12kv transformer isolated ground 6

[garetoo]

I am an electrician being asked to install a ground that appeared on the latest reved single line. The transformer is a 480V wye step-up to 12kv delta. The ground wire is to be insulated and not be bonded to the equipment ground. Since the transformer resides in a common switchgear enclosure and is not insulated from it, I am concerned that a fault to the steel would not be a direct ground fault.

The ground rod for the center tap will reside a couple of feet away from the ground rod for the equipment ground. It appears to me that a fault to the metal enclosure would go out on the equipment ground and then through the earth to the center tap. I think there will be a voltage potential. Am I wrong to think that safety is at risk here? I would rather be wrong and be embarrassed then right and not say anything.

I uploaded a portion of the single line.

http://files.engineering.com/getfil...-b466-d5a7c63700ac&file=a_12kv_iso_Ground.pdf

Thank you.

 

[garetoo]

QUOOTE:
I would not corner ground or ground the midpoint of one winding of a 12 kV transformer. I can't find offhand where the NEC prohibits this, but IEEE Std 142-1991 recommends against midpoint grounding systems over 240V. Ø-ground voltage will be 12 kV on two phases if corner grounded, 10.4 kV on one phase if mid-point grounded. Insulation, connectors, and arresters must be rated appropriately.

If you want a grounded system, I recommend installing a zig-zag or grdY-delta grounding transformer.
END QUOTE

Thanks JG, we are proceeding with recommending changing the spec on the transformer to a wye secondary so as to do this as has been suggested by you and others.
I have attached my "interpretation of what is being asked for at present, it may be a tad confusing because I have been rushing.

gary

gare

 

[jghrist]

There is no detail of T1 where the diagram shows a corner ground. The T2 detail shows loadbreak elbows on the 12 kV side. If T1 is the same, I'd be interested to know how one of the bushings would be grounded. Use a feed-thru insert and a grounding elbow? You would have to use 25 kV class elbows on the ungrounded phases. 15 kV class elbows are only rated 8.3 kV to ground.

 

[garetoo]

I have NEVER installed a 12 kv system, so the ratings of bushings and elbows are all a new learning experience. We have subbed this portion of work out to someone who does medium voltage work, I am watching everything to have a better grasp if it ever arises on another project. It is an interesting item to have in a residential application.

We have requested a detail for T1 as they have provided for T2. They made the comment that they should not have to detail everything for us, perhaps that is correct. We are thinking they are more prone to ordering a different transformer now, but who knows just yet.

I modified the drawing a little as the GC wanted all of my comments clouded. I will post the resolution when it is made, hopefully tomorrow.

Thank you for monitoring the thread and providing the input you have. I have forwarded on the information with no reference to you, as I want all liability in appropriate courts. I take no credit for anything, I merely state "Others I have communicated with have...."

Is this ok with you?

gare

 

[busbar]

 

You would have to use 25 kV class elbows on the ungrounded phases.

If I follow this correctly, wouldn't 25kV cables be required too?

 

[davidbeach]

No, it wouldn't necessarily require 25kV cables, but it would require more than 100% insulation on the 15kV cables. Which higher insulation level would be determined by the maximum duration of the overvoltage. For the wild leg that duration would be continuous. I believe there is a 175% insulation level (or something close) that would work. 25kV cable might be a more practical solution though.

 

[apowerengr]

T2 says 3ph-3w however it appears to be wired like 3ph-4w according to how I read the conductor call-outs. If you change T1 to 12kv 3ph 3w wye you don't have a circuit path for ground faults (see previous threads regarding ungrounded services). Since this seems to be someone's fix for running a 1700' line without significant voltage drop why not take a 12kV primary feed 4w from the utility and primary meter the entire site? With all the up and down transformations you're going to wind up with so much voltage drop and such a weak system due to all the series impedance load service will probably be an issue. Coordination will be difficult at best too. I agree with other posts that the 12kV ungrounded system would require higher levels of insulation, a ground-fault detection system (voltage sensing) and other items not shown on the diagrams provided.

 

[jghrist]

We have subbed this portion of work out to someone who does medium voltage work, I am watching everything to have a better grasp if it ever arises on another project. It is an interesting item to have in a residential application.

This is a residential application? I'd hate to be the homeowner who is stuck trying to find someone to maintain a corner grounded 12 kV system. I suspect that your MV subcontractor will think someone is out of their mind if they are asked to put a grounding elbow on a 12 kV bushing of an energized transormer. So will any maintenance contractor the homeowner hires. I've only been dealing with MV distribution for 40 some years, so maybe someone with more experience has heard of a corner grounded or midpoint grounded 12 kV system. I haven't. Is there anyone out there who has?

 

[garetoo]

QUOTE apwrengr: "why not take a 12kV primary feed 4w from the utility and primary meter the entire site? " END QUOTE

The utility said they will not do this because "there is not enough demand" Even we electricians thought the waste through multiple transformers was just that, a waste.

QUOTE jghirst: I've only been dealing with MV distribution for 40 some years, so maybe someone with more experience has heard of a corner grounded or midpoint grounded 12 kV system. END QUOTE

I have made use of most of the input from here and they are now reconsidering a different transformer and better grounding, Thank you all very much.

May I reference the thread to them? I have deliberately not referenced it specifically so that all responsibility rests on us as the sub, not on any comments from anyone in here. I would sure love to give them the link. If one person says no, I will not provide it.

gare

 

[jghrist]

I don't think there is any problem referencing a thread. Nothing on a public forum with anonymous contributors should be taken as a definitive reference, but it can be helpful.

 

[garetoo]

Thanks jghrist.

I want to honor the spirit of the forum, seeking/providing great information without the ever present liability issue in the back of someone's mind. At the same time, credit rightfully belongs to those who expense their knowledge, in this case at no cost to others.

I will wait this day out and barring no "no's" I will reference the thread to all parties involved at my end. I am convinced it is contributing to an installation that will not be as much a hazard to life as it started out. Sadly, I do not think the EOR would have stepped up and said, "oh, wait, this does not appear safe". Thanks to all of the input from here, they have backed off from crucifying me.

gare

 

[waross]

A couple of suggestions;
The utility may consider primary metering if you offer to pay the cost of the metering transformer(s).
A wye:delta transformer is often problematic. If the primary neutral is left floating, (recommended) there may be transient overvoltages due to switching surges when the transformer is energized. If the primary neutral is connected, the transformer becomes intolerant of voltage unbalances and phase angle errors. It will overheat on PU unbalances of the same order of magnitude as the transformer PU impedance. If a primary phase is lost, the bank will back feed full voltage into the "lost" phase until the transformer primary fuses blow.
A wye:wye transformer is a good choice.It provides a neutral ground point.
A delta:wye step down at the other end is a good choice. If you use a wye:wye to step down, you have to run 4 wires instead of 3 wires. (Not including the controversial ground wire.)
As well as posting a link, invite your engineer to join this site.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

I guess I'd missed one little detail until reading Bill's post above. As a utility we would not accept a customer installing a grounded wye (source)-delta (load) transformer without extensive protection on the wye side to trip the transformer off line should there be any ground fault on the utility system.

 

[garetoo]

QUOTE davidbreach:
As a utility we would not accept a customer installing a grounded wye (source)-delta (load) transformer without extensive protection on the wye side to trip the transformer off line should there be any ground fault on the utility system. END QUOTE

How on earth could any one person ever have a complete grasp of all of this? What would constitute "extensive" protection?


gare

 

[davidbeach]

Three-phase interrupting device, aka a circuit breaker, with voltage and current input to a relay that could trip the breaker on reverse overcurrent, reverse zero sequence overcurrent, phase overvoltage, phase undervoltage, zero sequence overvoltage. Maybe more; hopefully obnoxious enough to make customers never want to install a grounded wye-delta transformer to serve load. They shouldn't even be considered for load, though they are entire appropriate for generation and as generation would include all of the appropriate protection anyway.

 

[garetoo]

The first change has been made, they are changing T1 to a delta/wye. Thanks Everyone!
gary

gare

 

[waross]

The next change should be to connect all ground systems in the area together. That will be a big step towards code compliant and safety.
I would suggest, (if you can) extending your main ground grid to include the area of the transformer and then connecting the transformer wye point to the main, extended grid.
One lethal touch potential can ruin the rest of your life, however short it may be.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[garetoo]

Thanks waross, we are waiting to see what the final changes will be before anything else is said. I hope they have accepted the invitation to at least view this thread.

gare

 

[busbar]

 
If I had my druthers, I’d install a 480:12,000 ?-? on the “street” side, allowing the 480V overcurrent device to simultaneously protect the “street” transformer, the cables and the building transformer. Then, put a 12,000:480 grdY-grdY transformer at the building that would allow for “system” grounding of the 12kV circuit. {In the Article-240 tap rules, now NEC allows 3-wire ?-? transformers to be protected by one overcurrent device.}

Also, run a buried bare copper conductor to bond all of it together.

 

[davidbeach]

With out a ground source on the 480V side, or a delta tertiary, the 12,000:480 grdY-grdY won't provide much of a system ground for the 12kV circuit.

 

[waross]

And, a wye:wye transformer needs a connection to the system neutral, (NOT GROUND) to provide balanced and stable voltages on the secondary. If you are feeding this from a delta;delta you do not have a system neutral to connect to. The secondary voltages get a little weird.
There is another thread facing this problem. No neutral connection on a wye primary and unbalanced secondary voltages as a result.

Bill
--------------------
"Why not the best?"
Jimmy Carter