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
<al...@sindelarsolar.com <mailto:al...@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/m^2 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*
al...@sindelarsolar.com <mailto:al...@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*
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