One string or two?

[Rosco]

I am having a PV system professionally installed next week. The system will consist of 12 Siliken 255 watt poly panels (3.06 kW DC) and a grid tied Aurora PVI 3.0 inverter. The panels will be installed on the same roof with the exact same orientation. Shading will not be an issue. The Aurora inverter has two independent MPPT's that can report the output from 2 separate strings independently. I would like to wire the panels in 2 separate strings of 6. This would allow me to compare the output of each string. In theory, the two strings should perform identically. If one string falls significantly below the other, that would indicate a problem on that string. The "Stringtool" calculator on the Aurora website allows the panels to be configured both ways: as one 12 panel string, or as two 6 panel strings. When configured for 2 strings, they say to reduce the activation voltage from the default of 200v, to 181v.

So.... my question is: Will the overall net performance be less with 2 strings because the voltage will be lower? Will that cause the inverter to "cut in" later, or "cut off" sooner in low light conditions than it would with one string of 12?

The specs for the panel: Voc = 37.9, Isc = 8.96, Vmp = 30.51, Imp = 8.38.

The specs for the inverter - Input voltage range = 120 to 530 volts DC, Activation voltage = Adjustable 120-350v

Any opinions?

[Naptown]

The only problem I could see is if the voltage fell on a very hot day.
Does Aurora have any preference?

[Mike90250]

I agree, hot weather may reduce PV below MPPT levels. Often an inverter has 2 input specs:
Active. &
MPPT. Often are 20 - 50 v different

[Rosco]

Thank you both for your response. The Stringtool on the Aurora website asks for the maximum and minimum temperatures for your area, and I put in 95 F as the max summer temp. The print-out that Aurora provides calculated a max panel temp of 158 F, so they seem to take temperature into consideration when configuring the panels per string. The tool appears to allow a string with as few as 4 panels, with the activation voltage set down to 121v.

I did send the question to Aurora technical support 2 days ago, but have not yet received an answer. If I don't get an answer, I will probably ask my installer to wire the panels for 2 strings of 6. If I experience any problems, it should be easy to rewire the 2 strings in series at the j-box at the inverter.

Thanks again. This forum is a great resource. I am looking forward to making some electricity!

[Wy_White_Wolf]

... If I experience any problems, it should be easy to rewire the 2 strings in series at the j-box at the inverter. ...

I would really be worried about overvolting the inverter on a cold winter day doing that. How much do you need to compensate for cold/highlight conditions. Here it's 33%. Max VOC I could string them up to is 349.8V.

You can always decease the activation voltage a little more to compensate for hot weather. Say 175 instead of 181 if needed. I wouldn't though unless you have problems. Take a look at what the all time record high is for your location and use that in there calculator instead of the 95 F. Same for the low temp.

WWW

[Naptown]

I would really be worried about overvolting the inverter on a cold winter day doing that. How much do you need to compensate for cold/highlight conditions. Here it's 33%. Max VOC I could string them up to is 349.8V.

You can always decease the activation voltage a little more to compensate for hot weather. Say 175 instead of 181 if needed. I wouldn't though unless you have problems. Take a look at what the all time record high is for your location and use that in there calculator instead of the 95 F. Same for the low temp.

WWW

33% of VOC that's like a 100 Degree C difference between STC and low. Must get cold where you are. Most modules have a temp coefficient of between .33% and .37%