Voltage (Voc) and Current (Isc), but no Current when Load Applied

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balee123
balee123 Solar Expert Posts: 86 ✭✭
in Solar Beginners Corner #1
I have 4 identical 12V panels hooked up in series. When I measure the Voc and Isc, the values are what one would expect based on the specs on the back of the panels. However, when a load is applied, there is no current measured with a DC clamp on meter. This set of panels had worked in the past with no problems producing expected Vmp and Imp values under load. Can anyone suggest what might have happened to the set up and any suggestions on how to troubleshoot.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    First question: you have Isc per panel or Isc for all four ins series (4X Isc)?
    Because if they can produce the latter they certainly should be putting current into the load.

    Second question: what is the load?
    If the resistance of it is high enough the current flow may be too small to measure.
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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #3
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    The Isc is for all 4 panels (4x the value on the back of one panel). The load is a grid tie inverter.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied
    balee123 wrote: »
    The Isc is for all 4 panels (4x the value on the back of one panel). The load is a grid tie inverter.

    What grid tie inverter?
    A typical 12 Volt panel has a Vmp around 17-18 Volts. Four in series would be 72 Volts at most. If this is not in the range of input Voltage for the GTI it won't start, and as such no current will flow.
    Usually a GTI has input Voltage requirements in the hundreds. There are a few that run on <100 Volts, but they tend to be older design and/or junk.
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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #5
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    This is an older grid tie inverter whose input voltage is in the range of 60-80V. This set of 4 panels is one of many that are paralleled into the grid tie. This set for some reason is not functioning properly. As mentioned above, I get the expected Voc and Isc measurements from the 4 panels in series, but have a problem with the current. I can tell you that each of the other sets of 4 panels are inputing ~4 A each into the grid tie during peak production time of the day.

    I have some new information regarding this set of 4. The input current from this "bad" set was ~ half compared to any of the other sets (i.e. ~2A) when the current was first measured (mid-morning). The by midday, the current was non-existent. By late afternoon, the current returned, however, it was still ~ half of a normal set of 4 panels.

    Does this look like a bad solder joint that may be worse after a panel heat up, but allows some current to flow when the panel cools in late afternoon or morning temperatures?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    So you have one string out of four that is at best underperforming?
    With a current pattern of "low in AM, none at noon, low in PM" it sounds like a shading issue actually. The odd part is it being able to produce Isc (presumably at midday?). That leads me to suspect something in the difference of connections between when you check Isc and when you connect it up to the rest of the panels to feed the inverter.

    These should be connected with a combiner box and fuses/breakers. I'd inspect them very carefully for corrosion and loose wiring. It may sound ridiculously simple, but it is often a problem. At least you will be able to rule that out.
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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #7
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    Actually its one string (4 panels in series) out of 6 strings total that is underperforming. There is no shade issue going on, all in direct sun. Yes, Isc was measured at midday. The inverter has an internal combiner (Trace SunTie STRX2500 GridTie inverter). I have check all connections, but not the fuse connections inside the internal combiner, which I will do now. After inspecting one of the panels I noticed some corrosion at one of the main "connecting tabs" for the solar cells. Not sure if this could be the issue though.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    Another thing to try: switch the 'bad' string with another string at the combiner and recheck the current on both. If the 'bad string' still doesn't provide power there is something wrong with that string. But if it is any string connected at "#4 position" (as it were) then there is a problem with that connection point.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #9
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    balee ..

    Out of curiosity, what brand PV modules are these, and the model number please. Thanks Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #10
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    Astropower AP-130

    Rated Power: 130 Watts
    Rated Voltage: 18.9 Volts
    Open Circuit Voltage: 24.6 Volts
    Rated Current: 6.9 Amps
    Short Circuit Current: 7.6 Amps
    Max. System Open Circuit Voltage: 600 Volts
    Series Fuse: 15 Amps
    Bypass Diode: 8 Amps
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #11
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied

    most grid tie inverters want to sense the grid's (240VAC) stable presence for at least 5 minutes before they will start up. And it has to be grid, not another inverter or generator.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #12
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied
    balee123 wrote: »
    I have 4 identical 12V panels hooked up in series. When I measure the VOC and ISC, the values are what one would expect based on the specs on the back of the panels. However, when a load is applied, there is no current measured with a DC clamp on meter. This set of panels had worked in the past with no problems producing expected Vmp and Imp values under load. Can anyone suggest what might have happened to the set up and any suggestions on how to troubleshoot.

    If one of the panels is bad (with one or more cells that have low current output) then each panel will still measure full VOC because the bad cells will produce the same voltage, just not enough current. And when you short the panel (or the string) to measure ISC, those bad cells will be bypassed (by the bypass diodes) so that they will not choke off the current produced by the other cells.

    But if you use a variable load and measure the string VLOADED, you will see that once you go past a particular (small) current level the voltage will suddenly drop. That is the point at which that set of cells is effectively taken out of the string by the bypass diodes. If this voltage is too low to work your load (charge your batteries or run your GT inverter) then you will see no current at all.

    Now you may say "Well, if I have one bad cell, won't that just drop to voltage by .5 volts?" The answer is no, since each bypass diode is typically connected across 1/2 or 1/3 of the panel and so you will lose the voltage of all of the cells in the section that contains the bad cell.

    The first troubleshooting step is to measure the string voltage while the load is connected. If it is zero you have a connection problem somewhere. If it is close to the voltage of just three of the four panels, (or 3 and 1/2 panels) then you have a bad cell in one panel. You can determine which panel it is by measuring the voltage of each panel in the string while the load is attached.
    Once you find the bad panel, you can inspect for visible damage and also look at the connections of the tabs to the bypass diodes and output leads in the junction box of that panel (if it is accessible.) You will probably find that you just have to replace that panel.

    Some DIY types may try to rescue a bad panel by drilling holes into it to try to wire a bypass around just the failed cell(s). But this is unlikely to succeed and even if it does will drastically shorten the remaining life of the panel because you cannot evacuate all of the moisture and seal it up completely when you are finished with the repair.

    PS: The nameplate says "Bypass diode 8 amps". That could mean that there is only one bypass diode for the whole panel, in which case you will lose up to 24 volts when you reach the current limit of the bad cell(s). Or it could mean that you are supposed to provide the bypass diode yourself, in which case you will lose the output voltage of the whole string if no bypass diode is attached.
    If there is a bypass diode internally and it failed, then again you will lose the entire voltage of string past a certain current level. However if things are that bad, you would probably not see the normal ISC
    SMA SB 3000, old BP panels.
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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #13
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied
    balee123 wrote: »
    Actually its one string (4 panels in series) out of 6 strings total that is underperforming. There is no shade issue going on, all in direct sun. Yes, Isc was measured at midday. The inverter has an internal combiner (Trace SunTie STRX2500 GridTie inverter). I have check all connections, but not the fuse connections inside the internal combiner, which I will do now. After inspecting one of the panels I noticed some corrosion at one of the main "connecting tabs" for the solar cells. Not sure if this could be the issue though.

    I had an extra panel laying around, so I replaced the panel that appeared to have some corrosion (see pictures) and everything is normal now. The bad string is now producing the same as all other strings.

    That being said, can I inject some dielectric substance to help reduce the corrosion and increase the conductivity, if this corrosion is indeed the problem?

    Attachment not found.

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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #14
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    Re: Voltage (Voc) and Current (Isc), but no Current when Load Applied
    balee123 wrote: »

    That being said, can I inject some dielectric substance to help reduce the corrosion and increase the conductivity, if this corrosion is indeed the problem?

    No.
    You can try to prevent corrosion, but there is no practical way to reverse it. The only option is to try to bypass the corroded conductors if you can get at the inner ends.
    Probably better to just trash the panel or take it apart for experimenting.
    SMA SB 3000, old BP panels.
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