[RE-wrenches] 4 Wire Delta

[Drake]

Hello Wrenches,

I'm picking this thread back up now that more 
information has become available.

What I learned from talking with an AEP engineer 
is that the issues with the delta configuration 
on local AEP systems is that the transformers are 
wye-delta. The AEP system is wye on the utility 
side and delta on the load side. Originally it 
appeared that the issue was that the customer had 
requested 120/240 V loads, so they center tapped 
one phase of the 240 V delta 3 phase and grounded 
it to create a neutral. Apparently, if they 
ground the delta, they won't ground the wye.

According to the engineer I spoke with, the wye 
windings, with no ground reference, could be kept 
energized indefinitely from inverter backfeed 
into the delta side in the event that the utility 
lost one phase. I would think the voltage would 
go high on the open line and any connected 
inverters would shut down from high voltage. This 
may not be quick enough to meet UL 1741 requirements.

He said that "all sorts of weird problems" can 
occur if both sets of windings are grounded. He 
didn't go in to what the problems would be.Does anyone know?

AEP also said that having an ungrounded delta 
system, and creating 120 V power through a 
transformer would not be acceptable. At that 
point, I was told that the utility will not 
supply an ungrounded delta system. Problems I can 
see with an ungrounded delta include the issue 
that a ground fault on one leg would not activate 
overcurrent protection. (Maybe they could have a 
ground fault alarm system). Another problem is 
that a neutral could not be run to the service, 
as required by the NEC, since there would be no neutral.

Has anyone backfed a wye-delta transformer? Does 
any utility supply an ungrounded delta? What are 
the problems with grounding the center point of 
the wye and grounding the delta?

Thanks,

Drake

At 11:50 AM 6/5/2017, you wrote:
>We have done many high-leg delta installations 
>with both single phase and three phase inverters here in the Bay Area.
>Â
>Besides the normal stinger-leg type issues of 
>trying to install a single phase 120 volt load 
>across the high leg, I’m not aware of any 
>particular problems. Certainly we see many 
>existing old services where the high leg spots 
>are left blank and only two of the three phases 
>are used for branch circuits. It seems like 
>perhaps this would cause load balancing type 
>issue with the utility. I’m by no means 
>familiar with the ins and outs of what causes transformer problems, however.
>Â
>August
>Luminalt
>Â
>From: RE-wrenches 
>[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jeff Clearwater
>Sent: Monday, June 05, 2017 8:19 AM
>To: RE-wrenches 
><<mailto:re-wrenches at lists.re-wrenches.org>re-wrenches at lists.re-wrenches.org>
>Subject: Re: [RE-wrenches] 4 Wire Delta
>Â
>Hi Drake,
>
>Souther Cal Edison allowed us to do a High-Leg 
>Delta - sharing this in the chance like William 
>that at least you can say others have - can't imagine what their issue is.
>
>Best,
>
>Jeff
>
>Drake wrote:
>
>Hello Wrenches,
>
>I've recently learned that AEP, our utility 
>company, believes it is unsafe to backfeed a 4 
>wire delta transformer. Does anyone know what the issues are?
>
>Thanks,
>
>Drake
>
>Drake Chamberlin
>Athens Electric LLC
>OH License 44810
>CO License 3773
>NABCEP Certified Solar PV
>740-448-7328
><http://athens-electric.com/>http://athens-electric.com/
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>Jeff Clearwater
>Village Power Design
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>Ecovillage Design Consultant
>
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