CAPACITIVE LEAKAGE CURRENTS AND TRANSFORMERLESS PV INVERTERS

[marconi]

I have spent some time reading into PV panels and inverters while trying to help with a current post on nuisance RCD tripping at a PV installation on a farm. I happened upon this technical paper which explains capacitive leakage currents from PV arrays and the trouble they could cause, especially when transformerless inverters are in use. I thought others would be interested in reading it and learning from it. Helpfully, a compensation scheme is suggested at Section 6 and a way of estimating the capacitance at Section 1.

http://files.sma.de/dl/7418/Ableitstrom-TI-en-25.pdf

 

[Pete999]

I have spent some time reading into PV panels and inverters while trying to help with a current post on nuisance RCD tripping at a PV installation on a farm. I happened upon this technical paper which explains capacitive leakage currents from PV arrays and the trouble they could cause, especially when transformerless inverters are in use. I thought others would be interested in reading it and learning from it. Helpfully, a compensation scheme is suggested at Section 6 and a way of estimating the capacitance.

http://files.sma.de/dl/7418/Ableitstrom-TI-en-25.pdf

Thanks Marconi, an interesting read.

 

[The Ghost]

That is worth a bookmark indeed. We have a PV system at one of the sites I work. I confess to knowing very little about them. This does trip here and there and I have been asked to look at it. When I say I know nothing about them I understand the underlying principles of generation and conversion etc. But the info on that sheet while a bit heady is of value perhaps. Happily the PV has stopped tripping... for now.

 

[marconi]

My take -aways are that the displacement current effect is stronger when the upper panel surface is wetted or has moisture on it, and the combination of make/model of panels with make/model/technology of inverter can matter. Plus, the capacitive leakage current can bias the rcd towards its trip threshold-hence the compensator scheme of shunt capacitor and 4 pole rcd.

 

[The Ghost]

4 pole rcd

Ok now that threw me when I read that, I thought it was talking about single phase, so where does a 4 pole come into it?

 

[Wilko]

Hi V, I think it's to feed the compensating cap current through the RCD and allow the whole thing to be diss.

 

[marconi]

Yes, that's right Wilko. A three pole RCD (if they make them) would suffice too. It's a way injecting into the RCD sense coil a current(magentic flux) in antiphase to that created by the capacitive leakage current. The way the compensator capacitor is connected in circuit and to the terminals 2 and 1 achieves this. I think its a neat simple remedy.