GF technical question

[Federico Faina]

Hi Guys,

I am new to the forum and I would like to share with the "experts" in the forum something that I have heard today.

We had an installation where the GFP fuse in the inverter experienced a fault and therefore melted.

I was told that in order to quickly verify if either the + or - leads of my string are at fault, I should measure the + and the - leads voltage with respect to ground and, if a fault is present, my voltage reading should go from my series operating voltage to an intermediate value above 3VDC.

If a fault is not present in one of the leads, my multimeter reading between the lead and ground should be aroung 3VDC.

I was trying to find a logical explanation to this, but honestly I cannot.
The way they were explaining me this reminded me what we use to do to discharge capacitors using a resistor between the + and - terminals.

You guys have extensive experience with solar installations and I would like to know if could help me in explaining why, if the string is NOT at fault, I should read around 3VDC between the positive or negative terminals of my string and ground.

Thank you in advance.

----UPDATE----

Hi guys,

I have found a great article by SolarPro magazine that outlines this.
Thank you,

FF

[Rob Beckers]

Hi Green,

Welcome at Green Power Talk!

Not a clue how that would work. I would say that a either a simple Ohm meter or, if it's insulation failure, a megger would find the fault (used cautiously, since PV cells are diodes and we don't want to blow them). What issues of SolarPro is this in? I read it, but don't recall anything about that topic and now I'm curious. Alternatively, maybe you can explain why this works.

-RoB-

P.S. We go by first and last names on this forum, as per forum rules. The hope is that this will lead to a friendlier environment since there's no 'handle' to hide behind. So far that seems to be working great! Now, unless your mom was really into renewable energy and named you "Green" :confused: I have to assume that's not your name. If you would be so kind to let me know what your name is I will change the account.

[Federico Faina]

Hi Bob,

My name is Federico Faina.

The article in question is in the latest release of SolarPro.
The description that I was given it is not completely correct and that is why I shared it with the experts. I just could not find a logical answer to the 3VDC figure.

Anyhow, the discharging factor is due to the coupling capacitance in the array components and that is why when a fault is not present your voltage readings will go to 0 while taking the measurament.

The article is going to be very helpful.

Thanks

[Rob Beckers]

Thanks Federico!
I've read through the article, and it's clear now: The idea is to disconnect both the positive and negative leads of the PV array, so the array is completely floating in voltage. Then use a digital volt meter (they are high impedance) to measure the voltage between each lead and ground. For an array that does not have a ground fault the meter will initially measure around half the array voltage, due to capacitive coupling between the array and ground. The voltage will decay (the rate depends on the input impedance of the volt meter), as the capacitor gets discharged by the meter. This should go to zero IMO, as there's no reason why it should be left at 3V (can take a long time though in case of a high-impedance meter). In case of a ground fault the initial voltage that is measured will depend on where the fault is, if it is a fractional voltage one can even work out approximately where in the array the fault is located. There's no change in voltage either, as the array is not floating.

-RoB-