Blown Bypass Diodes

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  • nwdiver
    Solar Fanatic
    • Mar 2019
    • 422
    #1

    Blown Bypass Diodes

    I have a 'toy' array in my back yard. I call it a 'toy' because it's really just there for me to mess with... to get a feel for how shade, and other configurations effect it and to compare PWM and MPPT charge controllers. I've got a 10.7kW grid-tie on my roof that provides all the energy I need. The ability to run a few things off the 8 panels in my back yard is just a perk.

    I previously had 2 sets of 2 in parallel but recently added 4 panels so I was running 8 60 cell panels in series. They were arranged with 4 in the front and 4 in the rear... mostly in the same orientation.

    I happened to notice that the MPP of the 8 panel string was ~120vdc. Which was a bit low since the MPP of each panel should be ~25v. Come to find out that the rear 4 panels were only contributing ~20% of the power. A FLIR image revealed several hot cells and a very hot J-box which was actually slightly deformed. I removed the bypass diodes from one panel and they were all shorted... I'm assuming the other 3 panels are the same. Panel appears to still work so I might just replace the diodes and carry on.

    Anyone have any theories? Lightning? Was the orientation off enough that somehow the front four were driving the rear four and over heating the bypass diodes? There is shade. Could the bypass diodes have overheated?

    These are 255w Astronergy panels...

    FLIR0051.jpg
    Last edited by nwdiver; 06-23-2019, 05:35 PM.
    Tags: None
    • ButchDeal
      Solar Fanatic
      • Apr 2014
      • 3802
      #2
      I have seen a few pv modules with blown diodes. It happens and often depends on the quality of the diodes. What MPPT are you running with a voltage so high as 200+v? That shouldn't contribute to the diode failure but just wondering.
      OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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        • nwdiver
          Solar Fanatic
          • Mar 2019
          • 422
          #3
          Originally posted by ButchDeal
          I have seen a few pv modules with blown diodes. It happens and often depends on the quality of the diodes. What MPPT are you running with a voltage so high as 200+v? That shouldn't contribute to the diode failure but just wondering.
          Morningstar 600v charge controller; I just think it's odd that I've never seen a blown bypass diode before and now I have ~12 all in a row.... Something happened.

          These are old panels that had been sitting in storage for ~3 years....

            Comment

            • ButchDeal
              Solar Fanatic
              • Apr 2014
              • 3802
              #4
              Originally posted by nwdiver

              Morningstar 600v charge controller; I just think it's odd that I've never seen a blown bypass diode before and now I have ~12 all in a row.... Something happened.

              These are old panels that had been sitting in storage for ~3 years....
              You can get cascade events where one blows and throws some spikes as it goes, causing the next few to go..
              OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

                Comment

                • bcroe
                  Solar Fanatic
                  • Jan 2012
                  • 5198
                  #5
                  Panel issues I have seen (from their first app) were working OK at lower sun intensity, but sections
                  going into bypass at maximum sun and output. I suppose this condition might lead to shorting
                  bypass diodes, the diodes I have seen were marginal for the job. Once failed, the condition will
                  be present all the time. I like to see a heat sink on a 10A diode. Bruce Roe

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