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Very slight mismatch in strings- what is the effect?

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  • Very slight mismatch in strings- what is the effect?

    I have a grid tie solar application in which one panel has been broken. I need to replace the one panel.

    The original specs were:
    2 strings of 11 panels (22 total) 230 watt nominal power at STC, Vmp 29.6 volts, Imp 7.78 amps, Isc 8.34 amps

    The closest match I have found has the following specs:

    225 watt nominal power at STC, VMP 29.4 volts, Imp 7.65 A, Isc 8.19 amps

    Now, if I replace one panel in one of the two strings, will the mismatch of 0.6 volts/0.13 amps matter?
    Lawrence Lile, PE

  • #2
    No not really because the replacement has a higher Imp in the series string, and the other string will will pull down the voltage .6 volts to match. Now if the Imp of the replacement panel was significantly lower, you would have something to worry about because it would limit the series string to the lowest value. Yours is higher so no worries. So in essence it is a perfect match. Good job!

    Just remember this with respect to solar panels.

    [LIST][*]In parallel; voltage is limited to the lowest value parallel string. [*]In series; current is limited to the lowest value in the series string. [/LIST]

    Final comment is a example. Let's say you found a replacement with the same Imp, but much higher Vmp. What I would do is buy 2 panels, one for each parallel string to raise both string voltages equally. But at .6 volts is no problem.

    Hope that helps.
    MSEE, PE