UL listed Current Limiter?

[svetz]

A while back I was looking at bifacial panels on a string inverter, but discarded the idea as the power output was too variable and it messed up the economics.

Example Depending on albedo, a bifacial panel's power output can be +30% the rated wattage; so a 350W panel can vary from 0 to to 455W at STC.

That absolute max power is a only possible a couple of days out of the year on a fixed-tilt system (e.g., equinox after a hard rain at noon). But on average it'll be far far less. So if you size the system for 455W you spent a lot of money to handle power you'll almost never see.

This is basically the same dilemma about how much power to clip, but due to the variability of the magnitude is greater.

But if there was a current limiting device, you could size the system for the non bifacial wattage and clip the excess. Economically you'd come out way ahead because the bifacials would gather more light under the same conditions as their counterparts. At least that's what the SAM numbers looked like based on available string inverter sizes and costs.

It struck me that an inexpensive device that limited the PV output power would be useful, but didn't see anything appropriate. If you could find a fuse/breaker guaranteed to trip at X amps, but not at .95 X that would probably work, but AFAIK they're just not that sensitive.

Something like this could be done with some CTs and power relays to shut off one or more strings to reduce overall total, and on the old forums @PHoganDive posted this circuit which seems like it would do the job:

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But DIY devices are not UL Listed devices. Any ideas on something out that that is approved that could do this at String level voltages and amps?

 

[Will Prowse]

What I like to do is get a SCC with built-in current limiting. So that it can "over-panel" to double the max rated PV input wattage. Such as the BN series from epever. That way my array can be 2x bigger than necessary. Then I can over panel even further (3x the rated pv max) and use a fuse or circuit breaker to keep it below the max overpanel limit of the SCC

This works extremely well in places like alaska where the sun is low and over paneling is a must.

Or you could build the circuit above. And for grid tie, no need to worry because those inverters will use the grid as a "dump" and push whatever they can. And they are cheap considering how much power they push, so I would see benefit in just spending the extra money and getting a bigger SCC.

But in places far from the equator where your panels only produce 50% what they would at the equator, you must over panel. But the further from the equator you go, the colder it gets which means the coefficient of your panels will cause increased output. So if you have a massive array thats freezing cold, and you get a good hit of morning sunshine, the voltage may rise so high as to hurt the SCC or something else. Usually its not a big issue and you can size it accordingly with some head room, but it is a problem to think about in colder climates with large arrays.

 

[svetz]

It seems like a no-brainer device, dead useful in a lot of applications. I'm sort of surprised I can't just go to digikey and order one.
Honestly @PHoganDive's circuit doesn't look like it would be expensive to build (although getting a UL listing would be expensive).
Ideally these would be din mountable and circuit-breaker shaped, then you could just put them in a combiner box.

... And for grid tie, no need to worry because those inverters will use the grid as a "dump" and push whatever they can.

Most devices can take a some over current, but even string inverters have heat limits. For example the Sol-Ark can take up to 11 kW PV, but it can only continuously push 9 kW to the grid (it can put the other 2 kW into the battery). More than 11k, "bad things happen" according to their chief engineer.

 

[svetz]

Here's another application where a cheap reliable current limiting device would be useful (assuming the math/science isn't all claptrap ): http://www.diysolarforum.com/index.php?threads/incrementally-adding-batteries.186/post-1797

 

[Dave1]

I've thought about this becasue the internal MPPT Charge controller on my MPP solar 12V pip can only handle 500W and I would love to put 6 100w panels on the roof in a 2p3s arrangement but in the colorado summer sun I imagine we could bump past that 500w limit on the right days.

 

[HighTechLab]

What I like to do is get a SCC with built-in current limiting. So that it can "over-panel" to double the max rated PV input wattage. Such as the BN series from epever. That way my array can be 2x bigger than necessary. Then I can over panel even further (3x the rated pv max) and use a fuse or circuit breaker to keep it below the max overpanel limit of the SCC

This works extremely well in places like alaska where the sun is low and over paneling is a must.

Or you could build the circuit above. And for grid tie, no need to worry because those inverters will use the grid as a "dump" and push whatever they can. And they are cheap considering how much power they push, so I would see benefit in just spending the extra money and getting a bigger SCC.

But in places far from the equator where your panels only produce 50% what they would at the equator, you must over panel. But the further from the equator you go, the colder it gets which means the coefficient of your panels will cause increased output. So if you have a massive array thats freezing cold, and you get a good hit of morning sunshine, the voltage may rise so high as to hurt the SCC or something else. Usually its not a big issue and you can size it accordingly with some head room, but it is a problem to think about in colder climates with large arrays.

The outback Flexmax charge controllers can be overpaneled. The FM80's are rated for 80 amps output, and it will not go over - They actually can be adjusted down from 5 amps to 80 amps with a user adjustable setting. So if you have 20,000 watts of solar on a 12 volt battery bank, you will still only get 960 watts out. If you up your voltage to 60 volts (they will work on a 60v nominal system) you can get 4800 watts out, even if you have 8000 watts of solar it will limit itself so that it doesn't blow up.

The Flexmax is a MPPT controller as well so you get the most out of your panels.

 

[gnubie]

Victron's can do current limiting too. I have more panels on my controller than it is rated for and should they ever get to those dizzy heights, such as just now!, it limits the output current to what ever. 50 amps in my case because my battery can handle that much current. Any higher production by the panels just results in the panel voltage going up.

 

[svetz]

Victron's can do current limiting too. I have more panels on my controller than it is rated for and should they ever get to those dizzy heights, such as just now!, it limits the output current to what ever. 50 amps in my case because my battery can handle that much current. Any higher production by the panels just results in the panel voltage going up.

Really!? Which Victron device?

 

[gnubie]

Any of their MPPT charge controllers, perhaps their PWM ones too if they still offer them. By default they are configured with their max rated current but you can configure them via the smartphone app / PC software / serial port commands to have a lower limit of your choice.

 

[svetz]

Oh duh! Sorry... was thinking it was a stand-alone current limiter rather than an MPPT.

 

[svetz]

The outback Flexmax charge controllers can be overpaneled.

Yep, and that's the problem. Well, a problem. Current limiters would also be useful in other applications other than solar panels where MPPTs wouldn't be appropriate; so they'd be handy little stand-alone UL devices.

I didn't see anything on the maximum current for the FM80 but generally all devices have some max input. For example the FM100 is limited to a maximum input of 64 amps in (ref). I imagine the Victron's have some hard ceiling too even if it's not listed.