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

Thu Aug 25 23:12:08 PDT 2016

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 <mailto: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/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*
>     allan at sindelarsolar.com <mailto: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*
>
>
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