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Thread: Solar Module Datasheet - "Limiting Reverse Current"

  1. #1
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    Solar Module Datasheet - "Limiting Reverse Current"

    Hi folks,

    Wondering if anyone has any knowledge of "Limiting reverse current" from solar module data sheets?

    Scenario: Paralleling 2 strings of a 140w module(SF140-L) with 2.1 isc and a max series fuse rating of 4a. (2 x 2.1 x 1.25 = 5.25a) This would then mean that the each string would need to fused at 4amps and fused at 8a in the combiner panel. On the datasheet above the max series fuse rating it has listed the limiting reverse current at 7a. Does this imply that 7a is the actual the maximum current that the modules can actual take before it suffers damage? I have never seen this on a datasheet and was curious if anyone knew?

  2. #2
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    I'm not smart enough to do the semi-conductor science math on it, but if you mess up string length or polarity on paralleling strings you can send a current the wrong way through a module. This is part of why string lengths have to match when they are paralleled to the same MPPT.

    Hypothetical example:
    A crew has two strings of 14 mods on a rooftop system. One string is divided up into two arrays of 2 and 12. (The other is just 14).
    Somehow the crew royally screws up the wiring so that they effectively parallel 3 strings 14, 12 and 2.
    Now the 2 modules in series see a voltage at their leads that is effectively about 11 mods worth of voltage the wrong way, wanting to push current backwards through them. (Average of the 12 and 14 is 13. 2 modules worth of voltage cancels out. 13-2=11.)
    Good sunlight overcomes the internal resistance of the cells, and pushes most of two strings of current backwards through the two modules, damaging them.

    So I'd say if your data sheet says 7A reverse current, that's effectively saying it can only handle one string worth of current being pushed the wrong way, and not a situation like the one above.

    There was a thread on here a while back about an... er... undertrained tech who attached two extra modules in parallel to a system with a bunch of longer strings, because he thought why not get some extra energy. Similar thing happened.

  3. #3
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    Quote Originally Posted by andykee View Post
    Hi folks,

    Wondering if anyone has any knowledge of "Limiting reverse current" from solar module data sheets?

    Scenario: Paralleling 2 strings of a 140w module(SF140-L) with 2.1 isc and a max series fuse rating of 4a. (2 x 2.1 x 1.25 = 5.25a) This would then mean that the each string would need to fused at 4amps and fused at 8a in the combiner panel. On the datasheet above the max series fuse rating it has listed the limiting reverse current at 7a. Does this imply that 7a is the actual the maximum current that the modules can actual take before it suffers damage? I have never seen this on a datasheet and was curious if anyone knew?

    It's unusual to have a separate value listed for the max series fuse and the max reverse current. When the modules are tested for UL 1703 the manufacturer provides the reverse current to be tested to and the test lab verifies that the module is safe at this level of reverse current. The manufacturer then lists a max series fuse value at or less than the tested reverse current. So it could be that the manufacturer had the module tested to 7A but decided to use 4A as the max series fuse, and then to put both values on the data sheet. That's my analysis anyway.

    Normally you can parallel 2 strings without fusing since the max reverse current is limited to Isc, unless the installation manual says all parallel circuits have to be fused.

    Your Imax calculation should be 2.1*1.25*1.25 = 3.3A per string and 6.6A out of the combiner.

  4. #4
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    Quote Originally Posted by pv_n00b View Post
    It's unusual to have a separate value listed for the max series fuse and the max reverse current. When the modules are tested for UL 1703 the manufacturer provides the reverse current to be tested to and the test lab verifies that the module is safe at this level of reverse current. The manufacturer then lists a max series fuse value at or less than the tested reverse current. So it could be that the manufacturer had the module tested to 7A but decided to use 4A as the max series fuse, and then to put both values on the data sheet. That's my analysis anyway.

    Normally you can parallel 2 strings without fusing since the max reverse current is limited to Isc, unless the installation manual says all parallel circuits have to be fused.

    Your Imax calculation should be 2.1*1.25*1.25 = 3.3A per string and 6.6A out of the combiner.
    Imax is 1.25X Isc to take into account possible insolation of more than 1000W/m^2. The second 1.25X is to determine minimum OCPD and minimum conductor 75 degree ampacity for continuous use, but it's not part of Imax.

  5. #5
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    I found this First Solar white paper to explain it quite well:

    http://www.firstsolar.com/-/media/Fi...oad.ashx?la=en

    "When reverse current flows into a module, instead of producing electricity the module acts as load and it will attempt to dissipate the energy flowing into it. When the reverse current passing through the module exceeds its maximum reverse current rating as shown in Table 1, RCOL occurs. When RCOL occurs, the module may experience high surface temperatures, and could crack, smoke, arc, or ignite itself or surrounding materials, depending on the length and severity of the RCOL condition."

    RCOL: Reverse Current Overload

  6. #6
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    Quote Originally Posted by ggunn View Post
    Imax is 1.25X Isc to take into account possible insolation of more than 1000W/m^2. The second 1.25X is to determine minimum OCPD and minimum conductor 75 degree ampacity for continuous use, but it's not part of Imax.
    OP did not say the equation was for Imax, they were talking about fuse ratings so I made a pretty safe assumption that they meant to apply that equation to find the fuse rating. I should have said Ifuse instead of Imax.

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