Hi August, 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. -Kelly
On Apr 3, 2015, at 16:02, August Goers <[email protected]> wrote: > Hi Kelly, > > 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? > > Best, > > August > > From: RE-wrenches [mailto:[email protected]] On > Behalf Of Kelly Keilwitz, Whidbey Sun & Wind > Sent: Thursday, April 02, 2015 3:19 PM > To: RE Wrenches listserve > Subject: [RE-wrenches] PV step-up transformer design > > Wrenches, > 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: > > - 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. > > - 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. > > 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. > > 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…. > > 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. > > 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. > > 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. > > 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? > Other changes we’re pondering include: > - 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. > - 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. > - Can/should we remove the neutral connection to the transformer on the > secondary side? There are no 277 loads or sources. > - Can/should we put OCP on the neutral to the transformer? > > 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. > > Thanks, > -Kelly > > Kelly Keilwitz, P.E. > Principal > Whidbey Sun & Wind > Renewable Energy Systems > NABCEP PV Installation Professional > WA Electrical Administrator > [email protected] > PH & FAX: 360.678.7131 > > > > > _______________________________________________ > 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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