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

[Allan Sindelar]

Mike,
Jay's suggestion to turn off string three, the low one, is good. Plus I 
can guide the homeowner by phone, thus avoiding the cost of another site 
visit, so that adds appeal.

I just spoke with him. While he lacks the DMM and DC amp clamp, he is 
going to record V and I in and out for each string, using the MX60 
display as his data source. I'll post any noteworthy results here. We're 
in monsoon season (afternoon cloud buildup and thunderstorms) so it may 
not be for a few days.

Re Jerry's comments: I think that you mean an I/V curve tracer test, 
rather than Solmetric, right? That's not a tool I have or will get for 
my current work, but it's a good idea, thanks. The client and I are 
pretty much in agreement that we're dealing with serious browning - 
indeed, more than I have ever seen with BP 75W modules of this vintage 
(my own home's 18 BP590 modules of similar age show very little browning 
and about 15% degradation, rather than 50%) and aren't worth the cost of 
a whole bunch of testing. Mostly, we're looking for any specific and 
correctable cause, as we have one spare matching module in storage, and 
I also simply posted to seek answers as to why the Vmp was so low.

Jerry, it was selling, so the voltage would be held around the sell 
level of 26.4V. The inverter is set to the default sell AC amps, 35A as 
I recall and the MX60 is defaulted to 60A output, so they aren't causing 
any restrictions. The batteries are known to be used up - indeed, we 
want to better understand the array issue in order to help determine 
whether to replace the SunXtender AGMs for backup power or just leave 
the system in a state of benign neglect. The client is considering our 
suggestion to install a separate grid-tied batteryless system once he 
has lived in the home for a year or so to determine consumption. This 
legacy system would pretty much just be backup, as with hardware that 
old a new array as well as batteries is hard to justify.

Thank you to all. More to come later if we learn anything worth sharing.
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 cell*

**

On 8/26/2016 10:14 AM, Mike Kocsmiersky wrote:
>
> Your measurements of the Isc for the four stings seems to be odd in 
> string 3 with only 2.4A.  However, when you measure the circuit under 
> load, you are getting 14.6A from all 4 strings, which averages 3.65A.  
> I’d say check that string again, perhaps the current clamp wasn’t 
> zeroed or something.
>
> First I would try reading the MPPT voltage and current for each string 
> individually connected to the load.  If one of the strings had a bad 
> diode, I’m not sure it would show up in a Voc test.  Perhaps a bad 
> bypass diode could give the MPPT algorithm a difficult time finding 
> the MPPT of the array, and it gets stuck on curve of the one string 
> that has a bad diode.
>
> Also, it could be more of an issue with the MPPT controller thinking 
> the array is a 24V Array, or some other controller failure.  I would 
> look there.
>
> Mike Kocsmiersky
>
> Principal
>
> Phone: 413 883-3144
>
> *Spirit Solar*
>
>
Can you do a solmetric type test, when you performed the amp test the 
volts were at near "0" so the test does not show an accurate value, you 
said the modules are brown this is not good but are they checked because 
if so you are done needs new modules. I would suggest doing some 
additional testing, is it selling, and at what amount, whats going into 
or out of the batteries at that time, what inverter is it, trace 24 I 
would assume, if it is selling is the inverter capable of selling more 
than it is or is at the limit, I need more numbers to get a better 
understanding if there is even a problem. You can adjust "Sell" down to 
perform a test and see if the DC climbs, also you can set the MX amp 
down too.
Jerry
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