[RE-wrenches] Low array voltage stumper on legacy system

Fri Aug 26 09:14:19 PDT 2016

Your measurements of the Isc for the four stings seems to be odd in string 3
with only 2.4A.  However, when you measure the circuit under load, you are
getting 14.6A from all 4 strings, which averages 3.65A.  I’d say check that
string again, perhaps the current clamp wasn’t zeroed or something.

First I would try reading the MPPT voltage and current for each string
individually connected to the load.  If one of the strings had a bad diode,
I’m not sure it would show up in a Voc test.  Perhaps a bad bypass diode
could give the MPPT algorithm a difficult time finding the MPPT of the
array, and it gets stuck on curve of the one string that has a bad diode.

Also, it could be more of an issue with the MPPT controller thinking the
array is a 24V Array, or some other controller failure.  I would look there.

Mike Kocsmiersky

Principal

Phone: 413 883-3144 

Spirit Solar

www.SpiritSolar.net

From: Allan Sindelar [mailto:allan at sindelarsolar.com] 
Sent: Friday, August 26, 2016 2:12 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Low array voltage stumper on legacy system

Steve,
Yes, in the same sequence: 75.5V, 73.8V, 73.9V, 74.0V. All just about what
I'd expect.

Brian,
I have seen that with KC120s over the years - good Voc and Isc but no MPP.
That's a known failure that Kyocera (to their credit) continues to warrant
15 years out. But I haven't seen it with the many 75W BPs we installed in
that same time period. So it's possible, but unlikely, especially with all
four strings behaving in a similar manner.
Thank you. I'm off on field work now, will respond to any more replies
tomorrow.

Allan

On 8/25/2016 8:24 AM, Steve Bell wrote:

Hello Allan, 

Did you check the Voc of each string?  Is it possible that there could one
or more shorted bypass diodes in the modules?

Just a thought.

Steve Bell

Morningstar tech support

On Thu, Aug 25, 2016 at 8:04 AM, Allan Sindelar <allan at sindelarsolar.com>
wrote:

Wrenches,
Here's one I haven't seen before. 1997 grid-tie battery backup system that
was one of my first, although actual installation was by another before I
was licensed. 16 BP 275, 36-cell 12V modules, the standard of that time.
Early Outback MX60 replaced C40 around 2005; array rewired to four 48V
strings of four modules at that time, serving a 24V battery bank. Tested
with an end-of-life 9-year-old AGM battery bank in the system, but in Sell
mode with a 26.4V Sell voltage setpoint.

At 11 am, 68ºF, 900W/m2 on the Daystar, with the array under charge, I
measured 3.5A, 3.4A, 2.4A, and 3.6A. Short-circuiting each string, I
measured 4.0A, 3.7A, 3.8A, and 3.7A. Original rating was 4.45A, so except
for the third string, current is what I'd reasonably expect from 20-year-old
module degradation. However, watching the periodic MPPT sweep from the MX60
at the array, the maximum power point voltage for the entire 48V array
settled at 14.6A at 37.2V, or about 540 watts from a 1,200W (originally
rated) array.

I have commonly seen the current reduce substantially with age-related
degradation, while the MPP voltage remains fairly close to original. I have
never seen the voltage drop this far - an average of 9.3V MPP per each 12V
module. All of the cells are seriously browned. I have other systems out
there with the same modules of similar age that don't exhibit this weird
behavior. Can anyone help me understand why the MPP voltage would drop so
far below what is typical?

There is a slight possibility of corroded buried input conductors, because
of a serious but corrected issue of galvanic corrosion from a ground fault
(a Romex connector pinching a conductor and allowing seepage to ground).
This was corrected years ago, but the input conductors were direct burial
for ~100' and there was never a bonding conductor between the array and the
house, so there is a potential for corrosion damage. However, the voltage
measured at both ends while charging was identical, so I think that would
indicate that high conductor resistance isn't the issue.

As usual, thank you for the collective and individual wisdom shared here.
Allan

-- 

Allan Sindelar
allan at sindelarsolar.com
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder (Retired), Positive Energy, Inc.
505 780-2738 <tel:505%20780-2738>  cell

_______________________________________________
List sponsored by Redwood Alliance

List Address: RE-wrenches at lists.re-wrenches.org

Change listserver email address & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org

List-Archive:
http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html

List rules & etiquette:
www.re-wrenches.org/etiquette.htm

Check out or update participant bios:
www.members.re-wrenches.org

_______________________________________________
List sponsored by Redwood Alliance

List Address: RE-wrenches at lists.re-wrenches.org

Change listserver email address & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org

List-Archive:
http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html

List rules & etiquette:
www.re-wrenches.org/etiquette.htm

Check out or update participant bios:
www.members.re-wrenches.org

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20160826/64fc007f/attachment-0002.html>