Inverter DC Input Limit / Bifacial Gain

[Questionable Solar]

I am trying to understand how the DC input limit works for inverters that are being powerlimited due to the recent NEM applications. We have a few designs at our company that are being heavily affected by bifacial gain, limiting that amount of DC we are allowed to put on each of the inverters. One solution I had in mind was to take larger nameplate inverters (i.e. taking a SMA 50kW and upsizing to an SMA 62kW for example) and powerlimit them down to the correct AC size the original design has (taking the SMA 62kW and powerlimiting its AC output to 50.0kW).

The problem I need cleared up is this: if each of the inverters have a specific ratio of tolerance at the base nameplate AC output (i.e. the SMA 50kW having a max DC input of 75kW, making the DC to AC ratio 1.5) then is that ratio proportional to whatever new AC rating we powerlimit to (going from 50kW to 40kW, and the new DC input being 60kW DC due to the 1.5 DC to AC ratio) or is the DC input rating the same for the inverter no matter the AC output we are powerlimiting to? I believe I am overthinking this, but want to make sure this is a viable solution before presenting this option. Link to SMA Core1 inverters.

 

[jaggedben]

Have you not asked SMA? I think only they can definitive.

My guess is that your idea would work just fine because the max input rating of the inverter is what matters, and the power limiting has no effect on the max ratings of DC components. And on the AC side the power limiting is just to satisfy the utility, which does not really care about the DC side of the inverter components at all.

 

[Questionable Solar]

@jaggedben I just reached out to SMA to confirm, turns out regardless of AC output (even powerlimiting) we have to maintain the 150% DC input ratio. I am now curious if this is just an SMA design choice, or if other companies also force you to adhere to the DC to AC ratio for their inverters as well? Thank you for the suggestion, probably should have started with that in the first place.

 

[jaggedben]

@jaggedben I just reached out to SMA to confirm, turns out regardless of AC output (even powerlimiting) we have to maintain the 150% DC input ratio....

Makes no sense to me. Maybe try asking again.
That said, if they say so, they say so. Definitely could be something I don't know.

 

[Questionable Solar]

Might be a design issue? It could be something that might mess internally with the software being able to handle it rather than anything to do on the hardware side. A bit frustrating but it is what it is.

 

[solarken]

Might be a design issue? It could be something that might mess internally with the software being able to handle it rather than anything to do on the hardware side. A bit frustrating but it is what it is.

It is frustrating, and when inverter manufacturers specify a hard limit on DC to AC ratio, it totally omits any accounting for the specific characteristics of the site/installation. If you have an East-West design with half the DC capacity in each direction, it is completely different than a south-facing array with optimum tilt for the latitude. In the first case, you will have significantly less clipping than the second case, generally, so it should be acceptable to have a higher DC to AC ratio than the second case.

 

[jaggedben]

It is frustrating, and when inverter manufacturers specify a hard limit on DC to AC ratio, it totally omits any accounting for the specific characteristics of the site/installation. If you have an East-West design with half the DC capacity in each direction, it is completely different than a south-facing array with optimum tilt for the latitude. In the first case, you will have significantly less clipping than the second case, generally, so it should be acceptable to have a higher DC to AC ratio than the second case.

If there is a fault (say on the AC side) that briefly draws the full current of the array, the array orientation doesn't matter much. With the modules I-V curve there may be about the same fault current available regardless of orientation and irradiance (to a point, that is, but I assume not enough of a point to make a difference). The inverter needs to be able to withstand that gracefully, as it is presumably shutting down power output.

That said, I don't know why in a case like this they can't just state the max array in terms of total Isc instead of power ratio, let alone why power limiting the AC side would change the inverter's ability to withstand such a fault.