Odd amperage drop across breakers in combiner box

Raj174Raj174 Solar Expert Posts: 411 ✭✭✭✭
I know, sounds ridiculous right? I have a 3600 watt array, 12 300 watt panels, 6 strings of 2 panels each. I have had consistent output of 2100 to 2200 watts with clear skies, occasionally brief periods of 2400 to 2500 watts. Also, I have seen outputs upwards of 3000 watts with cloud edge effects. I have always thought this was low for this size array.

Today I was checking the amperage on the 6 strings with a clamp meter, and all 6 were between 6 and 7 amps, average 6.5.
So that adds up to about 39 amps. However, on the # 4 AWG cable terminated on the plate that combines the 6 breakers, I have 33 amps. This makes no sense to me. Next, I switched all the breakers off, except #1 and checked for amps on a single string. I got 6.4 on the string and 5.4 on the #4 CC input cable. Did this with all the other strings with similar results. 39 amps on the PV strings and 33 amps on the #4 cable. I am perplexed by the 1 amp drop across the Midnite 15 amp breakers. Seems to me that, if something were dragging the amperage down, it would happen on both sides of the breakers. The voltage at the time was consistent on all cables at 61 volts. The combiner box is a Midnite MNPV12.

Anyone have any ideas?      

Rick
12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.

Comments

  • mvasmvas Registered Users Posts: 298 ✭✭✭
    edited September 18 #2
    You say that you have 300 Watt Panels - NOCT or STC rating ?
    And then there are Real World Watts.
    Are the panels hot ?

    I have heard of Voltage Drop, but not "Amperage Drop".
    It sounds like you cannot accurately measure the DC Amps with your meter.
    Is there any metal near the #4 cable?
    It appears that you are reading only ~90% of actual on the #4 cable.
    Amps IN must equal Amps OUT.
    You have a measuring problem.
  • Raj174Raj174 Solar Expert Posts: 411 ✭✭✭✭
    @mvas
    They are 300 watt STC of course, and obviously less in the real world. I have a good DC clamp meter and I trust it.
    I do agree with you, amps in must equal amps out. This evening I will temporarily put all the positive wires on an insulated buss bar for testing.

    Rick 
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • HorseflyHorsefly Registered Users Posts: 251 ✭✭✭
    edited September 18 #4
    You have something like 6 amps per string, and you are missing 6 amps when it comes out of the combiner. Sounds pretty clear to me: One string is not going to the output of the combiner.  

    I don't know where it could be going except for a short circuit on that one string. If you turn off all the breakers in the combiner box, are the open circuit voltages all the same on all six strings, or nearly so?
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
  • Raj174Raj174 Solar Expert Posts: 411 ✭✭✭✭
    Have to run an errand, but I'll check that tomorrow, thanks for the suggestion.
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • mvasmvas Registered Users Posts: 298 ✭✭✭
    Horsefly said:
    You have something like 6 amps per string, and you are missing 6 amps when it comes out of the combiner. Sounds pretty clear to me: One string is not going to the output of the combiner.  

    I don't know where it could be going except for a short circuit on that one string. If you turn off all the breakers in the combiner box, are the open circuit voltages all the same on all six strings, or nearly so?
    I don't see how this can be "one string is shorted".
    He says, he measured each string, one at time and each string is ... 6.4 amps "in" but and only 5.4 amps "out".

    Shouldn't he have seen 0 Amps in #4 cable for the shorted string?
    That did not happen.
    This sounds like a measuring error.
  • HorseflyHorsefly Registered Users Posts: 251 ✭✭✭
    mvas said:
    Horsefly said:
    You have something like 6 amps per string, and you are missing 6 amps when it comes out of the combiner. Sounds pretty clear to me: One string is not going to the output of the combiner.  

    I don't know where it could be going except for a short circuit on that one string. If you turn off all the breakers in the combiner box, are the open circuit voltages all the same on all six strings, or nearly so?
    I don't see how this can be "one string is shorted".
    He says, he measured each string, one at time and each string is ... 6.4 amps "in" but and only 5.4 amps "out".

    Shouldn't he have seen 0 Amps in #4 cable for the shorted string?
    That did not happen.
    This sounds like a measuring error.
    @mvas, a couple of things. First, I admit that in my haste I didn't read the details about the per-string part of the test (the "Next, I switched..." part). So my suggestion seems less likely given that set of details that I left out.

    Second, unless I'm reading it wrong, the #4 cable is on the combined side, going to the charge controller. Fact is, I can't see it making sense any other way. So I don't think there is a "#4 cable for the shorted string", but rather one #4 cable for all the strings.

    For both current and voltage, it all has to add up to the same total. For voltage, if you see a lower voltage on one end, there has been some voltage loss along the way. Likewise, all the current has to go somewhere, so if you have less at one end than at the other, there is another path that it has taken.

    In the end it could be a measurement error, but I can't see how that error would repeat consistently across all the strings.

    Quite a mystery.

    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
  • Dave AngeliniDave Angelini Solar Expert Posts: 3,257 ✭✭✭✭
    Something is not righty tighty :) Time to disconnect wires and eyeball. Or as mentioned, measurement error which includes the connection quality of the measuring device.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail offgridsolar@sti.net

  • Raj174Raj174 Solar Expert Posts: 411 ✭✭✭✭
    Yes, connection quality is suspected, we will see, when I bypass the breakers and plate connections tomorrow.

    Rick
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • mike95490mike95490 Solar Expert Posts: 7,249 ✭✭✭✭
    That's why I like breakers instead of fuses in combiner boxes.  Simple to cut strings back to just 1, check the power, then switch to string 2, check power, check 3, switch to 4, check 4
     Either they are all the same, or some or one is different, and different would be bad,
    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 ,

  • Dave AngeliniDave Angelini Solar Expert Posts: 3,257 ✭✭✭✭
    Adding on to Mike, "that is why I like high voltage DC" no strings and no switching/connections outdoors! At the most 2 strings in a very large 6kw array and a Disco. Another reason grid-tie solar is so very reliable.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail offgridsolar@sti.net

  • VicVic Solar Expert Posts: 2,811 ✭✭✭✭

    Just a couple thoughts;

    First,   I DO question the accuracy/repeatibility of many/most Clamp DC Ammeters.

    Second,  assume that the Classic was in Bulk - MPPT,   but still,  the Solar conditions could have changed,  causing changes in the amount of PV power available.

    If the CC was in Absorb,   and you were feeling that a fairly rapid series of measurements would essentially equate to constant charge current conditions,    this could be a factor.

    However,   I have NO experience with LiFePO4 batteries,   so some of the above may just be NOISE,
    FWIW,    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.
  • mvasmvas Registered Users Posts: 298 ✭✭✭
    Second, unless I'm reading it wrong, the #4 cable is on the combined side, going to the charge controller. Fact is, I can't see it making sense any other way. So I don't think there is a "#4 cable for the shorted string", but rather one #4 cable for all the strings.

    Let me re-phrase ...
    IF there was a "Shorted String" ( as someone else had suggested ) then
    the amps measured in the #4 cable would have been "0 Amps"
    when the breaker for only that "shorted string" was ON.

    But that did not happen, therefore there is no "shorted string".

    Amp in must equal amps out,
    so how can a bad connection change the amps flowing through the circuit?
  • Raj174Raj174 Solar Expert Posts: 411 ✭✭✭✭
    Correct, there are no shorted strings. I did temporarily jump out the breakers and breaker buss bar yesterday morning by connecting all positives to an insulated buss bar, but had an overcast day with poor production. Need a clear day to to match the previous test conditions, which looks like this weekend, hopefully sooner. If the problem is connection related, there should be a consistently noticeable increase in amperage at the CC. The few spots of sun I got yesterday looked encouraging, but I can't rule out cloud edge effect at this point. If I find that power production is higher, I will wire the breakers back in to the circuit without the the breaker buss bar by wiring the breaker outputs to the insulated buss bar, then test again.  
    Thanks for your comments,
    Rick     
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
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