Overpanelling string inverter limits -- my theory on why the limit exists.

[hex4def6]

So I've seen some debate / discussion on why there are practical limits on overpaneling. I've seen SolarEdge recommend 135% / 150% / 175% depending on model, for example.

What I haven't found is "why" they have these limits, but I have some theories.

1. They are concerned about the inverter developing a fault condition (eg, internal short). If Isc is significantly above the design conditions, you might get some nasty situations develop before any OCP devices activate. (wires melting / OCPD vaporizing, etc).
2. They might just choose a reasonable practical limit to test their design to. No point in testing something no-one will use in practice.
3. There are actual edge cases in which over-paneling may damage something.

I've been thinking about #3, and here's something I think could potentially damage an MPPT controller:
Imagine you have 4x the max power array. Let's say the inverter is rated for 8kW, so this is a 32kW DC panel array.
You've kept under Voc limits, so the only difference is Imp could be significantly larger than the 8kW rating (or whatever the inverter is). In normal situations, when the MPPT is tracking correctly, this is not an issue. It approaches the (Vmp, Imp) point from the VOC direction, but reaches Imax before it reaches that point. No problem, it operates fine at this point (V_at_imax, Imax) = 8kW.

Now, imagine you have sudden shading occur. The MPPT tracker will now start hunting for Pmp. That may result in it choosing a current that is significantly higher than what it's designed for is maxed out (say 20A) but since the shading is resulting in reduced voltage, it's still under it's max power limit. 30A * 250VDC = 7.5 kW (say) Everything is still fine.

Now, let's imagine the clouds go away suddenly. Suddenly the array is capable of a full 32kW. Still 20A, but the array voltage rises from 250VDC to (say) 500VDC. Because the MPPT is still at the shaded MP point, it's suddenly overloaded -- maybe significantly over its maximum. That 7.5kW is now suddenly 15kW. It takes time for the MPPT to react (I'd be curious about how quickly this is? 100's of milliseconds?)

In this time, the FETs, etc are subjected to 2x+ the rated maximum, and you potentially burn things up.

Thoughts on this theory?

 

[wpns]

So I've seen some debate / discussion on why there are practical limits on overpaneling. I've seen SolarEdge recommend 135% / 150% / 175% depending on model, for example.

What I haven't found is "why" they have these limits, but I have some theories.

1. They are concerned about the inverter developing a fault condition (eg, internal short). If Isc is significantly above the design conditions, you might get some nasty situations develop before any OCP devices activate. (wires melting / OCPD vaporizing, etc).
2. They might just choose a reasonable practical limit to test their design to. No point in testing something no-one will use in practice.
3. There are actual edge cases in which over-paneling may damage something.

I've been thinking about #3, and here's something I think could potentially damage an MPPT controller:
Imagine you have 4x the max power array. Let's say the inverter is rated for 8kW, so this is a 32kW DC panel array.
You've kept under Voc limits, so the only difference is Imp could be significantly larger than the 8kW rating (or whatever the inverter is). In normal situations, when the MPPT is tracking correctly, this is not an issue. It approaches the (Vmp, Imp) point from the VOC direction, but reaches Imax before it reaches that point. No problem, it operates fine at this point (V_at_imax, Imax) = 8kW.

Now, imagine you have sudden shading occur. The MPPT tracker will now start hunting for Pmp. That may result in it choosing a current that is significantly higher than what it's designed for, but since the shading is resulting in reduced output, it's still under it's max power limit (Let's say, it's tracking at 7kW). Everything is still fine.

Now, let's imagine the clouds go away suddenly. Suddenly the array is capable of a full 32kW. Because the MPPT is still at the shaded MP point, it's suddenly overloaded -- maybe significantly over its maximum. It takes times for the MPPT to react (I'd be curious about how quickly this is? 100's of milliseconds?)

In this time, the FETs, etc are subjected to 3x+ the rated maximum, and you potentially burn things up.

Thoughts on this theory?

Makes sense.

 

[Ampster]

1. There are actual edge cases in which over-paneling may damage something.........Thoughts on this theory?

You are correct in the sense that voltage is the big risk in over paneling.
Most charge controllers inside inverters and in stand alone situations are designed with components which have voltage limits. That is why over voltage is the biggest risk of over paneling. Current is not so much a risk because the charge controller circuitry will only pull the current it can process. Maximum short circuit voltage is the most an MPPT controller would see under normal conditions in any of the MPPT sweeps. The only time voltage could exceed that is because of colder temperatures. Most panel have temperature compensation factors so that can be calculated when one designs a system to not exceed the charge controller voltage input limits.

 

[AntronX]

If your MPPT controller is designed by real analog hardware engineers then it will limit its maximum current in hardware way before the micro controller can react. But if mosfet gate driver is driven direct from PWM output of micro controller then you are at the mercy of software PWM regulation loop.

 

[hex4def6]

If your MPPT controller is designed by real analog hardware engineers then it will limit its maximum current in hardware way before the micro controller can react. But if mosfet gate driver is driven direct from PWM output of micro controller then you are at the mercy of software PWM regulation loop.

Hmm. That makes sense to me. If you're not relying on the MCU + algo for OCP, then I agree. Even something simple like a shunt resistor and comparator controlling the gates would be pretty darn quick.

But then, where does the idea these limits come from?

Is it purely from an abundance of caution?

 

[Mattb4]

Hmm. That makes sense to me. If you're not relying on the MCU + algo for OCP, then I agree. Even something simple like a shunt resistor and comparator controlling the gates would be pretty darn quick.

But then, where does the idea these limits come from?

Is it purely from an abundance of caution?

My theory is Duty cycle. It is one thing to run at 100% for a couple of hours. It is quite another to run for a lot more hours at full capacity. Being massively over paneled can result in the SCC being unable to last.

 

[hex4def6]

My theory is Duty cycle. It is one thing to run at 100% for a couple of hours. It is quite another to run for a lot more hours at full capacity. Being massively over paneled can result in the SCC being unable to last.

Maybe...

But that could also be dealt with using some sort of over temperature foldback. Reduce the available maximum power generation ability when the device gets hotter than (say) 80 degC. Continue to derate to 0% when it reaches (say) 100 degC. Throw some fan speed control in there, and voila.

Bonus is that you're not hobbling inverters that are operating in winter / inside cool garages vs outside in Vegas.

 

[Mattb4]

Maybe...

But that could also be dealt with using some sort of over temperature foldback. Reduce the available maximum power generation ability when the device gets hotter than (say) 80 degC. Continue to derate to 0% when it reaches (say) 100 degC. Throw some fan speed control in there, and voila.

Bonus is that you're not hobbling inverters that are operating in winter / inside cool garages vs outside in Vegas.

All things can be dealt with for more money. But a simple specification on maximum amount of panel wattage in the manual is essentially free.

 

[1201]

I have a 133% DC to AC ratio. On sunny days this time of year, the inverter is maxed out from 10:30am till 3:30pm.

It gets hot! I have to have a fan blowing at it to try to manage the temps.