fancourt avatar image
fancourt asked

Prevention of PV Potential Induced Degradation (PID) with Multi RS Solar/Inverter RS Smart Solar

I'm still learning about this topic and hoping for some assistance from those more knowledgeable in this area than me...

Given that the RS series solar charge controllers do not allow Earthing/Grounding of the PV DC conductors and that these SCC are designed for array voltages that apparently approach/exceed the voltage threshold where PID of PV arrays becomes an issue, is there anyone from Victron (or elsewhere) who can explain to me either:

1) how this issue is not applicable to these RS products or

2) how to prevent it occurring in systems using these RS products?

I would rather not have to resort to "Anti-PID boxes" that apply a large reverse potential to the array at night to attempt to 'undo' the PID effects caused during daylight operation.

The DC PV circuit on standalone MPPT RS controllers is obviously galvanically isolated from the battery connections, so it should not be possible for any potential between the array and earth/ground to exist.

However I have only just learned that there is NO galvanic isolation between the DC PV array circuit on Multi RS Solar and the AC output, so if the AC output is not configured as IT (ie any of TN-S, TN-C or TN-C-S or TT) then there will likely exist a potential between the PV DC and Earth/Ground.

There is no mention of this lack of galvanic isolation in the Inverter RS Smart Solar manual or datasheet (that I could find), but I am still waiting for confirmation whether the same lack of galvanic isolation as exists on the Multi RS Solar also exists here, but I suspect it does.

I'd be very grateful for any clarification anyone can provide.

mppt rsMulti RSinverter rsrs inverter
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1 Answer
Alex Pescaru avatar image
Alex Pescaru answered ·

Hi @fancourt

From my knowledge:

1. The PID is in fact a leakage current / positive ions generated by high DC voltages that move from the glass, frame and installation structure to the solar photovoltaic cell. But no one is forbidding you to ground the PV frame. And when grounding the frame, there is very little possibility of PID.

2. In general, PID effect is noticeable from 1000V strings and up. I doubt many will have more than 400V strings (most under 350V), this also because of the effect of the negative temperatures on string's VoC.

Just my 2 cents...


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fancourt avatar image fancourt commented ·

Regarding Point (1), see my reply to @Duivert NL below. Grounding the frame causes PID, not prevents it.

Regarding Point (2), there seems to be no consensus on the minimum voltage threshold at which PID occurs. Some sources say 600V, others say a lot less - for example the SMA document linked to below refers to MPPT ranges of 300 - 500V which overlaps very significantly with the range of array voltages connected to RS Series MPPT controllers. Unless you're installing in a climate that gets colder than maybe -10 C in Winter then your "most under 350V" comment is unlikely to be true.

A large proportion of Victron's global market would never see temperatures colder than -10 C and therefore I would suggest array Voc of >400V would be reasonably common in these climates. In the specific case I have in mind here Voc (@STC) is 410V and unless Climate Change dramatically messes with Winter minimum temps then this array should never, ever hit 450V.

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Alex Pescaru avatar image Alex Pescaru fancourt commented ·

Hi @fancourt

I stand corrected. You may be right... Sorry.

What I can say is that the power output of my panels connected to the Multi RS Solar is in no way "reduced by up to 70%" how they are saying on those documents. The solar power curve is as it should be for the conditions. Sometimes even bigger than NOCT specifications.

But my panels are not grounded at all, despite safety rules, I know...

More of that, the PV panels are the cheapest thing from a solar system. And at 20 cents or less per watt and their longlife specifications, you can change them when you suspect "foul play".


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fancourt avatar image fancourt Alex Pescaru commented ·
I would love to get some official word from Victron on how they factored these issues into the design of these products (if indeed they did?)... I assume Victron engineers are more versed on topics such PID than we are and so insights into their thinking and design choices would be really helpful here.

All the research into PID seems to point to it being an incompletely understood phenomenon with many examples of large solar farms cited where only seemingly random sections of the installed array are affected by PID whilst other identical sections escape from any PID effects at all.

If your panel frames are not grounded at all then it seems you are probably safe from PID. Curious on your thoughts on why you chose not to ground the frames?

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Alex Pescaru avatar image Alex Pescaru fancourt commented ·

Many taller buildings and trees around the place where the PVs are situated. Luckily on their north side, so no shading problem. For sure the lighting will hit them before. Anyway, we'll see... :-)))

Also, slim chance for anyone to touch them, because they are in a secluded area.

And many recommendations like the one below on similar inverters.

WARNING: Because this inverter is non-isolated, only three types of PV modules are acceptable: single crystalline, poly crystalline with class A-rated and CIGS modules. To avoid any malfunction, do not connect any PV modules with possible current leakage to the inverter. For example, grounded PV modules will cause current leakage to the inverter. When using CIGS modules, please be sure NO grounding.

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Duivert NL avatar image Duivert NL commented ·
i agree with that, grounding the frame and installation brackets will prevent PID
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fancourt avatar image fancourt Duivert NL commented ·

Everything I have read precisely and completely contradicts this.

Assuming everything I have read isn't completely wrong, the cause of PID is actually the very fact that the PV module frames and brackets are grounded.

Grounding the frames allows a current to flow from the cells themselves to ground via the frame changing the properties of the cells and degrading their output.

I could add many more links, but that should suffice I hope.

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Duivert NL avatar image Duivert NL fancourt commented ·
Yes you are right, but i still partly disagree, all the studies i have seen are on a single earthpoint on the installation frame, and panels making contact with that frame by clamping them, in large systems (a lot of panels in series) i do think PID is a real problem

i believe if you earth all the individual installation frames and the panels themself (most pv panels have dedicated earth connection points) chance of PID is minimal, also most victron installs are with low power MPPT’s (200v max)

Local regulations and safety are the reason why all my panels are individually earthed to a single point in the complete installation (i have 24 panels in 2s 2p config on 6 mppts, so voltage is always below 100v)

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