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

[Brian Mehalic]

If it is a module issue it could be bad solder joints, shorted diodes, or a
combination of both.  I just curve traced 12 12-year old KC120s - all
except 2 tested fine for Voc with a DMM (2 were totally dead).  However
with the curve tracer they all exhibited more of an IV "angle" than a
curve, going straightish from expected Isc to a Voc that represented only
1/2 a modules worth of voltage.  When we added it all up it made sense
based on the MPP that we initially observed the inverter to be operating
at.  They'll be coming off the roof soon and we'll be able to do better
testing (and check the accessible diodes).



Brian Mehalic
NABCEP Certified Solar PV Installation Professional™ R031508-59
520.204.6639

Solar Energy International
http://www.solarenergy.org

SEI Professional Services
http://www.seisolarpros.com


On Thu, Aug 25, 2016 at 8:24 AM, Steve Bell <sbell at morningstarcorp.com>
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 <505%20780-2738> cell*
>>
>>
>>
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