DC to DC Converters / Optimizers

[ggunn]

Optimizer data sheet... I attached it. scroll down on the 2nd page, all the way to the bottom... it's footnote #(10).

Ah. That's a newer cut sheet than the one I have for those optimizers. There is no way that they mean the differences have to be exactly 1000W and 2000W. Parallel strings will be the same voltage (basic electrical theory); the footnotes are saying that there is a limit to the optimizers' ability to lock into the same string voltage in two strings of different length if the power in the strings gets close to the limit of what they can handle.

 

[Carultch]

Let's say the difference between string A and string B is 500 watts... can you make each string a max of 6500 watts? or does the difference between the strings have to be exactly 1000 watts, instead of 500 watts? By saying 'maximum power difference of 1000 watts' seems to imply anywhere from 1 to 1000 watts.

It can be up to 1000W of STC power difference among the strings, or it can be completely identical strings with no difference at all in nominal power. Given typical module power ratings available today, and assuming a uniform module selection, 1000W at STC would typically be a difference of 1 or 2 modules, and at most be a maximum difference of 3 modules. You'd have to have modules that are either 250W (technology of 8 years ago) or 500W (enormous modules today) to get a difference in power ratings of exactly 1 kW, and that is an unrealistic expectation.

The takeaway here is that you have less flexibility for diversity among the strings, when one of your strings is between the standard upper limit of 6kW and the fine print upper limit of 6.5kW. The inverter has to increase the voltage above 400V, in order for a string to operate at 6.5 kW. All strings that are paralleled need to operate at the same DC voltage to meet Kirchhoff's voltage loop law. The higher the voltage, the harder it is for your smaller strings in the same parallel group to catch up with the larger strings, to achieve the same voltage.

 

[Grouch1980]

The takeaway here is that you have less flexibility for diversity among the strings, when one of your strings is between the standard upper limit of 6kW and the fine print upper limit of 6.5kW. The inverter has to increase the voltage above 400V, in order for a string to operate at 6.5 kW. All strings that are paralleled need to operate at the same DC voltage to meet Kirchhoff's voltage loop law. The higher the voltage, the harder it is for your smaller strings in the same parallel group to catch up with the larger strings, to achieve the same voltage.

I see. Understood about the smaller strings. So let's if i have this right... All strings can be 6.5kW, as long as I'm not exceeding the max DC power rating of the inverter (all the string kW ratings added together). Or, just throwing numbers out there, I can have my smallest string at 5.5kW (but no less), and the other strings all at 6.5kW.... and this should work since i'm not exceeding the 1000 watt max power difference from that footnote.

 

[ggunn]

I see. Understood about the smaller strings. So let's if i have this right... All strings can be 6.5kW, as long as I'm not exceeding the max DC power rating of the inverter (all the string kW ratings added together). Or, just throwing numbers out there, I can have my smallest string at 5.5kW (but no less), and the other strings all at 6.5kW.... and this should work since i'm not exceeding the 1000 watt max power difference from that footnote.

Yes, with the caveat that it is true for the optimizer, inverter, and service voltage combination(s) for which the footnote applies.

 

[Grouch1980]

Thanks everyone.