I've searched here and cannot find any topics relating to this. If this has been covered elsewhere please direct me to the thread.

 

What's the nett effect of parallelling strings of like panels with strings of like panels but different characteristics?

E.g. 3 x 315W, Voc 40.53, Isc 9.98 panels connected in series parallelled with 3 x 350W, Voc 46.6, Isc 9.51 panels connected in series. I'm referring in particular when they are connected in parallel using a combiner box.

I hope this makes sense because I'm having difficulty phrasing the question properly.

Edited April 26, 20197 yr by VisN

my understanding is that the system will perform to the lowest wattage but i could be mistaken?

I have done this before with 300W and 275W panels.

My total PVin was that of all panels (9 in total, 3 strings of 3) connected as if it were all 275W.

This is my experience though.

Yes, it generally performs at the lower panel ratings, i.e. 315W in this case. 

So in my above example what is the input voltage that the MPPT charge controller sees? I expect it sees (9.98 + 9.51) = 19,49 as the input current, correct?

15 minutes ago, VisN said:

So in my above example what is the input voltage that the MPPT charge controller sees? I expect it sees (9.98 + 9.51) = 19,49 as the input current, correct?

The input voltage would be the lowest of the parallel connected strings. 

 

Basically, when you have a string of 3x 315W (let's say  109.8V) and another string of 3x 350W (let's say 118.2V), and the two strings connected together in parallel, the whole circuit will run at the lower voltage, i.e. 109.8V in this example. The Amperage, as you have it, will be added together. 

I used the Canadian Solar CS6X-315P (315W) and CS6U-355P panels for my figures, so their amperages are 8.61A and  9.02A respectively, so you would sit with a total of 17.63A in this case.

 

Thus, in simple terms, it would perform at the lower panel wattage, "plus some" due to the slightly higher ampere. 

Do you need three pannels in series would it work with two. So 2 ( one 315w and one 350w) in series and 3 sets in parallel. You would loose less power in that configuration.

Just hang on a bit here...

53 minutes ago, SilverNodashi said:

Yes, it generally performs at the lower panel ratings, i.e. 315W in this case. 

Close, but not quite. It performs at the lowest CURRENT of the series panels. For example, if I string up a 300W 72-cell- and a 150W 36-cell module in series, I will get the full 450W. Why... well you can see from the cell count that the doubling in power is based purely on the doubling in voltage, the current for the modules are similar.

41 minutes ago, VisN said:

what is the input voltage that the MPPT charge controller sees?

The open circuit voltage will be the sum of the open circuit voltages of the individual panels.

The Vmp is more difficult to work out. It's going to depend on the I-V curves of the panels and where each one ends up on its own curve. Similarly, the maximum power is going to depend on what Vmp is eventually settled on, multiplied by the lowest Imax value.

8 minutes ago, plonkster said:

Just hang on a bit here...

Close, but not quite. It performs at the lowest CURRENT of the series panels. For example, if I string up a 300W 72-cell- and a 150W 36-cell module in series, I will get the full 450W. Why... well you can see from the cell count that the doubling in power is based purely on the doubling in voltage, the current for the modules are similar.

 

Yes, that's for a single string of two panels. 

But he mentioned two strings in parallel, so then it's a bit different. 

 

11 minutes ago, plonkster said:

Just hang on a bit here...

Close, but not quite. It performs at the lowest CURRENT of the series panels. For example, if I string up a 300W 72-cell- and a 150W 36-cell module in series, I will get the full 450W. Why... well you can see from the cell count that the doubling in power is based purely on the doubling in voltage, the current for the modules are similar.

The open circuit voltage will be the sum of the open circuit voltages of the individual panels.

The Vmp is more difficult to work out. It's going to depend on the I-V curves of the panels and where each one ends up on its own curve. Similarly, the maximum power is going to depend on what Vmp is eventually settled on, multiplied by the lowest Imax value.

@plonkster I am not asking about mixing the panels in series. I am asking about having the same panels in series i.e. a string of 3 x 315W panels parallelled to a string of 350W panels. I hope my terminology is correct.

31 minutes ago, seant said:

Do you need three pannels in series would it work with two. So 2 ( one 315w and one 350w) in series and 3 sets in parallel. You would loose less power in that configuration.

A valid point.

But he mentioned two strings in parallel, so then it's a bit different. 

Right! My bad.

I'm not sure the answer is that it will pull down to the lowest of the three. It's going to end up at a voltage somewhere between the lowest and the highest one, depending on what voltage favours the strongest string the most. However, the drop-off on the back of the P/V curve is so dramatic that running past the Vmp of the lower-voltage string would penalise the combination more than running the higher-voltage string lower on the front side of its P/V curve. So you will likely end up at the lower voltage.

This is an interesting case. The voltage of the lower module would probably be something like 34V (just taking 5/6 of the open circuit voltages you provided). At 34V the larger module would make 34*9.5 = 325W. So it's not pulling down QUITE as low as 315W... but it's close.

Is there any difference between the 2 wiring diagrams or will they both achieve the exact same thing?

Forgive the kindergarten drawing.

Just now, VisN said:

Is there any difference between the 2 wiring diagrams or will they both achieve the exact same thing?

You're asking if you can leave out the combiner box? Well, that depends on some factors, such as whether surge protection is a requirement in your area (in my area it is not), and whether you want to fuse them individually (a requirement, though we have argued before it only really becomes essential at 3 strings), and if it will sign off like that (if sign-off is something that is going to be done soon, of course).

On an electrical level there is no difference though.

5 minutes ago, plonkster said:

You're asking if you can leave out the combiner box? Well, that depends on some factors, such as whether surge protection is a requirement in your area (in my area it is not), and whether you want to fuse them individually (a requirement, though we have argued before it only really becomes essential at 3 strings), and if it will sign off like that (if sign-off is something that is going to be done soon, of course).

On an electrical level there is no difference though.

I meant on an electrical level. Thanks.