modules per string per mpp tracker

[lanpark]

I am receiving all sorts of differing advice on the above subject. Can you advise me on the correct way to calculate how many pv modules are required per string per mpp tracker in order to ensure that the inverter operates efficiently.

 

[BruceB]

There is a long answer, but the short answer is to use the software provided by inverter manufacturers.
Regards
Bruce

 

[Graeme Harrold]

Ill second that. For example 3.92Kw operated more efficiently on a SMA SB400TL over 2 MPP trackers than the single offered by the SMA SB3800 even though this is the one recommended by the suppliers website. The software is free to download, and I tend to use Fronius and SMA inverters. Just remember to update the software and panel database once installed

 

[lanpark]

Thanks to BruceB and Graeme for your replies. I have tried some of the manufacturers online configuration tools but without much success, probably due to my computer incompetence. I shall have to persevere it seems. I see there is another thread running on the modules per string topic, so it seems to be puzzling other people as well as myself. I would still like to have a better understanding of the issue from first principles so if anyone can recommend an authoritative source or some useful reading matter I would be grateful.

 

[dansk]

Hi Ianpark.

The quickest answer to your question is this:

1) - Work out how many panels fit on the roof.
2) - Ditermine the Kwp of the system.
3) - Find out the Voc of the Panel @stc
4) - Mulitply all the panels by the Voc @ stc.
5) - example: Voc 45v @ stc x 8 panels = 360v
5) - Look at the input voltage of the inverter and check its within boundrys.
6) - But its more technical than that, then you need to check the voltage at -10d and at 25d 1250w radiance. and make sure the panels are still within the boundrys, this will ensure on the brightest coldest day of the year the system will fire up and on the hottest brightest days the system will still produce.

Im not an expert of those calcs thats why i use PV SOL to help me.

Remember - Add the voltage up to get in range of the DC input, and then add more strings in parrale, but not exceeding the maximum input current.

When adding more strings the voltage stays the same and its only the current (amps) that increases.

Visa versa, when adding panels in a string the current stays the same, but the voltage increases.

Hope this makes sense.

And please anyone else add to this or correct me if im not 100% correct.

 

[BruceB]

Ianpark

Try Sunny Design. I find that an easy one - some of the spreadsheet based ones are not always obvious.

 

[lanpark]

Thank you Dansk, that was very helpful! The next part of the question is this. Where the inverter has two MPP trackers, unequal strings may be used which produce unequal voltage. Is the input voltage to the inverter then the sum of the input voltages to the two trackers, or an average of the two or what?

 

[lanpark]

Thanks again Bruce

 

[dansk]

Without looking at the inverter im led to believe there independant, so think of duel MPP trackers as two inverters in one, and make sure the strings drop inbetween each MPP voltage range.

TBH ive not had much to do with Duel MPP, so maybe someone else can give you a more qualified answer.

 

[BruceB]

Some inverters have inputs which are effectively paralleled up inside, which saves having a dc junction box external to the inverter. Where they actually have dual mppt, then the inputs are completely separate.
Regards
Bruce