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.
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 issignificantly 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?
What I haven't found is "why" they have these limits, but I have some theories.
- 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).
- They might just choose a reasonable practical limit to test their design to. No point in testing something no-one will use in practice.
- 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
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?
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