Voltage between array negative and ground

TroySmith80 · April 2018

I recently wired up a combiner box, where several strings of panels come together, and are combined into a single set of +, -, G wires that run to the inverter. While poking around in the combiner box with a multi-meter, checking for any weirdness, i'm finding that there is some voltage between the array negative output and the ground wire. When i test there, i see 35v or so very briefly, and the voltage rapidly (2-3 seconds) drops to 15v or so, and then continues to drop more slowly, approaching zero. The rate of voltage drop seems to change, perhaps on some kind of exponential curve such that it drops a lot at first and then more and more slowly, and may take a long time to get all the way to zero. Note that this is with the array circuits open, and no current flowing. I'm not sure what to make of this. The way that the voltage seems to dissipate when measured makes me think that there's not really much energy there, but i really don't understand it, so i don't want to dismiss it without further investigation.

Any ideas on what could be causing this, or how to determine if it constitutes a problem? Should I not turn on the inverter before resolving this?

More information if helpful: The array is 40 panels, 65W each. They are arranged in 10 series strings of 4 panels each. The 10 strings are then combined into two series groups of 5 strings each, and the two groups are paralleled and fed to the inverter in a single wire pair. Voc of the final output is 410V. The array ground is connected to a grounding rod and the house distribution panel ground.

mike95490 · April 2018

Yikes. That's not supposed to be there, and will likely activate any GFP gear installed.

Any way you can draw a schematic of the array wiring?

They are arranged in 10 series strings of 4 panels each.

The 10 strings are then combined into two series groups of 5 strings each,

and the two groups are paralleled and fed to the inverter in a single wire pair. Voc of the final output is 410V.

I'm just not able to follow that.

Estragon · April 2018

Is the DC negative tied to ground anywhere? If so, where?

Are the panels rated for that string voltage? If I'm reading right, it's essentially 20 panels in a 410v series string, in parallel with another 20 panel string.

The array ground goes to both the AC panel and the ground rod? Is AC neutral bonded to ground in the panel?

TroySmith80 · April 2018

I don't think that the DC negative is grounded anywhere... if it was, there wouldn't be any voltage between those points, right?

Yes, basically 2 strings of 20 panels each.

There is a grounding rod, and both the AC panel and the solar array grounds are tied to it. Yes, neutral and ground are bonded in the AC panel. As far as i can determine, the array DC(-) is not connected to anything.

The panels are rated for a max system volltage of 600v.

The reason the module strings are wired somewhat oddly is that it was originally an off-grid system with a 48v battery bank. There were 10 strings, each made of 4 modules in series. The system voltage was 82Voc. I'm switching it over to grid-tied and the new grid tie inverter wants much higher DC input voltage, so i took the 10 original strings of 4 panels and made it into 2 strings of 20 panels to bump the voltage from 82v to 410v.

TroySmith80 · April 2018

Here's a diagram of the wiring. Each of the blue blocks is a sub-array of 4 modules in series. Final output is equivalent to two strings of 20 panels each.

I don't think that the configuration is relevant to the issue, but i could be wrong.

When i put a meter between ground and the final DC- I expect to see zero, but instead i see the voltage described in the original post.

Image: https://us.v-cdn.net/6024911/uploads/editor/rq/7ce91r8hdxd4.gif

mcgivor · April 2018

What you are most likely seeing is a phantom voltage, if using a digital multimeter, the impedance of these meters are extremely high, in the mega ohms per volt, which often produce some strange unstable readings. Using an analog meter, which is in the kilo ohms per volt, my bet is it would read nothing. Using digital meters for finding ground faults in fire alarm systems is extremely difficult for this exact reason, naturally I'm not saying there is not a fault, just something to be aware of.

mike95490 · April 2018

Generally, wouldn't the PV - be grounded at the combiner box or charge controller, otherwise the whole array is floating at some potential ?

Estragon · April 2018

It may be worth checking the inverter manual to see what, if any, provisions it has for DC-GFP. IIRC, my Classics have internal GFP which, with a jumper properly set, makes the bond between ground and DC negative. If there is no DC-GFP in the inverter or otherwise making the bond, and they aren't bonded directly, it makes sense they could be a different potential because of galvanics (eg copper ground + aluminum pv frame) etc.

It could be a phantom reading, as McGivor suggests, or a real voltage which diminishes over time as the meter in essence creates the missing DC negative to ground bond.

BB. · April 2018

Get a 120 VAC filament light bulb (~20-100 watt) and clip it between your two grounds. The bulb will safely either bring the voltage difference to zero volts (floating system), or provide a safe/small load and allow you to figure out where the phantom power source is connected.

-Bill

Voltage between array negative and ground

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