voltage limiting

[xcentric]

I'm aware that going over the PV input voltage limit for inverters is a no-no, but it seems strange to me that we have to size arrays for the max possible input (low temperatures, blue sky, bright sun) when in reality, especially in the UK, panels are at suboptimal angles, not directly facing the sun, it's warmer than the minimum, and it's often hazy or cloudy, ands we're running at much less than this theoretical capacity.

In other scenarios one would size for more common conditions, and lose the excess when it happened on the odd occasion, but not with solar. Is there no device that exists to limit PV voltage on those rare but most powerful of generating days, allowing me to run more panels and my inverter at max for more of the rest of the time? E.g. if at the moment I size my array to run at just under the max input, then for most of the year I'm probably generating at about 50% of this max. Whereas if I could limit the voltage, I could size the panel array so that I was at 80-90% max, say, for most of the time. Since this seems sensible, why hasn't someone invented something like this?

 

[KnobCountry]

Voltage limiter between panel and controller for occasional voltage spike

I am using a 3kW Stackable 48V 150VDC 80A Off-Grid Inverter by Growatt, which has a Maximum PV Array Open Circuit Voltage of 145VDC. My panel array sits about 110-125V most of the time, but I had one time where there was a cold sunny day where it spiked above 145VDC and triggered a fault in the...

diysolarforum.com

PV String DC Voltage Limiter

Need a way to limit PV string DC voltage to that allowed by a hybrid inverter. Unique scenario. Imagine a situation where you are unable to change your panels or the string size of the panels (permits, zoning, etc) and you need to limit the DC voltage to a lower value than the string can...

diysolarforum.com

Any way to clamp PV voltage above a certain amount?

This is probably a dumb question, and I'm sure it's been asked in different contexts, but I couldn't seem to fine an answer. I have a Sol Ark 12k and am planning my array (currently, at least) so that under summer temps would be about 485 volts on each of the two MPPT controllers (at 11amps)...

diysolarforum.com

Its already invented. Its a "buck" converter. MPPT adapts to the sunlight available. A buck converter will incur losses, heat and doesn't adapt to the changing sunlight conditions.

Easier to just panel at 85-90% voltage and slightly over panel for current.

 

[wpns]

Well, first of all, do you want to size your system so it only blows up 'occasionally', or 'never'?

Second, panels have a certain Voc (plus temperature and other effects), but you can't pick 9.5 panels in your string, so you are 'losing' voltage with 9, but still the MPPT will harvest the maximum power from all the panels in that string, so the string of 9 will do just fine. It's all a tradeoff between number of panels you can fit in a space, Voc (plus plus) for those panels, and inverter details.

If you want to overpower (oversize) your array, just put more panels in parallel at less than peak breakdown voltage, and then your Isc will exceed your MPPT input, but it won't draw more than that, so it'll get more MPPT power for longer during the day (and for cloudier days and other less idea conditions).

Note, not Victron, they don't allow overpowering Isc for some reason...

 

[KnobCountry]

Well, first of all, do you want to size your system so it only blows up 'occasionally', or 'never'?

Second, panels have a certain Voc (plus temperature and other effects), but you can't pick 9.5 panels in your string, so you are 'losing' voltage with 9, but still the MPPT will harvest the maximum power from all the panels in that string, so the string of 9 will do just fine. It's all a tradeoff between number of panels you can fit in a space, Voc (plus plus) for those panels, and inverter details.

If you want to overpower (oversize) your array, just put more panels in parallel at less than peak breakdown voltage, and then your Isc will exceed your MPPT input, but it won't draw more than that, so it'll get more MPPT power for longer during the day (and for cloudier days and other less idea conditions).

Note, not Victron, they don't allow overpowering Isc for some reason...

I don't think that is exactly true re: victron.

Matching solar modules to MPPT charge controllers - Victron Energy

Life used to be so simple; in a 12V battery system you took a ‘12V’ solar module, watched carefully that the maximum PV current would not exceed the charge controller maximum current and the system would work. Unfortunately due to the fact, that with PWM controllers the PV module is not feeding...

www.victronenergy.com

Oversizing a PV Array (within max Voc and Isc) can do any damage to MPPT ? - Victron Community

community.victronenergy.com

 

[kommando]

It's not the rare most powerful days you need to worry about, the higher temperatures that go hand in hand will limit the voltages. The days to worry about are early to late mornings on a cold day as you get full sun just after a cloud has passed. I have seen 330w from a 270w panel in those conditions.

 

[wpns]

I don't think that is exactly true re: victron.

I don't make the news, I just report it:

https://www.victronenergy.com/upload/documents/Datasheet-SmartSolar-charge-controller-MPPT-100-30-&-100-50-EN.pdf

Max.PVshortcircuitcurrent 2)
[...]
2) A PV array with a higher short circuit current may damage the controller.

 

[efficientPV]

Actually, techluck makes a PV voltage limiter. It is a variation of the water heater board. It is still absolute crap like their water heater board. It is prone to totally destroying itself with a voltage spike.

 

[hwy17]

The problem statement is correct: panels typically operate at partial power, so why should systems be designed for peak output.

The proposed solution is non productive; Trying to design longer strings. The voltage profile of the panels doesn't even change that much based on the output. Over voltage events actually typically occur at quite low power outputs during cold winter mornings, not peak output.

There is a productive solution: Over panel in parallel, not series. Instead of trying to shove a 9th or 10th panel onto the string, picture doubling the array and running 2 strings of 8 in parallel. Over amping is completely electrically acceptable and aligns with the real production output curve, which is the amps.

 

[sunshine_eggo]

I'm aware that going over the PV input voltage limit for inverters is a no-no, but it seems strange to me that we have to size arrays for the max possible input (low temperatures, blue sky, bright sun) when in reality, especially in the UK, panels are at suboptimal angles, not directly facing the sun, it's warmer than the minimum, and it's often hazy or cloudy, ands we're running at much less than this theoretical capacity.

Voltage is not capacity. An MPPT has an output rating. It is very common to have MORE array power on an MPPT than it can ever deliver, and this can easily be done without risking exceeding Voc limits.

It seems strange because you do not understand that light intensity has very little effect on voltage. Key concepts:

1) light gives voltage
2) intensity of light gives current

Even with only 20% max irradiance on a panel, the Voc is going to be about 1-2% off the max.

Check out a typical power curve for a PV panel:



Note how the Vmp point barely changes between 20% of max irradiance and 100%.

In other scenarios one would size for more common conditions, and lose the excess when it happened on the odd occasion, but not with solar. Is there no device that exists to limit PV voltage on those rare but most powerful of generating days, allowing me to run more panels and my inverter at max for more of the rest of the time?

No. You can't effectively limit voltage and again, you're operating under the flawed concept that more light = more voltage and that higher voltage = more capacity.

You also seem to think that for some reason, you need more voltage. This just is almost never the case. All MPPT are OUTPUT limited - typically by current, so in almost all cases, you should be able to fully utilize all of your MPPT's capabilities (and often notably exceed them) without challenging the Voc limit.

E.g. if at the moment I size my array to run at just under the max input, then for most of the year I'm probably generating at about 50% of this max.

No you are not. Even if your total power output is reduced to 50%, your Voc based on irradiance will vary only a small amount year round as will Vmp.

It's the current that changes. If your array is only outputting 50%, it's doing so at temperature adjusted Vmp with about 50% the peak CURRENT.

Whereas if I could limit the voltage, I could size the panel array so that I was at 80-90% max, say, for most of the time. Since this seems sensible, why hasn't someone invented something like this?

Again, the idea that hitting max Voc to get max output is a flawed assumption and limiting voltage is essentially impossible with simple components, and it gains you nothing.

 

[wpns]

Over amping is completely electrically acceptable

Except for Victron, so be aware if you use their components.

 

[sunshine_eggo]

Except for Victron, so be aware if you use their components.

They have caveats on this. The published limit is for PV reverse polarity protection as that's the max current the contactor can handle. If you're willing to forego reverse polarity protection, there is no practical limit.

The exception is their RS450/X00 line. They publish Imp, Isc and don't you dare values for PV input current on each MPPT.

 

[KnobCountry]

Except for Victron, so be aware if you use their components.

Like a dog with a bone even after being corrected. Are you OK?

I along with others have for example 24 amps on a 100/20 and it just clips the extra.

Spreading misinformation intentionally seems sus

 

[wpns]

Like a dog with a bone even after being corrected. Are you OK?

I along with others have for example 24 amps on a 100/20 and it just clips the extra.

Spreading misinformation intentionally seems sus

You linked to:

Matching solar modules to MPPT charge controllers - Victron Energy

Life used to be so simple; in a 12V battery system you took a ‘12V’ solar module, watched carefully that the maximum PV current would not exceed the charge controller maximum current and the system would work. Unfortunately due to the fact, that with PWM controllers the PV module is not feeding...

www.victronenergy.com

which says:
/*

Determining the maximum PV short circuit current​

Get the maximum PV short circuit current from the PV Panel datasheet. Multiply by the number of panels in parallel in the array. Having more panels in series does not change the number.
The result of the calculation may not exceed the Max PV short circuit current as specified in the MPPT Datasheet.
*/
and:

Oversizing a PV Array (within max Voc and Isc) can do any damage to MPPT ? - Victron Community

community.victronenergy.com

Where there's a lot of uncertainty, and then the moderator replies:
/*
Yes, there are unspecified conditions that the MPPT controller will short circuit the PV array.
*/

Always happy to be corrected (that's how I learn), but doing the above seems <ahem> risky to me.

 

[RCinFLA]

Maximum Voc has little to do with illumination shortcomings.

As long as there is enough illumination to overcome panel self-leakage shunt current the panel will go to Voc.

 

[Brucey]

You linked to:

Matching solar modules to MPPT charge controllers - Victron Energy

Life used to be so simple; in a 12V battery system you took a ‘12V’ solar module, watched carefully that the maximum PV current would not exceed the charge controller maximum current and the system would work. Unfortunately due to the fact, that with PWM controllers the PV module is not feeding...

www.victronenergy.com

which says:
/*

Determining the maximum PV short circuit current​

Get the maximum PV short circuit current from the PV Panel datasheet. Multiply by the number of panels in parallel in the array. Having more panels in series does not change the number.
The result of the calculation may not exceed the Max PV short circuit current as specified in the MPPT Datasheet.
*/
and:

Oversizing a PV Array (within max Voc and Isc) can do any damage to MPPT ? - Victron Community

community.victronenergy.com

Where there's a lot of uncertainty, and then the moderator replies:
/*
Yes, there are unspecified conditions that the MPPT controller will short circuit the PV array.
*/

Always happy to be corrected (that's how I learn), but doing the above seems <ahem> risky to me.

Here's the critical post in that thread:

"Hi @thanar,

The maximum Isc (input short circuit current on the PV panels) is a limitation of the reverse polarity protection within the MPPT for the PV array.

If you connect a PV array in reverse polarity that is below the short circuit current limit, then the MPPT has a protection circuit that will allow you disconnect the PV array, reconnect it and have the MPPT continue to operate without any damage.

If you exceed the PV array input current limit AND connect the PV array in REVERSE POLARITY, then there is likelihood of damage to the MPPT, and this damage in not a manufacturing fault and will not be covered by warranty.

Connecting a PV array in correct polarity that exceeds the PV input current limit is possible, and in some cases desirable, but comes with potential risks of damage to equipment if incorrectly installed, or later modified or adjusted by someone who is not aware of the situation."


Basically don't exceed the amp rating to avoid damage if you so happen to connect them in reverse polarity. Assuming you understand the risks, you can exceed that limit, but you are on your own if you make a mistake

 

[wpns]

And in the other thread, they admit that there are circumstances which can cause properly polarized panel strings to blow up your controller if your Isc exceeds the spec limit on the controller.. Not testing it on my Victron when I buy it.