Corner Grounded Systems

[steve66]

I got a call from a client asking about hooking a generator up at a site with what sounds like a corner grounded service. I don't think I've ever seen one of these.

My question is this: If you ground the corner of the delta, and one phase has a center tap to get 120V loads, where is the neutral wire connected??

It seems like the neutral wire must come from the center tap, which would be at 120 volts with respect to the ground. Is that right?

 

[raider1]

If you corner ground a delta transformer then there is no center tap and you won't be able to get 120 volts.

A corner ground delta would have a single voltage secondary.

Chris

 

[kellytshort]

no slash ratings

no slash ratings

also for your corner grounded delta make sure the breakers don't have a slash rating like 120/208v because the phase to ground voltage will be 208v not 120v

 

[jumper]

also for your corner grounded delta make sure the breakers don't have a slash rating like 120/208v because the phase to ground voltage will be 208v not 120v

208v or 240v?

 

[raider1]

208v or 240v?

A corner grounded delta secondary would have 240 volts phase to phase and phase to ground.

Chris

 

[steve66]

If you corner ground a delta transformer then there is no center tap and you won't be able to get 120 volts.

A corner ground delta would have a single voltage secondary.

Chris

So with a corner ground, I would assume the corner that is grounded is brought to the service, and that becomes the "neutral" wire , and is bonded to ground? Is that correct??


Assuming there was a corner ground, and a center tap. Then what? The center tap would be at 120V to ground, and couldn't be used as a neutral. Does that sound right?

 

[raider1]

So with a corner ground, I would assume the corner that is grounded is brought to the service, and that becomes the "neutral" wire , and is bonded to ground? Is that correct??

The conductor that is grounded would be a grounded conductor, but in the case of a corner grounded system it would not be a neutral.

Assuming there was a corner ground, and a center tap. Then what? The center tap would be at 120V to ground, and couldn't be used as a neutral. Does that sound right?

It would not be code compliant to corner ground a center tapped delta secondary where 120 volts is used.

250.20(B)(3) would make it required to ground the center point of a delta transformer where the midpoint of one phase is used as a circuit conductor.

Chris

 

[cadpoint]

... I don't think I've ever seen one of these....

Google, Image, Here

Noting that no power size was given I like the first Image that refernces a "typical 3 phase 3-Wire systems that are permitted to be grounded"

IF one where to use your exact opening title phrase, plural by adding the "s" one would get a sligtly different google image response.

Also a reminder that the IAEI.org is the govenors of above 600 Volts.

 

[SG-1]

The delta is either corner grounded with no center tap or it is grounded through the center tap of one of the transformers.

It is one way or the other, it cannot be both.

 

[broadgage]

A corner grounded delta service can be designed for any desired voltage, but 240 is most common.
Only one voltage is available from such a system, there would be no choice of 120/240 or 120/208 or 277/480 as would be available from other service types.

The grounded corner will be the neutral and should be white or perhaps gray, no fuses or single pole switches or breakers may be inserted in this conductor, just as with any other neutral.
The two hot conductors may be coloured most any colour except white, grey or green, but red and black is very common.

In theory one could have a center tap on one or more of the windings, but this center tap COULD NOT be grounded, since one corner of the delta is already grounded.
With a 240 volt grounded delta and a center tap in one winding, it would be possible to obtain 2 wire 120 volt service with grounded neutral but this is very uncommon and not normally done.
With such a non standard and rare arrangement great care would have to be taken. For example if a 120 volt load was connected to the "wrong" end of the center tapped winding it would have 240 volts to ground and be potentialy dangerous.
I have heard of such services but they are very rare.
The usuall aplication is large, remote pumping stations that are either unmanned or very lightly manned. In such cases hundreds of HP will be required for pumping plant and for this a corner delata service is ideal.
Any small 120 volt 2 wire loads such as lighting or small appliances are supplied between the grounded corner of the delta and a center tap.

Some corner grounded deltas have center taps on all 3 transformers with nothing connected to the center taps. That is so as to use standard stock transformers intended for single phase 120/240 volt service.

 

[Volta]

... In theory one could have a center tap on one or more of the windings, but this center tap COULD NOT be grounded, since one corner of the delta is already grounded.

I think a case could be made that "Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts" 250.20(B)(1), that it shall be grounded in that manner.

Of course, the EC can't control what the POCO does, so that might be unenforceable for a service. But for a SDS, I feel a mid-point tapped secondary could not be used for a compliant corner-grounded delta.

 

[steve66]

With a 240 volt grounded delta and a center tap in one winding, it would be possible to obtain 2 wire 120 volt service with grounded neutral but this is very uncommon and not normally done.
With such a non standard and rare arrangement great care would have to be taken. For example if a 120 volt load was connected to the "wrong" end of the center tapped winding it would have 240 volts to ground and be potentialy dangerous.

I haven't seen this service, but this sounds like what the client is describing: a 4 wire service, with an open delta, corner grounded transformer bank, with one center tap used for 120 V loads.

Steve

 

[steve66]

I haven't seen this service, but this sounds like what the client is describing: a 4 wire service, with an open delta, corner grounded transformer bank, with one center tap used for 120 V loads.

Steve

It's also possible that this is an ungrounded service. The client said they couldn't get any continuity between ground and neutral, but wasn't sure if the main had been shut off when they tried this.

Steve

 

[RICK NAPIER]

If there is a center tap there can be no corner grounding and the center tap is the required ground and neutral.

 

[Volta]

If there is a center tap there can be no corner grounding ...

At least it can not be done correctly . . .

 

[broadgage]

It is possible (though very uncommon) to have a corner grounded delta with a center tap on one winding.
However this center tap could NOT be grounded, since grounding both the center tap and one corner of the delta would result in a short circuit.

Consider three transformers each with a 240 volt winding, connected in corner grounded delta.
Presume that the A corner is grounded.
Any phase to any other would be 240 volts
Phase B or C to ground would be 240 volts.
Now consider a center tap on the winding between the A (grounded) and B phases.
This would give 120 volts between the grounded corner of the delta and the center tap. This would be just like any other two wire 120 volt service, and would work 120 volt lamps and appliances just fine.
It would however be a rare arrangement, the drawback being that all 120 volt loads must be connected to the same half of the winding.
It is however an economical way to supply limited 120 volt loads from a system designed mainly for 3 phase pumping plant.

For significant 120 volt loads it would be usuall to ground the center tap, and NOT to ground the corner of the delta, this then becomes not corner grounded delta at all, but the more common 3 phase delta for 120 volt and 240 volt service.

 

[LarryFine]

This would give 120 volts between the grounded corner of the delta and the center tap.

Basically, using that secondary like an auto-transformer.

For significant 120 volt loads it would be usuall to ground the center tap, and NOT to ground the corner of the delta, this then becomes not corner grounded delta at all, but the more common 3 phase delta for 120 volt and 240 volt service.

Also known as a high-leg Delta, with 208v to the neutral.