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Thread: Unbalanced Strings - loss calculation

  1. #11
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    I certainly do understand Voc measurements aren't what you project forward for system production.
    The real reason for my origonal post was more academic than anything else. I do plan to replace the non functioning panel. Again, academically I had figured that the panel replacement wasn't too critical as to matching electrical characteristics, as long as Imp & Vmp are equal or greater,and the same basic setup,IE neg ground etc.. I thought greater would be a tiny advantage?

    I was curious and went back to the site to get Vmp measurements;

    string of 11 - 288Vmp
    string of 11 - 288Vmp
    string of 10 - 260Vmp
    combined - 280Vmp
    These measurements are averages of the displayed readouts that fluctuated continuosly.

    The inverter readouts Were a little higher than my tester.

    So according to this limited test of a moment out of the year, and following the simple formula of V x A = W, having an unbalance of the strings doesn't seem to adversly affect efficiency very much in this case.

  2. #12
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    Yes, not surprisingly the combined Vmp is closer to that of the intact source circuits. The MPPT algorithm is going to lock in on a Vmp that is set by those longer strings. The systemic inefficiency results from the fact that Vmp of the string of 10 modules does not equal the Vmp of the of two other source circuits, as you have measured.

    So what happens when you operate a source circuit off of its Vmp?

    The answer is that that source circuit no longer outputs its maximum power.

    In this case, something like 30% of the array is being operated off of its maximum power point under all operating conditions. What does that mean at the end of the year? The effects of this are not easily quantifiable, but you can estimate it in a number of ways.

    Jaggedben's suggested approach is the simplest. But it is also the most conservative. With one module missing, the system will perform better than if you simply had three source circuits of 10 modules each. However, the losses will definitely be more than 1/33rd of the estimated annual production; the losses are not linear—because that source circuit with the missing module will never be operated at its Vmp.

  3. #13
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    Quote Originally Posted by SolarPro View Post
    However, the losses will definitely be more than 1/33rd of the estimated annual production; the losses are not linear—because that source circuit with the missing module will never be operated at its Vmp.
    But since by the very nature of a Maximum, the slope of the power graph is zero at that point, you can move a little bit either way without changing the output by nearly the same percentage.
    So the two strings of 11 operate a little lower than MPP, but within 1%, and the string of 10 operates at a little higher than Vmp, but within 1%. Net result:within 1% of 32 panels full output.

  4. #14
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    That's optimistic. There's a 9% difference between the Vmp of the 10-module string and that of the 11-module strings.

  5. #15
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    Put another way: If your analysis is correct, why do inverter manufacturers bother providing string inverters with multiple MPPT inputs?

  6. #16
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    Quote Originally Posted by SolarPro View Post
    That's optimistic. There's a 9% difference between the Vmp of the 10-module string and that of the 11-module strings.
    Which means that the composite value is not much more than 5% away from either of them.
    If the slope of the power versus voltage were 45degrees, that would make a 5% difference. But since the slope of the power curve is close to zero, a 5% change in voltage will not cause anywhere near a 5% difference in power compared to the MPP value.

  7. #17
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    Quote Originally Posted by SolarPro View Post
    Put another way: If your analysis is correct, why do inverter manufacturers bother providing string inverters with multiple MPPT inputs?
    Partial shading, vastly different numbers or types of panels, very different temperatures at a time when both strings are producing high power, etc.....
    If you have matching panel types and numbers, and no partial shading within a string, you can even parallel panels facing in totally different directions without any power loss.
    Part of the reason that multiple MPPT inputs were provided is that people do not understand this and so it became a sales point.
    Part of the reason is people who have a lot of really different panel types or array sizes because they are upgrading an existing smaller system. Or are planning to do that someday.

    As long as there is a good voltage match (at Vmp) you can parallel different Imp strings with no problem.
    As long as there is a good current match (at Imp) you can put different individual panel types in series to create comparable Vmp strings to be paralleled.

    Last but not least, if two same-length strings of comparable panels are facing in different directions, the Vmp of both strings in isolation will be so close as to be insignificantly different, even when the corresponding current, Imp varies by a factor of 10 or more between the strings.
    Last edited by GoldDigger; 07-03-13 at 02:06 AM.

  8. #18
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    Quote Originally Posted by GoldDigger View Post
    But since by the very nature of a Maximum, the slope of the power graph is zero at that point, you can move a little bit either way without changing the output by nearly the same percentage.
    So the two strings of 11 operate a little lower than MPP, but within 1%, and the string of 10 operates at a little higher than Vmp, but within 1%. Net result:within 1% of 32 panels full output.
    One thing to consider is that the power graph is not symmetrical. A string that is operating at lower than its MPP voltage won't be affected as much as a string that is operating at higher than its MPP voltage.

    With a good meter with clamp current capability on site, someone could settle this bar bet.
    Last edited by ggunn; 07-03-13 at 09:03 AM.

  9. #19
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    Quote Originally Posted by So Cal View Post
    I certainly do understand Voc measurements aren't what you project forward for system production.
    The real reason for my origonal post was more academic than anything else. I do plan to replace the non functioning panel. Again, academically I had figured that the panel replacement wasn't too critical as to matching electrical characteristics, as long as Imp & Vmp are equal or greater,and the same basic setup,IE neg ground etc.. I thought greater would be a tiny advantage?
    I think we all agree, given a system of 33 panels, that one loses far less production by temporarily leaving one non-functioning panel out of a string than by leaving out the whole string. But I can think of situations where the question is less academic. Suppose more than one panel starts to malfunction. At what number of dead panels is it better to disconnect the whole string? If there are dead panels removed from multiple strings, at what point is it better to take live panels out of the good string(s) to match the others?


    I was curious and went back to the site to get Vmp measurements;

    string of 11 - 288Vmp
    string of 11 - 288Vmp
    string of 10 - 260Vmp
    combined - 280Vmp
    These measurements are averages of the displayed readouts that fluctuated continuosly.

    The inverter readouts Were a little higher than my tester.
    Hm, no current readings? That would tell us what the real difference was.

  10. #20
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    Quote Originally Posted by GoldDigger View Post
    Last but not least, if two same-length strings of comparable panels are facing in different directions, the Vmp of both strings in isolation will be so close as to be insignificantly different, even when the corresponding current, Imp varies by a factor of 10 or more between the strings.
    Except that two same-length strings facing different directions are likely to be very different temperatures for much of the day, at least in a typical example where it's east and west on a pitched roof house. On a recent job, I measured a 40V difference between identical east and west strings. That was at 10am, and the house only had about a 20deg pitch. The east string was more than one panel Voc lower. After scratching my head for a bit I realized it was all due to temperature, since the sun at that time had been hitting the east string directly for a couple hours, but not the west string.

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