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 

The inverter did not show any error codes. 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 tested the line voltage and hertz - both are ok (244V and 60.5Hz).

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.

Photowhit

You could put a small load that would handle the voltage. perhaps a 12volt fan, and plug it into each to see how it will handle a load. 

lindq530

Update on diagnosis:

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.

Aguarancher

Are any of the inverter led's on or showing fault?

you can read through this: https://www.manualslib.com/manual/813653/Sunny-Boy-Swr-2500u.html?page=27#manual

Photowhit

These are low wattage panels, just out of curiosity, did you check the amperage in front of the inverter with an open circuit? It's only 7 amps, handheld meters should be able to handle this, Though perhaps others will chime in on doing this with such high voltage? Not sure if it could arc internally?

lindq530

Aguarancher said:

Are any of the inverter led's on or showing fault?

you can read through this: https://www.manualslib.com/manual/813653/Sunny-Boy-Swr-2500u.html?page=27#manual

No faults shown on the inverter. The green LED is on and constant (indicating everything is OK).
Thanks for the link. My manual is dog eared from being read cover to cover many times.

lindq530

Photowhit said:

These are low wattage panels, just out of curiosity, did you check the amperage in front of the inverter with an open circuit? It's only 7 amps, handheld meters should be able to handle this, Though perhaps others will chime in on doing this with such high voltage? Not sure if it could arc internally?

I'm a little embarrassed to admit that I tried to measure the array amperage (in front of the inverter) with my probes on the open circuit. Bright flash and a little smoke. Yikes.

I tested the Isc by disconnecting the DC conductors from the inverter, shunting them, and using my clamp on meter to measure the Isc when the sun came up. Got about 6A.

Do you think there would be a difference in current if I measured through a meter versus a clamp on with a short circuit?

mcgivor

So far you've checked the DC side, which appears good, replaced the inverter with a known working unit which resulted in no change, it's looking likely that there is an issue with the utility side. Conventional instruments, multimeter etc. can only provide a small amount of information, a power quality analyzer, or spectrum analyzer would be needed to see if the voltage is stable without sags or swells, frequency is stable without superimposed waveforms, harmonics are not excessive and so forth. Sometimes the crimp connections on the aluminum overhead cables go high resistance, has there been any development in the area such as new construction. Having the utility check /analyze the incoming lines may be a good starting point, you did say the frequency is 60.5 Hz, that is the high limit for the inverter, could it be fluctuating, just some thoughts perhaps worth considering.

lindq530

mcgivor said:

So far you've checked the DC side, which appears good, replaced the inverter with a known working unit which resulted in no change, it's looking likely that there is an issue with the utility side. Conventional instruments, multimeter etc. can only provide a small amount of information, a power quality analyzer, or spectrum analyzer would be needed to see if the voltage is stable without sags or swells, frequency is stable without superimposed waveforms, harmonics are not excessive and so forth. Sometimes the crimp connections on the aluminum overhead cables go high resistance, has there been any development in the area such as new construction. Having the utility check /analyze the incoming lines may be a good starting point, you did say the frequency is 60.5 Hz, that is the high limit for the inverter, could it be fluctuating, just some thoughts perhaps worth considering.

Thanks for the detailed response. I will attempt to request that PG&E (our electric utility) come a check the power. Based on previous experience, I'm not very hopeful that anything will happen.  

I believe I've done a very thorough check of the DC side. The last thing I can do is remove each panel and load test them individually.

With regards to the replacement inverter - I'm assuming that the person that provided me with the inverter was telling the truth. It seems very unlikely that my replacement inverter would have the exact same issue as my existing inverter, but it is possible. That would be terrible luck.

Does anyone have a suggestion for how to have my inverter professionally tested?

Photowhit

I like @mcgivor 's thoughts, the inverter will need a voltage and cycle rate in a certain ball park to sync.

mcgivor

An independent contractor who specializes in the field could run an analysis of the incoming power, at a cost of course, the utility should, in my opinion check, if a trouble ticket is filed under suspected fault, some SMA units have an arc fault detection which shuts the inverter off, I'm not 100% knowledgeable on the particular units, just what I've read.
http://www.smainverted.com/tech-talk-five-tips-for-afci-troubleshooting/

lindq530

Problem solved. It was a failed inverter. Argh.

Long story:
After measuring voltages, wire continuity, and currents (the easy stuff), everything indicated it was a failed inverter. Luckily (so I thought) I had an identical model replacement inverter. I swapped inverters and the problem didn't go away. Based on that I concluded (incorrectly) that the inverter wasn't the issue. That led me to further about 10 hours of work checking of wiring, individual panels, etc.

After I was exhausted checking everything else and not finding any issues, I decided to purchase a replacement inverter. Installed the new inverter and voila - the system worked great. 

I hope my experience can be helpful for someone. Thank you for everyone's responses.

BB.

I glad you found the problem... It is frustrating when you think you have done everything--And it turns out you had a bad replacement part.

-Bill