Grid tied solar array has voltage but no current

lindq530

I have a roof mounted grid tied system: 20 each Sharp ND-L3E1U panels (123W panels) connected in series and a Sunny Boy SWR2500U inverter. It's worked flawlessly for the last 15 years.

A couple of weeks ago I noticed that the the inverter display indicated that the array was producing 395V and 0 watts (I use the old fashion monitoring system of looking at the inverter display once a week or so). The voltage was consistent with historic values, but obviously the watts was not. Everything appeared to be working - except that I wasn't getting any power

I tried "rebooting" the system by turning off the AC and DC inputs. That didn't changing anything. I was fortunate to get an identical SB2500 inverter a few years ago form a working system that was being upgraded - I replaced the inverter and the results were the same - about 395 volts and no watts. I tested the continuity of the wires from AC connection to the house panel and the inverter to the roof - the wires are OK.

I visually inspected the panels and wiring connections and did not see any obvious issues. 

The last thing I've tried is disconnecting each panel from the array and testing Voc and Isc using my multimeter. Every panel's output voltage was close to the the manufacturer's spec of 21.3V. Current output varied from 3-6A for each panel (I tried to measure the output in direct sun, but that was hard due to time constraints - spec is 8.1A). I have not tested each panel under load (because I'm trying to avoid that until its absolutely necessary).

I've read on this and other forums that I might have a panel that has failed. If so, the failed panel is letting a small amount of current pass so I get voltage, but not enough current to pass to produce measurable watts.

Any suggestions about what is causing the issue?
If it is a bad panel, any suggestions on how to identify the bad panel without dismantling and load testing each panel?

Thanks in advance for your comments and suggestions.

mcgivor

Have you checked the grid voltage and frequency, any inverter error codes? To search for a bad panel a pair of mc4 cables could be used to bypass one panel at a time, remember to disconnect the load to prevent possible arcing.

lindq530

mcgivor said:

Have you checked the grid voltage and frequency, any inverter error codes? To search for a bad panel a pair of mc4 cables could be used to bypass one panel at a time, remember to disconnect the load to prevent possible arcing.

Thanks for the suggestion. I checked the grid voltage and frequency - they were OK (242V total (each leg was OK also) and 60.5Hz).

I may be at the point of isolating each panel with bypass cables. It will be a pain in the behind because of the how the panels are arranged, but it might be my last and final option.

lindq530

Update:

I measured the Isc of the total array (18 panels are still on the roof - I disconnected two panels to provide safe access) - measured about 6.5 amps with my clamp on meter. Voc was about 330V. That matched panel specs.

I finally resorted to the hardest option yet - bypassing individual panels. I thought I'd find a bad panel - when the bad panel was removed from the array I'd get watts. I used a jumper cable to bypass each panel individually. Each time I would bypass a panel, I would restart the system (it took about 4 hours). Each time the system voltage would be about 18 volts lower (as expected because of the array missing a panel) and still zero watts. It appears its not a bad panel. 

When I measure the voltage of the array leads (with the inverter connected and on) I get:
Pos-neg: 330V
Pos - ground: 330V
Neg - ground: 0.0V
It doesn't appear I have a ground fault.

Everything points to the inverter being bad. I'm having trouble accepting that because one of the first things I did was replace the inverter. It seems very unlikely that two separate inverters would have the same issue.

I'm out of ideas.