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

[Steve Bell]

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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