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Grid tied solar array has voltage but no current

Hello Everyone,

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 (the inverter wasn't showing any error codes) - 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 checked the line voltage - its OK (244V (122V each leg) and 60 Hz).

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

The last thing I've tried was 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?

Any Idea , Suggestions would be appreciated,

Thanks,

  • Registered Users Mitglied
    Employee Mitarbeiter

    Hi  thanks for your question. Maybe  or  might be able to help. 

  • Dear RyanCaleb,

    I know, this is really annoying.
    So, let’s troubleshoot the problem step by step.
    The system is formed of three main parts: the panels, the inverter, and the cables.
    Based on your tests and trials, the inverter seems to be working and the cables, as well. Otherwise, see the last note.
    So, the focus is to be directed to the panels.
    Getting a voltage from a PV string of several panels without a current has several possible reasons. These reasons include:
    1- All panels operate at their open circuit voltage with no output current
    2- Damage of one (or some) of the panels
    3- High shading is affecting one (or some) of the panels
    4- Faulty bypass diodes in one (or some) of the panels

    Regarding the first reason, the string output voltage should be 20×21.3=426 V. However, the inverter measures 395 V. This would yet mean that the maximum power tracking of the inverter is not working well. Nevertheless, let's assume that this reason is not of concern now.
    The second, third, and fourth reasons are mutually related. They arise basically from the physical structure of the PV panels with their bypass diodes.
    As per the datasheet, the used panel is equipped by a bypass diode. This bypass diode targets bypassing any panel when its produced current is less than the string current. This is to avoid hot spots and panel damage. The less produced current would be due to relatively higher shading effect or deteriorated panel performance. So, the second and third reasons are, so, clarified. For the fourth reason, the bypass diode itself could get damaged.
    In large PV plants, thermal photographing would be sufficient for determining the hot spots and the faulty locations. This is usually not suitable for individual applications.
    Accordingly, I would suggest doing the following to troubleshoot the problem.
    Try bypassing the panels one by one and watch the inverter display. This means that you bridge the first panel (remove it from the string) and keep the rest working and then watch the inverter. This is to be repeated until you detect the faulty panel.
    Another option is to measure the resistance between the panel positive and negative terminals. If the resistance is low, this means that there is a problem with the bypass diode. This option is, however, of limited capability. So, I would recommend following the aforementioned procedure of disconnecting the panels one by one. In a later stage, we would discuss, how to replace the eventually defect bypass diode.
    I hope, this would be helpful. I will be happy to hear once again from you.
    Greetings,
    Moustafa

    P.S.: I have some concerns with the maximum power tracking of the inverter. From my view point, its efficiency is not good at all. Let’s think of it later.
  • Als Antwort auf Moustafa Adly:

    I agree with Moustafa. One faulty solar panel "chokes" the output from all others in the string, they will produce voltage normally but little or no current. If all panels are OK it's likely an inverter issue.

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