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.  Besides the normal stinger-leg type issues of trying to install a single phase 120 volt load across the high leg, Iâm 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. Iâm by no means familiar with the ins and outs of what causes transformer problems, however.  August Luminalt  From: RE-wrenches [mailto:[email protected]] On Behalf Of Jeff Clearwater Sent: Monday, June 05, 2017 8:19 AM To: RE-wrenches <<mailto:[email protected]>[email protected]> Subject: Re: [RE-wrenches] 4 Wire Delta  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/  <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient&utm_term=icon> Image removed by sender. Virus-free. <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient&utm_term=link>www.avast.com _______________________________________________ List sponsored by Redwood Alliance  List Address: <mailto:[email protected]>[email protected]  Change listserver email address & settings: <http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org>http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org  List-Archive: <http://www.mail-archive.com/[email protected]/maillist.html>http://www.mail-archive.com/[email protected]/maillist.html  List rules & etiquette: <http://www.re-wrenches.org/etiquette.htm>www.re-wrenches.org/etiquette.htm  Check out or update participant bios: <http://www.members.re-wrenches.org>www.members.re-wrenches.org   -- Jeff Clearwater Village Power Design Renewable Energy Design & Installation Ecovillage Design Consultant Former NABCEP Certified - 2003-2012 Licensed Solar Contractor in CA - C-46 <http://www.villagepower.com>www.villagepowerdesign.com 413-559-9763 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Content-Type: image/jpeg; name="image001.jpg" Content-Disposition: inline; filename="image001.jpg" Content-ID: <[email protected]> X-Attachment-Id: 82cef4cbd5ee3dac_0.1 _______________________________________________ List sponsored by Redwood Alliance List Address: [email protected] Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/[email protected]/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
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