code issue

You are correct, this is a design choice.

One I did like this was for a Scout camp, the MV was run through the woods (buried) and across a river. With all of the 'critters' out there we were concerned a ground fault would not be noticed until it was catastrophic.

 

[infinity]

This came up the other day because someone asked me about running GEC's for these step-up transformers. When I took a closer look at the set up I realized that this was designed as an ungrounded system which yielded some strange looks. Sometimes electricians cannot understand how a system could be installed that is not grounded especially at 4160 volts.

 

[ActionDave]

.... Sometimes electricians cannot understand how a system could be installed that is not grounded especially at 4160 volts.

How does it work without a ground? has to be in the top five questions of all time. I can understand common folk asking that, but electricians should know better.

 

[electrofelon]

This came up the other day because someone asked me about running GEC's for these step-up transformers. When I took a closer look at the set up I realized that this was designed as an ungrounded system which yielded some strange looks. Sometimes electricians cannot understand how a system could be installed that is not grounded especially at 4160 volts.

Even though you arent grounding a conductor of the system, Dont you still need a GEC for the metal parts?

 

[infinity]

Even though you arent grounding a conductor of the system, Dont you still need a GEC for the metal parts?

For the step-ups the metal parts are connected to the EGC in the feeder which is anywhere from 5-8 sets of 4" EMT feeding the 208 volt primary. I would guess that if one phase of the 4160 were to fault to ground the system would still operated as a corner grounded system. For the step-downs the 208Y/120 secondary would be wired like a typical transformer with a EGC, GEC, SBJ and SSBJ.

 

[don_resqcapt19]

This came up the other day because someone asked me about running GEC's for these step-up transformers. When I took a closer look at the set up I realized that this was designed as an ungrounded system which yielded some strange looks. Sometimes electricians cannot understand how a system could be installed that is not grounded especially at 4160 volts.

I think the grounding electrode system and GECs are required. The only real difference between the rules for a grounded and ungrounded transformer is that the ungrounded one does not have a system bonding jumper.