[RE-wrenches] PV step-up transformer design

Jarmo.Venalainen at schneider-electric.com Jarmo.Venalainen at schneider-electric.com
Thu Apr 2 16:09:04 PDT 2015


Hi:

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.

The Xantrex GT / Conext TX have an RJ-11 wire connection called AC-sync 
which takes cares of this requirement.

JARMO 
_____________________________________________________________________________________ 


Jarmo Venalainen  |   Schneider Electric   |  Xantrex Brand  |   CANADA  | 
  Sales Application Engineer 
Phone: +604-422-2528  |   Tech Support: 800-670-0707  |   Mobile: 
+604-505-0291 
Email: jarmo.venalainen at schneider-electric.com  |   Site: www.Xantrex.com 
|   Address: 3700 Gilmore Way, Burnaby, BC V5G4M1 








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From:
"Kelly Keilwitz, Whidbey Sun & Wind" <kelly at whidbeysunwind.com>
To:
RE Wrenches listserve <re-wrenches at lists.re-wrenches.org>, 
Date:
04/02/2015 03:19 PM
Subject:
[RE-wrenches] PV step-up transformer design
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>



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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