panel failure more common with micros?

[Zee]

I have had almost no solar panels fail over the years. The few panels that had issues seem to be connected to micro-inverters and not string inverters.
1. Does anyone notice this pattern?
2. Is there any reason for it?
3. In the most current case V-DC from panel is low. It's partially producing, but way less than it should. Why would that be?
(micro has been replaced so clearly not the culprit)

 

[solar.stevepvip]

Bad bypass diodes within the panel. Possibly from water damage from wires being too tight. Whoever replaced the micro should have checked panel output

Sent from my SM-G955U using Tapatalk

 

[jaggedben]

How many failures are we talking about? And how disproportionate are the failures to the percentage of micros you've installed?

My experience does not corroborate any problems with micros. But then, I don't really have enough examples to make a comparison. I can only think of 3 electrical panel failures in the 7 years that I would have known about such a thing in our company. They were all with micros, but the majority of our installs at that time were micros so that essentially means zilch. We had one job where a half dozen panels started showing signs of cell cracking, but the distinguishing feature in that job was the brand of panel. (It was with micros; the panels showed signs of lowered output but none of them outright failed).

Regarding your current case, if the panel voltage is ~2/3 of what it should be, you have a failed internal string of cells. (It's not bad bypass diodes. The diodes are doing their job or the entire panel would fail.) Of the three failures I mentioned above, 2 were this.

One possibility is that you have the same failures with string inverters but you just have no way of noticing them. If you have panel level monitoring with micros and a panel's output drops by a third or two thirds, you notice, or your customer does. If you have a 13 panel string and the voltage drops by one 39th, then nobody notices, or can readily prove that it isn't just dirt or the weather or panel degradation.

 

[GoldDigger]

How many failures are we talking about? And how disproportionate are the failures to the percentage of micros you've installed?

My experience does not corroborate any problems with micros. But then, I don't really have enough examples to make a comparison. I can only think of 3 electrical panel failures in the 7 years that I would have known about such a thing in our company. They were all with micros, but the majority of our installs at that time were micros so that essentially means zilch. We had one job where a half dozen panels started showing signs of cell cracking, but the distinguishing feature in that job was the brand of panel. (It was with micros; the panels showed signs of lowered output but none of them outright failed).

Regarding your current case, if the panel voltage is ~2/3 of what it should be, you have a failed internal string of cells. (It's not bad bypass diodes. The diodes are doing their job or the entire panel would fail.) Of the three failures I mentioned above, 2 were this.

One possibility is that you have the same failures with string inverters but you just have no way of noticing them. If you have panel level monitoring with micros and a panel's output drops by a third or two thirds, you notice, or your customer does. If you have a 13 panel string and the voltage drops by one 39th, then nobody notices, or can readily prove that it isn't just dirt or the weather or panel degradation.

:thumbsup: to the last speculation.

And if a panel with three bypass diodes is delivering 2/3 of its nominal voltage, there are at least three possible explanations, which can be tested if you have access to the diodes inside the panel junction box:
1. One segment of the panel has an open (cracked?) cell or an open tab or bus foil. The bypass diode will conduct to allow the other two segments to deliver power.
2. The calls and busing are all intact, but one of the bypass diodes has failed with an internal short. This is, admittedly, the least likely failure mode for a bypass diode, and would probably result from the bypass diode being chronically overheated because of partial shading on the panel.
3. The panel is partially shaded, resulting in a lower potential current from one segment of cells, but this is only likely to cause that segment to be bypassed if the panel is in a string in which the string inverter MPPT seeks the full current of the unshaded portions. . This case would then be indistinguishable from the outside from case 1.

 

[Zee]

Thanks guys.

Solar Steve,
Yeah, I should have checked panel output in the field. However, I was able to online afterwards.
It shows about 20 VDC..... vs 30 VDC for other panels in array. Enphase has identified the panel as the culprit.

JBen,
I agree totally that it is likely just panel-level monitoring that allows us to *catch* more issues on micro-inverter systems.
Whereas, in a string it wouldn't even be noticed.
Hell, whole strings have failed for months w/o anyone batting an eyelid.

(Not sure of exact numbers of failures...just an unscientific feeling for now.)

GoldDigger,
Those are excellent points as to what might cause the problem.
So it can be the diode itself....... but most likely a break in a cell or bus bar foil.

Are you saying there are panels with 1 diode and panels with 3 diodes?
Also it sounds like panels have three circuits .....and they are in series?

------------------------------------------------------
In the meantime i just ordered a similar solar panel since the manufacturer is defunct and completely unresponsive.
25 year warranty notwithstanding.
I'll pay cash and be done with it.

 

[GoldDigger]

Thanks guys.
...
...
Are you saying there are panels with 1 diode and panels with 3 diodes?
Also it sounds like panels have three circuits .....and they are in series?
...

Without bypass diodes, partial shading of even one cell can lower the current from an entire panel OR cause excess current to be forced through the shaded cell, potentially damaging it.

There is a trade off between lots of segments (yes, in series) and lots of diodes to squeeze the largest possible amount of power out of a partially shaded panel and a smaller number of segments and diodes (yes, it is always one segment per diode as each diode is in parallel with one segment).
More segments and diodes costs more, requires more bus foil within the panel and thus raises the complexity and cost.
Fewer segments means greater losses. (Only 1 segment, 1 diode means that you lose the entire output of a that panel in a partially shaded string; 2 diodes, 2 segments means you may lose half the power. The most common compromise of these competing design goals is 3 segments, 3 diodes, but other combinations can be found.

 

[jaggedben]

Typical 60 and 72 cell panels have three segments arranged in U shapes the long way. i.e. two long rows of cells make a segment. I think this saves on manufacturing since all connections end up at one end near the jbox.

One effect of this is that if a shadow is creeping up a panel from the bottom, you'll get better production with a 'landscape' mounting than a 'portrait' mounting. I find in residential design there are usually other factors that determine my layout, but sometimes it figures in.

 

[Zee]

GD and JB, thanks !

I learned something.:dunce:

Ever replace a diode yourself? Should i just throw panel away?

 

[BillK-AZ]

The primary function of bypass diodes is to protect against hot spots that can occur with shaded cells. The secondary function is to allow current to bypass an open circuited or low current string of cells and allow a longer string of modules to continue to function with a failed or shaded string of cells in a module. With string inverters and module strings of 7-13 modules, the loss of a fraction of a module will in most cases still allow the inverter to function with only a corresponding power loss. With micro inverters the loss of 1/3rd or 1/4th of the voltage will likely severely impact operation and the micro inverter may not even start up in the morning.
With only one module on a micro inverter, the conditions that cause a hot spot are not likely to happen, therefore there will be little to no penalty to simply removing a shorted bypass diode. If the bypass diodes are in module junction boxes, it is usually easy to simply clip them out.

 

[Zee]

.......
With only one module on a micro inverter, the conditions that cause a hot spot are not likely to happen, therefore there will be little to no penalty to simply removing a shorted bypass diode. If the bypass diodes are in module junction boxes, it is usually easy to simply clip them out.

Correct, many times the module won't even have sufficient V to operate the micro.

Are you saying that removing a diode returns the module to full power?

 

[jaggedben]

Are you saying that removing a diode returns the module to full power?

I think he's only saying that for a shorted diode. That's not going to fix an opened circuit on a string of cells.