[RE-wrenches] PV step-up transformer design

August Goers august at luminalt.com
Fri Apr 3 16:02:57 PDT 2015


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:re-wrenches-bounces at lists.re-wrenches.org] *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
kelly at whidbeysunwind.com
PH & FAX: 360.678.7131
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