Busted bypass diode sanity check

[NJFred]

I have 33 (3 strings of 11) 300W 72 cell Canadian Solar panels each ran through Tigo Optimizers (which I love by the way) to an SMA 8000 inverter and one of the panels suddenly started putting out ~2/3 voltage, no change in current. Obvious suspect was bypass diode, in this case expected shorted based on my understanding of how panels with diodes are supposed to operate.

I took the panel down, removed all the gunk in the junction box to get to the three bypass diodes (only has 3 diodes in there, so apparently no blocking diode), to measure diodes but these measurements confuse me.

Looking at each string of cells on the panel (measuring across each diode) I get the expected short current and open voltage, indicating string of cells themselves should be ok. Measuring diode for continuity I get conduction in the expected direction and open in the reverse direction.... I tried to find a suitable load to hook across the panel itself to emulate what it does on my roof (should be ~37V DC at ~8A under full load or ~4.7 Ohms) but closest thing I had available was a 100W light bulb which measured ~20 Ohms with multi meter. With this load the panel still usually behaves ok (voltage split evenly between 3 strings and expected current) but I believe sometimes I actually measure 0V across one of the 3 strings and higher voltage across others but not usually.......

Is this at all reasonable or am I messing up my measurements? I thought this would be extremely simple, just find and replace shorted diode, I did not expect panel, or rather diodes on panels to have a mode or state when they would short out and stay there for any reason and then recover. Is this something reasonable, should I replace the suspected diode, or is this unheard of and I should just replace the entire panel.... I have fairly extensive experience with integrated circuits, as I do integrated circuit design for a living, and this behavior looks like what we refer to as latch up, but I thought this was just a diode.

[inetdog]

I have 33 (3 strings of 11) 300W 72 cell Canadian Solar panels each ran through Tigo Optimizers (which I love by the way) to an SMA 8000 inverter and one of the panels suddenly started putting out ~2/3 voltage, no change in current. Obvious suspect was bypass diode, in this case expected shorted based on my understanding of how panels with diodes are supposed to operate.

I took the panel down, removed all the gunk in the junction box to get to the three bypass diodes (only has 3 diodes in there, so apparently no blocking diode), to measure diodes but these measurements confuse me.

Looking at each string of cells on the panel (measuring across each diode) I get the expected short current and open voltage, indicating string of cells themselves should be ok. Measuring diode for continuity I get conduction in the expected direction and open in the reverse direction.... I tried to find a suitable load to hook across the panel itself to emulate what it does on my roof (should be ~37V DC at ~8A under full load or ~4.7 Ohms) but closest thing I had available was a 100W light bulb which measured ~20 Ohms with multi meter. With this load the panel still usually behaves ok (voltage split evenly between 3 strings and expected current) but I believe sometimes I actually measure 0V across one of the 3 strings and higher voltage across others but not usually.......

Is this at all reasonable or am I messing up my measurements? I thought this would be extremely simple, just find and replace shorted diode, I did not expect panel, or rather diodes on panels to have a mode or state when they would short out and stay there for any reason and then recover. Is this something reasonable, should I replace the suspected diode, or is this unheard of and I should just replace the entire panel.... I have fairly extensive experience with integrated circuits, as I do integrated circuit design for a living, and this behavior looks like what we refer to as latch up, but I thought this was just a diode.

You have left out one likely circumstance: That there is a problem with one or more cells in a protected section which is not limiting current when you are doing the short circuit test, possibly with cool panels, but is limiting the current when the panel gets hot.
If one section of the panel produces a lower Isc/Imp current than the others, then the corresponding bypass diode will conduct when the panel is under a load higher than the available current of the defective cell(s). The Voc will still be normal.

If you measure the normal Vmp and Imp from the panel when the optimizer is connected, but the output from the optimizer is only 2/3 of nominal power, then you have a defective optimizer.
If the Vmp is 2/3 of the nominal Vmp, then one bypass diode is conducting. And if the diode is not shorted, it must be conducting because of a defect in that section of the panel.

It would be theoretically possible for a bypass diode to break down at a voltage just above the 1.5V or 3.0V test voltage used by a multimeter but still below the 10-15V that the section should normally produce. That will be hard to test for except by metering inside the junction box while the panel is misbehaving.

Either a diode fault or a panel fault could be temperature or mechanical stress dependent and so appear different when you try to test the panel/optimizer pair.

[Mike90250]

Your bypass diodes are good, but there is a segment of your panel that is bad. Bummer.

Good luck on the warranty, since you opened it up.

[inetdog]

Your bypass diodes are good, but there is a segment of your panel that is bad. Bummer.

Good luck on the warranty, since you opened it up.

That still is hard to reconcile with seeing full Isc from each individual panel section when connecting across its bypass diode. It would have to be an intermittent problem of some sort.