<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">Hi August,<div>Yes, it’s difficult to understand what’s going on from my text description. I’ll forward a copy of the schematic to all who replied, when I get in the office tomorrow. Any wrenches lurking who’d like to look at the schematic let me know off-line.</div><div>-Kelly<div apple-content-edited="true"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; border-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><br></span>
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<br><div><div>On Apr 3, 2015, at 16:02, August Goers <<a href="mailto:august@luminalt.com">august@luminalt.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div lang="EN-US" link="blue" vlink="purple" style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div class="WordSection1" style="page: WordSection1;"><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);">Hi Kelly,</span></div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);"> </span></p><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);">Sounds like a doozey. As others have posted, it sounds like an imbalance issue. However, it might be possible to solve the problem without replacing the major equipment (could just be wire size, phasing, fuses, etc?). Do you have a single line diagram you can share?</span></div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);"> </span></p><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);">Best,</span></div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);"> </span></p><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);">August</span></div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);"> </span></p><div><div style="border-style: solid none none; border-top-color: rgb(181, 196, 223); border-top-width: 1pt; padding: 3pt 0in 0in;"><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><b><span style="font-size: 10pt; font-family: Tahoma, sans-serif;">From:</span></b><span style="font-size: 10pt; font-family: Tahoma, sans-serif;"><span class="Apple-converted-space"> </span>RE-wrenches [mailto:<a href="mailto:re-wrenches-bounces@lists.re-wrenches.org" style="color: purple; text-decoration: underline;">re-wrenches-bounces@lists.re-wrenches.org</a>]<span class="Apple-converted-space"> </span><b>On Behalf Of<span class="Apple-converted-space"> </span></b>Kelly Keilwitz, Whidbey Sun & Wind<br><b>Sent:</b><span class="Apple-converted-space"> </span>Thursday, April 02, 2015 3:19 PM<br><b>To:</b><span class="Apple-converted-space"> </span>RE Wrenches listserve<br><b>Subject:</b><span class="Apple-converted-space"> </span>[RE-wrenches] PV step-up transformer design</span></div></div></div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">Wrenches,</div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">We have issues with a 1-1/2 year old 25 kW grid-tied PV system utilizing 2 step up transformers from the array to grid. Actually it’s two identical, 12.5 kW systems (for incentive and financing reasons) on each transformer. Note that the inverter size and voltage selection was also dictated by incentives. Each of the two systems are designed as follows:</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">- 3, 3.8 kW, 240 Vac inverters (16A max output current) across each phase of the 240V, 3-ph Delta primary of a 15 kVA transformer. Each inverter 1-ph output has a fused disconnect. The output of all three inverters is combined into 3 phase delta configuration with terminal blocks before the transformer. Other than the inverter AC disconnects there is no OCPD on the 3-ph primary side of the transformer.</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">- Transformer secondary to grid is 277/480 wye. The secondary output has two fused disconnects (one at each end of the 600-ft run between array and line-side connection) each with 20-A fuses.</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">After more than year of operation we noticed one inverter, in one of the two identical systems, was down (monitoring showed it happened past December), indicating a grid problem. One fuse (of the 3) in the secondary (grid-side) fused disco had blown. Replaced that fuse, but then 2 fuses at the disconnect near the meter, 600 feet away, popped. </div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">Systematic checking of the system reveals no shorted conductors, but a problem at the transformer. Close inspection revealed that the insulation on our 90˚C neutral conductor had slightly melted where it touched the X-O neutral wire of the transformer (the splice looked fine). Now it gets more interesting….</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">In the last week, while troubleshooting the first 12.5 kW system, its adjacent twin system had an inverter quit with a ground-fault error (these inverters commonly will display ground fault errors for other reasons). One of the 20A fuses in the secondary (grid 277/480) side had popped. And, yup, the neutral wire had signs of overheating. We shut all three inverters down in that system. The transformer doesn’t seem damaged, hopefully because we caught it in time. But we’re looking at a replacement transformer for the first system.</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">Here’s what we think may have happened: The grid went down and, on coming back on, the in-rush current to the secondary side of the transformer popped a (undersized) fuse, taking out one leg of the 480. One inverter on the primary side saw a bad grid and dropped out, but the other two connected and kept producing. This unbalanced production resulted in the neutral of the secondary wye (grid) side of the transformer, being overloaded. Over 3 months this unbalanced operation damaged the transformer.</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">Initially we replaced the fuses near the line-connection with 60A (it’s a hot-swap and we don’t want those to go again unless the conductors need it), and the fuses near the array transformer secondary with 30A, thinking that will handle the inrush current to the transformer after a grid outage better than the 20A fuses. However, after a lot of discussion, research, and consideration of the risk (of replacing another transformer) we’re wondering if we need to alter the design more substantially. </div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">The main design change we’re considering is to replace the fused disconnect on the secondary side with a 3-phase breaker, that will shut off all phases of the grid supply simultaneously in case of an over-current fault. But, what if one inverter just plain quits and that breaker stays on? Will the neutral on the secondary still be overloaded from the other two?</div></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">Other changes we’re pondering include:</div></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">- Do we need a ganged 3-ph breaker on the inverter side, so that all three inverters go off at once? This will only be useful in case of over current on an inverter output, which isn’t likely, and is now addressed with the fused 240 AC 1-ph disconnects on the inverter outputs.</div></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">- Use a step-down transformer in reverse. I.E. use a 277/480 wye primary and 240 delta secondary and back feed the secondary with the PV. Some info from other sources suggest that this will reduce the inrush current from the grid after an outage. I think this would be inefficient at transmitting the PV power.</div></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">- Can/should we remove the neutral connection to the transformer on the secondary side? There are no 277 loads or sources.</div></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">- Can/should we put OCP on the neutral to the transformer?</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">Your suggestions and experience will be appreciated. This has additional importance as we are preparing to install another 75 kW at the same site utilizing 240Vac inverters to the 277/480 grid.</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">Thanks,</div></div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;">-Kelly</div></div><div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div><div><div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"><span class="apple-style-span"><span style="font-size: 10pt;">Kelly Keilwitz, P.E. </span></span><span style="font-size: 10pt;"><br><span class="apple-style-span">Principal</span><br><span class="apple-style-span">Whidbey Sun & Wind</span><br><span class="apple-style-span">Renewable Energy Systems</span><br><span class="apple-style-span">NABCEP PV Installation Professional</span><br><span class="apple-style-span">WA Electrical Administrator</span><br><span class="apple-style-span"><a href="mailto:kelly@whidbeysunwind.com" style="color: purple; text-decoration: underline;">kelly@whidbeysunwind.com</a></span><br><span class="apple-style-span">PH & FAX: 360.678.7131</span><br></span><br><br><br></div></div><p class="MsoNormal" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;"> </p></div></div>_______________________________________________<br>List sponsored by Redwood Alliance<br><br>List Address:<span class="Apple-converted-space"> </span><a href="mailto:RE-wrenches@lists.re-wrenches.org" style="color: purple; text-decoration: underline;">RE-wrenches@lists.re-wrenches.org</a><br><br>Change listserver email address & settings:<br><a href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org" style="color: purple; text-decoration: underline;">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a><br><br>List-Archive:<span class="Apple-converted-space"> </span><a href="http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html" style="color: purple; text-decoration: underline;">http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html</a><br><br>List rules & etiquette:<br><a href="http://www.re-wrenches.org/etiquette.htm" style="color: purple; text-decoration: underline;">www.re-wrenches.org/etiquette.htm</a><br><br>Check out or update participant bios:<br><a href="http://www.members.re-wrenches.org/" style="color: purple; text-decoration: underline;">www.members.re-wrenches.org</a></div></blockquote></div><br></div></body></html>