Max VOC - How strict?

Well it looks like there’s a way to make a voltage limiter, regardless it’s way out of my wheelhouse. Yes it obvious just size the panels correctly. But so many nice panels are approaching 50ocv and some charge controllers really respond better and have more day hours with running in the upper end of the range(but not over). MPP LV6548 inverters are that way but Victron seems to make good power even running in the mid range.

AntronX said:

But will you pass inspection? That's the main purpose of RSD. Once passed you can remove it.

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Not sure if UL Listing requires a single physical panel per input. I can connect 2 or 3 of some models in series and comply with voltage limits.
REC panels 2 in series would be pushing the 80V (STC) limit but remain under 90V cold. I have a bunch of AP120 (take down from old property to replace with SunPower 327W), could put 3 in series into each and easily meet 80V absolute max input.

RSD is promoted as fireman safety. I would not want to have an RSD switch and sticker without the function operating. I'd say remove the switch too if you're going to remove the RSD boxes, so as to not falsely represent it to firemen.

AFCI is supposed to be fire safety. Which may be more necessary with 2x the connections and possibly mating of incompatible species.
Fire safety is of interest to me. Considering that several Walmart had PV fires even after PV system was shut off (manual version of AFCI), I'm interested in AFCI triggering RSD.


"The product will offer outstanding system safety thanks to full integration with SMA’s world-leading AFCI function."

I like the sound of that.

NEC2020 calls for 80V limit within array boundary and 30V outside array boundary. So if your panels are <40Voc each then it would be compliant. But 60 cell panels have >40Voc so that will not pass inspection if 2 are in series. The only way to be within the 80V limit is to connect all panels in parallel. That would satisfy 80V in boundary limit but not 30V out of boundary requirement. I wonder if placing high current contactor within PV boundary will pass inspection. But then you have to run 4/0 cable 50 feet down from the roof and still have 4% power loss with 16 panels. Quite ridiculous method to avoid using RSD and remain within limits.

The kicker is that the mosfet becomes more sensitive in cold weather, while the solar panel produces a higher voltage in cold weather.

So even a 48 VOC panel, three in series for a total 144 VOC, can damage a 150VOC rated charge controller in February in Michigan. I've blown two charge controllers, and as best as I can tell, this is the reason.

There is value in adding a protection circuit, clamp style, and if well designed it would't have to burn much current. It just dissipates enough current to keep the voltage below 140V all the time. 0 current below 140V, up to as much current as the panels supply the higher it goes. Since the panels aren't efficient near their VOC, it should be a small amount of energy, but this is only going to happen when it's cold - dump it into a heater in the battery room.

When the charge controllers are operating properly, they will keep it well below the VOC, so this would only run when the charge controllers aren't operating correctly, or if the batteries are full.

stienman said:

There is value in adding a protection circuit, clamp style, and if well designed it would't have to burn much current.

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Bonus points if this shunt protection circuit is built into MPPT controller itself. This way any heat produced from shunting is used to warm up the mosfets and raise their breakdown voltage limit.

AntronX said:

NEC2020 calls for 80V limit within array boundary and 30V outside array boundary. ... But then you have to run 4/0 cable 50 feet down from the roof and still have 4% power loss with 16 panels. Quite ridiculous method to avoid using RSD and remain within limits.

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I had contemplated that (have a suitable combiner box.) But that would only be for a 48V DC coupled system, and I use AC coupling.

What I'm referring to now is high voltage series strings with RSD. Just trying to shoehorn in all the panels I can with as few RSD as possible, minimize added cost. If I bought newer higher wattage panels it would bite less.

I picked up more RSD cheaper so it is becoming more palatable.

Tigo TS4-R-2F dual panel RSD is $50 or $25 per panel or 7.5c/W extra per 330w panel. For me it would cost $400 extra for 16 panels (not counting PLC transmitter and hardware) and would add 3 months to payback period. I guess it's not worth the effort and headache of trying to engineer around this.

Since I paid $0.32/W and $0.15/W for greater number similar size panels, my pain (annoyance) threshold is lower.
I bought -2F for similar price, just picked up -F for about $11 apiece.

I'm just trying to squeeze all I can onto the RSD modules. I might have used the wrong data sheet before, was in danger of exceeding a hard 80V limit for -2F in cold weather. Reallocating my planning between two systems I think I've solved that, will use the -2F for 64Voc SunPower. The -F says 80V, but tolerates 90V which would be approached by REC in freezing.

I don't have enough REC for a planned 15.6kW system, only half. I'm considering using the (1/3 less efficient) AP120. In that case it will be 3s for 63Voc into each -2F RSD input.

Long time planning and gathering, so I change my mind a few times.

stienman said:

...

So even a 48 VOC panel, three in series for a total 144 VOC, can damage a 150VOC rated charge controller in February in Michigan. I've blown two charge controllers, and as best as I can tell, this is the reason. ...

Since the panels aren't efficient near their VOC, it should be a small amount of energy, but this is only going to happen when it's cold - dump it into a heater in the battery room.
...

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I know this is an old thread, but that's a good point.
You wouldn't want to rely on this obviously... and as stated in this thread, if you design your system correctly to begin with, you shouldn't even be experiencing this.

But if you HAD to implement something like this or just wanted to as an extreme failsafe, it'd be worth looking into.

Like stated it's likely you'd only ever be going over the VOC when it's cold out, so any "power" lost as heat would be reclaimed into the voltage limiters surroundings, to good use.