Wrenches,What a delight it is when a client is so fully able to understand and follow directions! Albert, the homeowner of the 1997 system with 16 BP 75W modules on a tracker, sent the results below. In short, he isolated each string and measured input and output voltage and current as shown by the display on the MX60.
The results suggest that while two of the four strings are worse than the other two, all are severely degraded, and none put out close to original MPP voltage or current.
Given that the tracker has recently stopped operating (due to an obviously failed internal electronic component, most likely from lightning damage) and replacement controllers are no longer available, SunXtenders that have reached the end of their life after nine years of operation, and these modules, I'm not recommending rebuilding this system with new batteries and modules. I have suggested a batteryless grid-tied system, and either keeping this as a modest backup resource for occasional outages (with new batteries, of course) or using a home standby generator to meet backup needs.
And both the client and I would be interested in any observations and further conclusions that can be taken from the numbers below and in the attached spreadsheet. It appears that string three pulls down the overall output beyond what it contributes. We have one spare module, if the client chooses to keep the existing array in operation. The other three strings put out between 50% and 61% of original nameplate current rating. All have decidedly low Vmp, which at STC would be around 68V.
Thank you, Allan *Allan Sindelar* al...@sindelarsolar.com <mailto:al...@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* ** -------- Forwarded Message -------- Date: Sun, 28 Aug 2016 09:11:31 -0600 Allan,I completed the test yesterday during late morning sun. Results are shown in the attached spreadsheet. It appears that both strings 3 and 4 gave lower Vmp, although 3 was worse.
Albert
PV Array Output
Observation date: 8/27/16
Observation time: 11:10 - 11:45
Values displayed on MX-60 charge controller @ MPPT
String* Input Output
V A W V A
1 52 3.7 184 26 7.1
2 46 3.4 153 26 6
3 38 3.3 122 26.2 4.7
4 37 4.2 152 26.2 5.9
1+2+3+4 41 16 610 26.2 23.3
1+2+4 47 12.3 544 26.2 20.9
*selected using breakers on combiner box at array
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.
Replacement is my thought.
Jay
Peltz power.
On Aug 25, 2016, at 7:04 AM, Allan Sindelar <al...@sindelarsolar.com
<mailto:al...@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 --
PV-array-output_Aug2016.xlsx
Description: MS-Excel 2007 spreadsheet
_______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
