Ground Fault on negative-ground system

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mohitap55
mohitap55 Registered Users Posts: 2 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
What can cause the sum of ground voltages to not equalling the line voltage?
I have a ground fault in my negatively grounded Solar PV system and at the DC strings,
L+ to G = 40V; L- to G = -40V; L+ to L- = 860 V; with module Voc = 40 V.

Until now, I did string tests and insulation resistance test.
This site is a mess ! Please help.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,461 admin
    edited November 2021 #2
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    You are reading 860 VDC L+ to L-? But only +40 and -40 Lx to ground?
    Have you confirmed that the meter has fresh batteries, is set to VDC volts (not AC volts)? Try meter on 12 volt car battery, etc. to confirm scaling is correct.
    What is expected Lx volts (80 volts or 800 volts)?
    First question is how are you getting -40 VDC between L- and Gnd? If you have a negative grounded system, you should be within a couple volts of Zero VDC...
    What brand/model of Solar Charge Controller do you have? Does it have a 1 amp fuse between DC ground and Safety ground? You tagged Schneider--And they typically Use a 1 Amp fuse internally to "ground" the DC GND wiring. If you had a short L+ to something like grounded metal--That can blow the "Ground Fault" sense fuse (open fuse=MPPT Solar Charge controller will shut down and you will lose your DC - Ground Reference.
    What did you use for the insulation test? A "Megger" meter can output 125 to 1,000 Volts or so for insulation testing... If you had DC devices connected (charge controllers, inverters, DC loads)--That may be causing issues with some devices (over voltage on DC buses)?
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mohitap55
    mohitap55 Registered Users Posts: 2 ✭
    #3
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    Hey Mr.Bill, I appreciate your quick reply.

    Yes, 860 L to L and 40; -40 L to G; that's something very wrong going at the site.
    I have a Hioki CM4376 multimeter which measures both DC and AC voltage on the same setting; and I confirmed that the meter scale is well as I tested on other good parts of the system.

    I agree L- to G should be close to 0 and not -40 V; while L+ to G should ideally be 860 V (same as L+ to L-).

    There is a GFDI breaker inside the Schneider Electric Conext Core XC-NA series central inverter cabinet which is in the OFF position due to a ground fault. I will be checking for the fuse on my visit to the site next week.

    It is an on-grid system. I did not notice a solar charge controller on the site.

    I used Fluke 1000 V insulation resistance tester for checking the underground DC wires between the DC cabinet and the Combiner boxes.


    These will be my approaches on my next visit at the site for the faulty inverter B1 :

    Isolate all strings of B1 and check input voltage inside the inverter cabinet. If line voltage ≠ 0, then inverter fault

    Find out the negative to ground connection structure at skid B

    Check continuity of main ground connection in skid B

    Check the junction boxes and diodes of solar modules

    Check if metal frames of all solar modules are grounded

    Disconnect a few modules to test strings in skid B

    Check GFDI functionality and Auxiliary fuse block of inverter B1


    Please add any recommended steps I need to perform to know the location of the fault.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,461 admin
    #4
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    Okay, this is a large central inverter installation (grid tied/utility interactive).

    I know nothing about those systems... So where/how the DC = to safety grounding is done is something you will have to look at and understand.

    Regarding True RMS reading meters--They are great for accurate measurements, but because the do RMS readings of the signal, they mix AC and DC readings together. So it can be difficult to understand readings when you have a mixed signal environment. In this case you have the DC of the solar array and (possibly) AC ripple current on the DC.

    I have taken older style non-RMS reading meters where when you switch to AC voltage, you only read AC voltage (they have a blocking capacitor). Short of an oscilloscope, that is the only "trick" I can think of to separate AC "noise" from DC signals.

    Regarding fault isolation--One trick that may help. A current clamp meter measures the total current flow through the clamp. You put one wire through the clamp you measure the current in the wire. If you put two wires through the clamp (say wire to light bulb and back), the total current is zero amps (current out to bulb equals the negative current back from bulb).

    Trying to track down a short circuit to ground (example)--You put clamp meter on the +/- wires from the array, and the total current should equal zero amps. Move 100' down the run and measure the current on the +/- wires, should still read zero amps--However, if you measure non-zero amps, then there is probably a short to ground/conduit somewhere between the first and second points. (you need current flowing, possibly system operating? Confirm current flow through each +/- wire and green wire and figure out what "zero amps" should be... At least in the smaller solar systems, the sense fuse/breaker is typically 1 amp).

    Don't know if this will help or not--But may be of help.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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