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Thread: Series String protection

  1. #1
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    Series String protection

    I would like to get your opinion as to what could be the cause of the below cases happening in the same solar plant.
    1.Case 1: burning of junction boxes of all PVmodules including its diode in a string of 21 PV mod.
    2.Case 2: In some cases the burning of MC4 branch (Y)connector that joins two strings in parallel or some cases the MC4 connectorsat PV modules.
    Initially it seems to be problems of improper terminations or crimping of MC4 but looking at how the strings are protected with fuse I beginning to see some other issues.
    There are 21 strings in a combiner box (with 21 module perstings with an Isc rating of 9A per module or the string) .These 21 strings are then wired in pairs.A total of 10 pairs of string (tied togetherwith a MC4 branch connector at the field) and a 30A fuse protection per pair atthe combiner box. Then the remaining one (1) single string is protected with15A fuse (although the drawings says 30A) I have attached a single line diagram on how it was wired.
    It seems that the 30A fuse will never protect a single string if a fault will occur in any of the string.
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  2. #2
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    My first guess as to what happened would be that the damaged string had its polarity reversed. You haven't mentioned any evidence of another kind of fault. So you have the string wired backwards you get current flowing through the string because it's in series with all the other strings. The string has some internal resistance so it's not a true short circuit, and I'm not sure if a 15A fuse would have blown where a 30A didn't, although I would guess it's much more likely.

    The diagram you posted ought to be fairly adequate if everything was wired correctly. A true fault from positive to negative on a string could draw the full current of the array and thus blow the 30A fuse. The other string it is paired with is not going to blow a 15A fuse by itself anyway, so that's why the pairing is considered okay by many people. The downside is that if you have a true fault on the pair you have two strings worth of current instead of one. And perhaps your situation demonstrates an additional downside.

  3. #3
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    huh
    CircuitRyder --- Unfortunately not all good ideas are code enforceable.

  4. #4
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    Quote Originally Posted by henryo View Post
    I would like to get your opinion as to what could be the cause of the below cases happening in the same solar plant.
    1.Case 1: burning of junction boxes of all PVmodules including its diode in a string of 21 PV mod.
    2.Case 2: In some cases the burning of MC4 branch (Y)connector that joins two strings in parallel or some cases the MC4 connectorsat PV modules.
    Initially it seems to be problems of improper terminations or crimping of MC4 but looking at how the strings are protected with fuse I beginning to see some other issues.
    There are 21 strings in a combiner box (with 21 module perstings with an Isc rating of 9A per module or the string) .These 21 strings are then wired in pairs.A total of 10 pairs of string (tied togetherwith a MC4 branch connector at the field) and a 30A fuse protection per pair atthe combiner box. Then the remaining one (1) single string is protected with15A fuse (although the drawings says 30A) I have attached a single line diagram on how it was wired.
    It seems that the 30A fuse will never protect a single string if a fault will occur in any of the string.
    Your fusing is inadequate. If you look on the back of the module it will say "maximum series fuse = " and most likely 15 or 20 amps. That means for each string, and you cannot combine two strings and double the fuse size. You obviously had some kind of a fault and the fuses did not protect the strings.

    Some guys will try this occasionally to try to save wire, but as you have found out, it doesn't work. It should not have passed inspection.

  5. #5
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    Quote Originally Posted by ggunn View Post
    ...It should not have passed inspection.
    The OP is in the Philippines, so it may not have been inspected to current NEC standards.
    But in any case the wiring should have respected the Maximum Series Fuse size on the panel documentation.

    To blow the bypass diodes, the ones in the terminal boxes of each panel, would require either a catastrophically high reverse voltage or current being conducted through the diodes because the string was connected backwards and reverse fed.

    If the bypass diodes were inadequate in the first place and the ambient temperature was too high, simple activation for a long period of time in full sun with one panel shaded could burn out that panel's bypass diodes, but should not have cascaded to all the panels in a string.

  6. #6
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    Quote Originally Posted by jaggedben View Post
    My first guess as to what happened would be that the damaged string had its polarity reversed. You haven't mentioned any evidence of another kind of fault. So you have the string wired backwards you get current flowing through the string because it's in series with all the other strings. The string has some internal resistance so it's not a true short circuit, and I'm not sure if a 15A fuse would have blown where a 30A didn't, although I would guess it's much more likely.

    The diagram you posted ought to be fairly adequate if everything was wired correctly. A true fault from positive to negative on a string could draw the full current of the array and thus blow the 30A fuse. The other string it is paired with is not going to blow a 15A fuse by itself anyway, so that's why the pairing is considered okay by many people. The downside is that if you have a true fault on the pair you have two strings worth of current instead of one. And perhaps your situation demonstrates an additional downside.

    I have attached an illustration which do you think may also happen? ... in relation to your comment on "a true fault from positive to negative on string could draw ....current......" that on an early morning or afternoon or even on a cloudy day when the solar irradiance is low (on my illustration ---around 166w/m2 to have an Isc of around 1.5A (1.5A x 20 strings = total about 30A). Those total reverse current will generate a dangerously high temp as it do exceed the module rating (module fuse rating of 15A) resulting to melting of junction box cover; burning of diodes...etc
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  7. #7
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    I'm not sure if you're trying to show a series bypassed module in your diagram, or a reverse wired string. A reverse wired or a short string would be most likely to conduct current in a bad way. In any case, the actual amount of current conducted will depend on irradiance on those panels as well. And if the inverter is operating it will also draw current and thus affect the current.

    Ggunn's point is well taken. It seems likely that you had between 15 and 30A of current through that string. In other words, more than the modules can handle but not enough to blow a fuse. Also if it was a reverse wired string the the series voltage was too high. You're fusing scheme would have protected against a full blown accidental positive-to-negative fault, but does not protect well against wiring mistakes. It's very important to commission the system properly to avoid that sort of thing, even if you do have proper fusing.

  8. #8
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    Quote Originally Posted by jaggedben View Post
    I'm not sure if you're trying to show a series bypassed module in your diagram, or a reverse wired string. A reverse wired or a short string would be most likely to conduct current in a bad way. In any case, the actual amount of current conducted will depend on irradiance on those panels as well. And if the inverter is operating it will also draw current and thus affect the current.

    Ggunn's point is well taken. It seems likely that you had between 15 and 30A of current through that string. In other words, more than the modules can handle but not enough to blow a fuse. Also if it was a reverse wired string the the series voltage was too high. You're fusing scheme would have protected against a full blown accidental positive-to-negative fault, but does not protect well against wiring mistakes. It's very important to commission the system properly to avoid that sort of thing, even if you do have proper fusing.
    FWIW, several times people I have worked with have come to me with this same scheme to save wire, since most of us use #10, which has a lot more ampacity that a single string needs. You can do it if you fuse the strings individually before you combine them or if the two combined strings and no others are going to their own MPPT input on the inverter. Otherwise, a faulted string gets the full current from the other strings up to the limit of the (too large) fuse PLUS the current from the other string it is paired with. Not good.

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