How to find bad panel in a multiple string system

[keepsake]

I've got 40 panels feeding an old Fronius IG5100. There are 10 panels (230w) in each string (4 strings). Mid day I will at times see the production drop from like 4800 watts to 3600 watts. I have separate breakers for each string. At times, I can kill one string and see no change at the inverter level. So I know which string is flaky.
Now how to find the culprit panel or the connection. It's intermittent. When its working, if there is a panel issue, maybe I can one by one block the sunlight from part of it to inflict a pv drop. At times that string will contribute less to the pv current than the other strings. I have a clamp on dc meter. I don't think that helps me at all. I have checked all connections at the panels, nothing found. I'd rather not try to measure each panel on a meter by itself.

Just looking for thought I haven't had yet !!

 

[AHTrimble]

I just did trouble shooting on a 24 panel system.
I tested each panel for the voltage, then each panel for amps. One stood out with significantly lower numbers. Hence, the "bad" panel...or at least the panel not keeping up with the others.

 

[keepsake]

My thought is the panel failing will show the least change, from a blocking of the sun action, on the entire string output. aka its already under producing and I am trying to flag it with the least effort on the hot roof.

 

[Shimmy]

Have you measured the isolated "bad" string's open circuit voltage? Have you isolated the "good" strings and looked at voltage and current on the string when running to the inverter?

You could try a non-contact temperature sensor for connections, but it isn't easy; a thermal camera is more likely to be helpful, both for connections and the panel itself.

Barring that, it is breaking down the suspect string and checking each panel-- open circuit voltage and short circuit current at a minimum. There are also handheld mppt panel testers that can give you a sense of degradation that might not show up with other tests.

First thing to confirm is that your inverter is not being curtailed by either settings or the utility...

 

[Shimmy]

I have never tried it, but I wonder if a low voltage non-contact voltage sensor would work to see a bypass diode in action.

 

[keepsake]

I have seen a 275v open ckt when the others strings are like 325v. But an hour later its back to normal.

 

[keepsake]

Have been up on the metal roof this past hour with a roofing guy. Florida 33523. Panels went up almost 12 years ago and about a 10 degree tilt. Every southerly panel edge has triggered rust to form on the roof below. About 5 inches from alum frame to roof surface. Seems like aluminum residue, contaminates rain water flow and causes a chemical reacting with the roofing deck. I am told its Galvalum. Its getting pretty bad in many places. Anybody have their own long term metal roof stories ?

 

[keepsake]

I guess I should start a new thread

 

[Shimmy]

Anybody have their own long term metal roof stories ?

Well, I wanted an aluminum standing seam roof, but the house has copper flashing and the cost of replacing that with aluminum was almost the same cost as going to a copper roof. Didn't want a copper roof though... so silicon coating it was.

The magic of galvanic reactions.

As for your original issue, I would be checking your wiring that is just laying on the roof to see if you have any insulation nicks; sometimes at high humidity periods you will get a low level ground fault if beasts (or wind) have been gnawing at the wire.

 

[dcg9381]

I just did trouble shooting on a 24 panel system.
One stood out with significantly lower numbers.

Both volts and amps, correct? Because voltage is a lot easier to get at..

 

[keepsake]

Well, I wanted an aluminum standing seam roof, but the house has copper flashing and the cost of replacing that with aluminum was almost the same cost as going to a copper roof. Didn't want a copper roof though... so silicon coating it was.

The magic of galvanic reactions.

As for your original issue, I would be checking your wiring that is just laying on the roof to see if you have any insulation nicks; sometimes at high humidity periods you will get a low level ground fault if beasts (or wind) have been gnawing at the wire.

Are you saying that a silicon coating is a way I can retard further rusting as in those photos? Can I just spray something on there? Does the ospho rust remover work? It is supposed to meld with the rust on a surface and turn into something that is paintable.

 

[Shimmy]

Are you saying that a silicon coating is a way I can retard further rusting as in those photos? Can I just spray something on there? Does the ospho rust remover work? It is supposed to meld with the rust on a surface and turn into something that is paintable.

My original roof was rolled asphalt, but my understanding is that you would need to completely remove the rust first and then you might stand a chance. I think there are high performance paints that can be applied after-market as well... but the sad reality is that it is likely cheaper to pull the steel and put in aluminum with a 20-year paint warranty.

 

[AHTrimble]

Both volts and amps, correct? Because voltage is a lot easier to get at..

This was my post...

I just did trouble shooting on a 24 panel system.
I tested each panel for the voltage, then each panel for amps. One stood out with significantly lower numbers. Hence, the "bad" panel...or at least the panel not keeping up with the others.

So yes, I tested both volts and amps as stated. And the numbers were notably off. Hence the bad panel.
The strings were made up of 4 panels each...3 read similar, 1 was way low on amps, slightly lower on volts. Bad panel, mystery no more, problem solved.

 

[keepsake]

My original roof was rolled asphalt, but my understanding is that you would need to completely remove the rust first and then you might stand a chance. I think there are high performance paints that can be applied after-market as well... but the sad reality is that it is likely cheaper to pull the steel and put in aluminum with a 20-year paint warranty.

So I guess from my photos ... wire brush rust where it is visible and paint will prevent the dripping rain from rusting anymore ? Or do I need to mess with silicon.

 

[sunshine_eggo]

This was my post...

I just did trouble shooting on a 24 panel system.
I tested each panel for the voltage, then each panel for amps. One stood out with significantly lower numbers. Hence, the "bad" panel...or at least the panel not keeping up with the others.

So yes, I tested both volts and amps as stated. And the numbers were notably off. Hence the bad panel.
The strings were made up of 4 panels each...3 read similar, 1 was way low on amps, slightly lower on volts. Bad panel, mystery no more, problem solved.

This is essentially the main way most identify issues - outlier behavior. We all learned this process while watching Sesame Street.

It comes in handy in all aspects of these systems...

DIY battery cells
Whole PV strings (different voltage/current between strings suggest a problem with the components)
Individual panels in a string
Different sensor values (MPPT, inverter and shunt measured voltages)

Probably several other things that slip my feeble mind.

 

[timselectric]

This is essentially the main way most identify issues - outlier behavior. We all learned this process while watching Sesame Street.

Be careful.
I'm pretty sure that the cookie monster will also eat waffles.

 

[sunshine_eggo]

Be careful.
I'm pretty sure that the cookie monster will also eat waffles.

I hear he's a type 2 diabetic, and he only eats vegetables...

 

[summit]

My thought is the panel failing will show the least change, from a blocking of the sun action, on the entire string output. aka its already under producing and I am trying to flag it with the least effort on the hot roof.

did you find your bad panel ? seems most likely the low voltage string. My first thought is to cut out a piece of cardboard the size of a panel, then cover one at a time while checking the string Voc.

 

[chrisski]

To troubleshoot, I shade a single panel in the string one string at a time, while watching the total power output, if the power output total drops that string is good, but if the power output stays the same when I Shad a panel I have found the bad string. Once I found the bed string, I disconnect the panels one at a time and check connectors. If the connectors are bad like broken bent recessed corroded, I fix the connectors. if the connectors a good I check for good voltage. I keep going down the string till I've done that with all the bad panels.

Unfortunately this will only work if you can access all the connectors in a string. My my 3s 3p portable panel system is easy to troubleshoot this way, a rooftop system might not be.

 

[Shimmy]

So I guess from my photos ... wire brush rust where it is visible and paint will prevent the dripping rain from rusting anymore ? Or do I need to mess with silicon.

The aluminum from the PV and the steel from the roof will react via rain water. My copper flashing will lose enough copper to corrode the steel rebar in the concrete slab below, despite not being in the direction of mass flow.

I'd talk to a pro, but I am guessing you need to at least coat the steel with something that will prevent the two from reacting.