GEC required for 208-480 delta transformer?

[GoldDigger]

Why is it okay for the utility?

Utilities often run wye-wye transformers, which do not have the same problem.
And when they use wye-delta they are in a position to measure, analyze and most importantly do something about voltage imbalance in the wye.

POCOs also have more pressing reasons to use wye primaries since, I am told, it reduces the likelihood of ferroresonance problems at high voltages.

 

[electrofelon]

I had an incident with a delta wye transformer wired in reverse last year. We talked about it, some of you may recall. The transformer had a neutral run to the wye point "primary" and a system bonding jumper was installed. It had been working for a decade or two, but I happened to notice it while doing some work on the premises so I removed the neutral. However the light was bad, stuff in the way, it was real dirty, etc so I didnt notice the bonding jumper. I dont remember the time frame, but it was a least a few weeks later that the EMT raceway got warm enough for long enough to compromise the insulation in the transformer feeder and a conductor faulted to the EMT. Oh wait, its coming back to me....No What happened first was we lost a phase from the utility and I believe that imbalance caused a large current flow on the EMT raceway which was connected to the Y point via the bonding jumper. I believe that theory matches up with goldie's explanation.

 

[jap]

This is not the first time this has come up here, though I don't recall exactly why, but a single core transformer like the dry types most of us use for under 600 volts has more problem with connecting the wye point of a wye primary to the supply neutral, then a bank of transformers where you have multiple cores - like what is typical for POCO pole top transformer banks.

Most people that have posted here with problems and we find out they connected the neutral on a wye primary are not burning the thing out in just seconds but they definitely are usually having high amounts of current flowing and causing problems, I still think when Jap said he had one go out really quickly there was more to it then just the supply neutral being connected, maybe there was another fault somewhere that nobody ever noticed?

A neutral was not connected to the primary. 3 ungrounded conductors (208v) were terminated to X1 x2 and x3 And there was not a fault somewhere else.
But XO was bonded to the case and the EGC via an insulated conductor.
As soon as the power was turned on the insulated bonding jumper got so hot it melted the insulation off the conductor.
With this bonding jumper was removed it was fine.

There is something to do with it being a hard connection one way when backed fed and this being an induced connection normally.

Jap>

 

[GoldDigger]

A neutral was not connected to the primary. 3 ungrounded conductors (208v) were terminated to X1 x2 and x3 And there was not a fault somewhere else.
But XO was bonded to the case and the EGC via an insulated conductor.
As soon as the power was turned on the insulated bonding jumper got so hot it melted the insulation off the conductor.
With this bonding jumper was removed it was fine.

There is something to do with it being a hard connection one way when backed fed and this being an induced connection normally.

Jap>

Even though the EGC is not intended to carry "normal" current, connecting the X0 to the EGC is electrically just as bad as connecting the neutral to the X0.

For the possibility of generating a wye winding imbalance the voltage of the neutral and the voltage of the EGC are close enough that either could cause problems.

The case should be bonded to the EGC and the X0 should be left unconnected to anything.

 

[greenspark1]

FYI, for this application Eaton recommends not landing the LV neutral conductor and removing the XO-enclosure bonding strap. They don't speak to the HV 480 delta side.

 

[Smart $]

FYI, for this application Eaton recommends not landing the LV neutral conductor and removing the XO-enclosure bonding strap. They don't speak to the HV 480 delta side.

You can ground, or not, the 480 delta side any way you want. You have what seems to be four choices, but it really only amounts to two. Technically we could add a third by connecting a grounding zig zag transformer, but somebody went to sleep if that's the plan, as the installation would be better off with a 480Y/277V SEC.