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

[Jay]

Hi Allan,

I would try turning off string #3 and see what happens to the vmp. 
I'm guessing it'll go up to a more normal voltage. 

I just had a very similar experience, also BP modules, very brown. 
2 sets of 4 billion n series to a 12v system. One set was totally dragging down the VMP, I disconnected it, all normal.  I tested voc of all modules after they were replaced,(190 watt 72 cell) customer wanted more watts, and all were normal and the same. I didn't spent the time to do a ISC. 

Replacement is my thought. 

Jay
Peltz power. 








> On Aug 25, 2016, at 7:04 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 cell
> 
>  
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