Corey, What make of panel please. Forgive if I missed it...
Dave Angelini Offgrid Solar "we go where powerlines don't" http://members.sti.net/offgridsolar/ e-mail offgridso...@sti.net text 209 813 0060 > Corey, > > I have been studying IR diagnostics quite a lot, but I haven't had a whole > lot of opportunity to "get my hands dirty" with it, so I'm not sure how > much help I'll be. I would like to know how many modules are in each > string. > > I'm going to share some random thoughts on this, but nothing here is at > all conclusive, and if anyone can correct some flaws in my logic, I'd love > to hear it! > > It doesn't look like a simple, obvious problem. Your curve trace has a > predicted Isc, with a dampening current to the knee, it has a sharp knee, > then it has a low voltage at Vmp, and it stays really low all the way to > Voc. A high series resistance would explain the dampening current to the > knee, but it wouldn't explain the low voltage at all. If there was high > resistance in the cells, you should see signs of diodes activating unless > all of the cell groups had relatively uniform resistance. > > Generally, the hot cells in the checkerboard pattern are underproducing > compared to the cooler cells. Because they can't pass on as much current, > they are actually wasting some of the energy that the other cells are > producing. This wasted energy turns into heat. The often times, they waste > more energy than they are producing themselves. > > > Alternatively, there could be other issues (or damage) at the hot cells. > > The fact that the checkerboard pattern goes away when you shut off the > inverter may be helpful. If you short out a module, it will almost always > look like a checkerboard in an IR shot, and if a diode shorts out, that > third of the module will generally look like a checkerboard. > > This is the confusing part... Because the checkerboard appearance goes > away, that would most likely indicate that it's not an issue of a short > circuit failure (such as short-circuited diodes, or short-circuited > modules). On the other hand, your curve trace looks somewhat like what > you'd expect if you had some short-circuited diodes. > > It looks like you have a calculated Voc of around 860, and a measured Voc > of around 725. That's about 84%. My initial first guess based on this > curve trace alone would have been that perhaps roughly 16% of the cell > groups might be shorted out. So if you had 20 modules per string, that > would be 30 cell groups (assuming 3 diodes per cell group). With these > hypotheticals, you would have somewhere around 5 shorted cell groups. > > > > > > > > Now that I'm looking at this closer, the knee of the curve is very quite > sharp. I wonder if this could be caused by a higher resistance short > somewhere out in the array (or in a conduit somewhere). (Perhaps this > could be from compromised insulation or water intrusion somewhere??) > Assuming that this isn't an intermittent problem, this would be easy to > test under Voc. Just take a good DC amp clamp into the array while the > inverter is shut down, and test the PV wires several places to try to find > current in the array. > > Under this hypothesis, as the curve tracer starts pulling current, and the > measured voltage falls from Voc, it gets to a point where suddenly, the > fault isn't passing as much current. This would cause the sharp knee on > the curve trace. As the voltage gets lower, the current at the fault would > also get lower and the measured curve would increasingly get closer to the > predicted curve. This model would also explain why the checkerboard > phenomenon goes away when you turn off the inverter... it would be because > the current going across the fault would be much lower than Isc, so when > it's resting, there isn't enough current flowing to produce the > checkerboard effect. Using this hypothesis, the checkerboard phenomenon > might not be indicating a problem in the modules at all. If there is a > high resistance short somewhere in the array, then that would cause the > actual current in the modules to be above the predicted Imp because the > current is being lost before it gets to the inverter. Because nearly all > modules get the checkerboard phenomenon at Isc, it wouldn't be surprising > if many normal modules would also get the phenomenon if they're operating > between Imp and Isc. > > > > Again, take these thoughts with a grain of salt. If anyone sees somewhere > that I'm going wrong, I'd love to hear from you! Hopefully, something here > helps! > > Thanks, > Kienan > > > Maxfield Solar > maxfieldso...@hotmail.com<mailto:maxfieldso...@hotmail.com> > (801) 477-0-SUN (477-0786) > (801) 631-5584 (Cell) > ________________________________ > From: RE-wrenches <re-wrenches-boun...@lists.re-wrenches.org> on behalf of > Corey Shalanski <co...@joule-energy.com> > Sent: Thursday, May 17, 2018 4:19 PM > To: RE-wrenches@lists.re-wrenches.org > Subject: [RE-wrenches] PV Cell Temperature Variation > > On a recent utility-scale PV installation we noticed a strange phenomenon > on a single string on a single inverter. An IV curve (attached) reveals > that the measured performance characteristics for this string (solid line) > are significantly less than the expected values (dotted line). For > reference, the ambient temperature was 31C and the irradiance was 1010 > W/m. > > The phenomenon that more so caught our attention is apparent on a thermal > image (also attached) of the modules in this string. For lack of a better > term I would describe the distribution of cell temperatures as resembling > a "checkerboard" or "scattershot" (random) pattern, ranging between > roughly 55C and 70C. Interestingly this phenomenon was only apparent > while the inverter was operating, i.e. with the inverter turned off the > modules revert to a much more uniform temperature distribution nearer to > 55C, instead varying by only a couple degrees across the entire > module/string. > > There was no apparent physical damage to the modules. > > Can anyone offer any suggestions about what might be causing this > phenomenon? > > -- > Corey Shalanski > Joule Energy > New Orleans, LA > _______________________________________________ > 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 > > _______________________________________________ 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
