What make of panel please. Forgive if I missed it...

Dave Angelini Offgrid Solar
"we go where powerlines don't"
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
> (801) 477-0-SUN (477-0786)
> (801) 631-5584 (Cell)
> ________________________________
> From: RE-wrenches <> on behalf of
> Corey Shalanski <>
> Sent: Thursday, May 17, 2018 4:19 PM
> To:
> 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 31œC 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 55œC and 70œC. 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
> 55œC, 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
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