Has anyone seen this DC Isolation problem before?

[LarryD]

I have 15KW Fronius Primo in a system with 2 inverters that is giving DC isolation faults at weird times. It gets down below 2kohms according to inverter error. The Primo doesn’t seem to monitor the DC isolation in realtime, only at startup. The only times I get the error is after a hot day “100+ deg” with NO dew overnight. If it’s raining or heavy dew it does fine.

I’ve tried to catch it when the isolation is low but the lowest reading I can get from the Primo is 8.5k. The inverter has four 12 panel strings on it and my only next step that I can think of is to isolate the strings and see if one is lower than the others but kinda wonder if that will really point to the problem string since I can never catch it doing it.

[SunEagle]

Hello LarryD and welcome to Solar Panel Talk

I might suggest you find a recording meter like a Fluke that can pinpoint the exact time an event happens. Otherwise unless it is a real problem I would suggest not worrying about it.

[LarryD]

I looked for some test equipment that can read the DC isolation with the string energized but it was crazy price. I don't think you could log with it either.
These don't have and rapid shutdown or optimizers so when the sun's out they have 550v.
I would ignore it if I could but when the primo errors out after a couple of self test then it won't start up. Everytime I get there after it won't start up the the isolation value reads fine and I just restart it.
I've seen DC isolation problems with wet conditions but nothing dry and only with curtain temperature conditions. The only thing I could think of is some sort of thermal expansion. But I can't imagine a thermal expansion problem not being able to be triggered by water.
I'm just stumped and if I dig into it I have no idea if I can even narrow it down unless it's under it's perfect conditionssl..

[bcroe]

I am going to assume, you have a fault to ground in either an
array or the wiring. Here is how I would check that. A super
high impedance voltmeter tends to be mislead by static charges,
and normal, very small, insulation leakage. The tool I use is 4
75W 120V bulbs in series, which can respond to a fraction of an
amp at up to array voltage. The impedance is low enough to put
some load on the panels, while instantly draining static charges
and ignoring normal tiny insulation leakage. It operates at system
voltage, in case that is needed to reveal faults.

PVtestLt.JPG

Another variation is connect an about 50,000 ohm 10 watt resistor
in parallel with your DC voltmeter, the resistor will eliminate false
readings, but expect it to heat up.

For 550VDC you might need 5 series bulbs. Your array will supply
the voltage to reveal the fault, that your hand held meter cannot. If
the fault is fairly high impedance, the incandescent bulbs might
need to be the night light 6W or 7W size.

With the arrays disconnected from the feed wires and some sun,
first use the bulb test from ground, to either a +, then a - array
output lead. Both should give no response, otherwise that array
has a ground. The voltage on either side should give an indication
of how far down the string, the fault is. Test connection to the
array leads should light the bulbs.

Next connect the feed wires to the array, but not connected to the
inverter. Do the same test from ground, if one wire lights bulbs,
the other wire has the ground fault. Bruce Roe

[LarryD]

That is a good idea. I don't know why I didn't think of that. I'll have to make that test rig up. Now I'm just hoping the string is on an outside row and not in the middle! I've never had good luck with SS fasteners working twice even with anti-seize on them.

[LarryD]

Got the parts in to make the string voltage tester and finally got a day to go test it. I still have some tweaking to do. I thought I bought regular filament bulbs but looking closer at them they have some efficiency rating. Regardless they would not light up. It wasn't a sunny day and the inverters were only putting out 3000W so it might have been current limited testing the individual strings. The rig did at least put a small load on the string so that I could take voltage measurements with a real meter easy. Problem was there was absolutely nothing wrong with the strings. There are 4 equal strings. All within a volt of each other. No cross voltages and a fast sinking voltage (real meter only) on all leads to ground. It's been 5 days since a DC Isolation error that it was able to test out of and eventually start up and it has only been 4 times that it would not test out and start up. So it may be fixing itself. But I don't know of anyway to find the problem unless it's doing it when I'm there to even get a start of what the cause is.
Guess it's a waiting game...
Thanks for the input though!

Btw. The meter on the box is in parallel with a 1/10 voltage divider so that 100v on the meter would be 1000v.

20220816_150626a.jpg

[bcroe]

That looks great, but you need incandescent bulbs to assure some
load over a wide voltage range. Incandescents have a non linear
resistance which gives a relatively constant current flow over a very
wide voltage range. Bruce Roe

[LarryD]

Ya, short of breaking one of these open I can't tell what's in them. They do measure like a incandescent bulb with only 16ohms but since there is no way to make an incandescent bulb more efficient then lowering the wattage I don't know how they would be considered efficient. Regardless they should have lit at least a little with 536vdc in series I would think.as there should have been around 750w of current available.

[azdave]

Zooming in I can see the bulbs are marked as 72W halogen so must be an incandescent design. Did they just use a lower wattage filament than the standard 75W incandescent and claim some questionable energy savings compared to a bulb that consumes 75W?

[bcroe]

I have not measured halogens, but would expect them to be similar. Something
is wrong if 500V from 8A panels can not begin to light 0.6A bulbs. Bruce Roe