Fronius IG to Primo

[lind]

I'm trying to replace my Fronius IG4000 with a Fronius Primo 3.8, but I'm confused about the PV grounding. I have a 10+ year old grid-tie system and the Fronius IG inverter has gone bad, as diagnosed by a solar electrician. He offered to replace it but only if he sold me the inverter for twice it's price, and I thought it would be easy enough to do myself. Unfortunately I'm stuck now. My existing system has a ground coming from the PV modules, which is bonded to a junction box, and then passed to the existing bad inverter, where it's bonded and passed to the house ground. The new Primo inverter doesn't have a transformer and requires an ungrounded DC connection. So I don't know what to do with the existing ground line from the solar cells. I've read that I could just remove the ground, but that seems odd. The manual just assumes my system is not grounded. Any advice?

 

[BentleyJ]

I have an old IG Plus 3.0, still working. Don't recall if the either the PV + or - is grounded, usually its not. Most systems the EQUIPMENT ground is only connected to the panel frames, mounting rails, EMT and metalic J boxes.

When you say "bonded" are you referring to the ground wire being connected to a ground lug on the aluminum inverter chassis? Here again this would only be the Equipment ground not a System ground. Most systems, the DC voltage is insulated from ground.

Please clarify.

 

[lind]

I have an old IG Plus 3.0, still working. Don't recall if the either the PV + or - is grounded, usually its not. Most systems the EQUIPMENT ground is only connected to the panel frames, mounting rails, EMT and metalic J boxes.

When you say "bonded" are you referring to the ground wire being connected to a ground lug on the aluminum inverter chassis? Here again this would only be the Equipment ground not a System ground. Most systems, the DC voltage is insulated from ground.

Please clarify.

Thank you for responding. To clarify a bit, I have three wires coming from the PV array: black, red, and green. The green/ground is connected to a ground lug on the chassis of the DC disconnect before the inverter. It is then passed through to the Fronius IG inverter, where it wired to the ground screw as well as to the chassis of the inverter. That is all then passed to the house ground.

I tested for continuity across the green and both DC+/- wires and there wasn't anything between ground and red or ground and black.

 

[Hedges]

Green/ground bonded to chassis is good, always want that.
If you measure voltage Black to chassis, does it hold steady voltage, or decay to zero?
If you measure Red to chassis does it hold steady voltage, or decay to zero?

How old are the panels? Some older panels have to be negative grounded (typically in the inverter) and some have to positive grounded, otherwise they rapidly decay due to PID.

"PID free" panels are necessary for use with transformerless inverter.

 

[lind]

The panels are about 10 years old. Voltage from the panels: ground to red = 320V steady, ground to white = 0V steady.

I miss-stated above about the continuity from the panels: ground to red has no continuity, ground to white has continuity.

 

[Hedges]

OK, white is bonded to ground somewhere. Need to disconnect that for use with transformerless inverter.

Start by disconnecting from inverter.

Safest to shut off inverter (if operating), open any breakers, switches, or touch-safe fuses.
Then disconnect PV+ and PV- MC4 connectors at array.
Confirm zero voltage on black and white wires. Check for continuity white to ground, figure out where their bonded and disconnect.

My older inverters bond PV- to ground through a fuse or maybe an electronic circuit. Earliest ones were a 1A fuse in screw-in fuse holder. (There was a recall issue involving wrong hot wire going to fuse holder body instead of recessed contact.) Later model had plug in touch-safe fuse holder.

 

[lind]

OK, white is bonded to ground somewhere. Need to disconnect that for use with transformerless inverter.

Start by disconnecting from inverter.

Safest to shut off inverter (if operating), open any breakers, switches, or touch-safe fuses.
Then disconnect PV+ and PV- MC4 connectors at array.
Confirm zero voltage on black and white wires. Check for continuity white to ground, figure out where their bonded and disconnect.

My older inverters bond PV- to ground through a fuse or maybe an electronic circuit. Earliest ones were a 1A fuse in screw-in fuse holder. (There was a recall issue involving wrong hot wire going to fuse holder body instead of recessed contact.) Later model had plug in touch-safe fuse holder.

Thanks this is very helpful. To sum up I'm just trying remove the continuity from DC white to ground, and by disconnecting this bond I should be okay to use the transformerless Primo inverter. There will be no ground used/required from the panels if I understand correctly.

 

[Hedges]

Yes except, you need ground wire bonded to panels and to chassis of inverter, with a wire running between them.

That prevents a shock hazard in case of damage. And for some brands, prevents a shock hazard in normal operation (some superimpose AC on PV+/- wires, and this capacitively couples to PV panel frames.

Some types of PV panels aren't compatible with transformerless inverters due to high capacitance and therefore AC ground leakage current. I think those are unusual thin-film types.

It is PV+ and PV- wires that aren't grounded.

 

[lind]

I think I see. So I'm removing the ground bond with the PV+/- wires, but still need a ground wire from the panel frames, bonded to the junction shutoff chassis, and to the inverter chassis?