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. Id say check that string again, perhaps the current clamp wasnt 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, Im 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 www.SpiritSolar.net From: Allan Sindelar [mailto:[email protected]] Sent: Friday, August 26, 2016 2:12 AM To: RE-wrenches Subject: Re: [RE-wrenches] Low array voltage stumper on legacy system 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 <[email protected]> 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 [email protected] 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 _______________________________________________ List sponsored by Redwood Alliance List Address: [email protected] Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/[email protected]/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Redwood Alliance List Address: [email protected] Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/[email protected]/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
_______________________________________________ List sponsored by Redwood Alliance List Address: [email protected] Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/[email protected]/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
