<font size=2 face="sans-serif">Hi:</font>
<br>
<br><font size=2 face="sans-serif">You've run into one of the reasons why
single phase inverters which are operated as a 3-phase cluster, are supposed
to have built in circuits/comm wires which send a shutdown signal from
one to the others so that if one stops, they all do.</font>
<br>
<br><font size=2 face="sans-serif">The Xantrex GT / Conext TX have an RJ-11
wire connection called AC-sync which takes cares of this requirement.</font>
<br>
<br><font size=2 face="sans-serif">JARMO <br>
</font><font size=3 color=#008000>_____________________________________________________________________________________</font><font size=3>
<br>
</font><font size=1 face="Arial"><b><br>
Jarmo Venalainen</b> | </font><font size=1 color=#008000 face="Arial"><b>
Schneider Electric </b></font><font size=1 face="Arial"><b> | Xantrex
Brand</b> | <b>CANADA</b> | <b>Sales Application
Engineer</b> <b><br>
Phone:</b> +604-422-2528 | <b>Tech Support:</b> 800-670-0707
| <b>Mobile:</b> +604-505-0291 <b><br>
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www.Xantrex.com</u></font></a><font size=1 face="Arial"> |
<b>Address:</b> 3700 Gilmore Way, Burnaby, BC V5G4M1 <br>
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<table width=100%>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">From:</font>
<td><font size=1 face="sans-serif">"Kelly Keilwitz, Whidbey Sun &
Wind" <kelly@whidbeysunwind.com></font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">To:</font>
<td><font size=1 face="sans-serif">RE Wrenches listserve <re-wrenches@lists.re-wrenches.org>,
</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Date:</font>
<td><font size=1 face="sans-serif">04/02/2015 03:19 PM</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Subject:</font>
<td><font size=1 face="sans-serif">[RE-wrenches] PV step-up transformer
design</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Sent by:</font>
<td><font size=1 face="sans-serif">"RE-wrenches" <re-wrenches-bounces@lists.re-wrenches.org></font></table>
<br>
<hr noshade>
<br>
<br>
<br><font size=3>Wrenches,</font>
<br><font size=3>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:</font>
<br>
<br><font size=3>- 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.</font>
<br>
<br><font size=3>- 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.</font>
<br>
<br><font size=3>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. </font>
<br>
<br><font size=3>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….</font>
<br>
<br><font size=3>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.</font>
<br>
<br><font size=3>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.</font>
<br>
<br><font size=3>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.
</font>
<br>
<br><font size=3>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?</font>
<br><font size=3>Other changes we’re pondering include:</font>
<br><font size=3>- 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.</font>
<br><font size=3>- 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.</font>
<br><font size=3>- Can/should we remove the neutral connection to the transformer
on the secondary side? There are no 277 loads or sources.</font>
<br><font size=3>- Can/should we put OCP on the neutral to the transformer?</font>
<br>
<br><font size=3>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.</font>
<br>
<br><font size=3>Thanks,</font>
<br><font size=3>-Kelly</font>
<br><font size=3> </font>
<br><font size=2>Kelly Keilwitz, P.E. <br>
Principal<br>
Whidbey Sun & Wind<br>
Renewable Energy Systems<br>
NABCEP PV Installation Professional<br>
WA Electrical Administrator</font><font size=2 color=blue><u><br>
</u></font><a href=mailto:kelly@whidbeysunwind.com><font size=2 color=blue><u>kelly@whidbeysunwind.com</u></font></a><font size=2><br>
PH & FAX: 360.678.7131</font><font size=3><br>
<br>
</font>
<br>
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