Series String Wiring Question

[Kramerica Industries]

Was wondering if all series strings need to be the same voltage?

Example....

I have 15 solar panels wired in strings of 3, each panel is rated for 37 Voc. So 111 Voc per string x 5.

I wanted to wire them up in strings of 4, but that would leave me with a string of only 3 panels. Would the string of 3 bring down the total voltage of the other strings?

My Solar Charge Controller can handle 150 Voc And I would like to use less wires and Circuit breakers.

Kramer.

 

[MichaelK]

What are your winter low temperatures like? Solar panel voltage is very dependent on temperature, and though the Voc might be 37V in the summer, below freezing they are likely to be more like 40V. So, with snow on the ground, your four series panels might be outputting 160VDC.

You can use Midnight's voltage calculator to determine what the voltage of your string will be at a certain temp. Just click on the "Classic Sizing Tool" at http://www.midnightsolar.com/

Alternatively, you could upgrade your controller to one that handles higher voltage. I'm running four panels in series exactly as you describe, however, I am using Midnight's 200V controller.

 

[Forbisher]

Was wondering if all series strings need to be the same voltage?

Example....

I have 15 solar panels wired in strings of 3, each panel is rated for 37 Voc. So 111 Voc per string x 5.

I wanted to wire them up in strings of 4, but that would leave me with a string of only 3 panels. Would the string of 3 bring down the total voltage of the other strings?

My Solar Charge Controller can handle 150 Voc And I would like to use less wires and Circuit breakers.

Kramer.

4 panels are 148Voc which is too close to your 150V SCC which you do not give a brand and model.
If your location never gets colder than 75° you would be ok with 4 in series.

 

[Kramerica Industries]

What are your winter low temperatures like? Solar panel voltage is very dependent on temperature, and though the Voc might be 37V in the summer, below freezing they are likely to be more like 40V. So, with snow on the ground, your four series panels might be outputting 160VDC.

You can use Midnight's voltage calculator to determine what the voltage of your string will be at a certain temp. Just click on the "Classic Sizing Tool" at http://www.midnightsolar.com/

Alternatively, you could upgrade your controller to one that handles higher voltage. I'm running four panels in series exactly as you describe, however, I am using Midnight's 200V controller.

Thanks for the info, I’m in Tampa Fl, it only gets cold here over night in Feb.

Would the string of 3 be a problem?

 

[Kramerica Industries]

4 panels are 148Voc which is too close to your 150V SCC which you do not give a brand and model.
If your location never gets colder than 75° you would be ok with 4 in series.

Correct! Thanks for the info...

How about the string of 3? How will the lesser voltage in that string play into the bigger scheme of things?

 

[mopat]

I believe your orignial thought of 5 parallel strings of 3 seems like the way to go; wire is cheap.

 

[Kramerica Industries]

I believe your orignial thought of 5 parallel strings of 3 seems like the way to go; wire is cheap.

Thanks for the reply, but it still doesn’t answer my original question...

 

[mopat]

Would the string of 3 bring down the total voltage of the other strings?

Yes.
Can you buy another panel? Odd numbers (15) are odd in many situations.

 

[mopat]

Was wondering if all series strings need to be the same voltage?

Yes, that would be best.
I guess this is your "original question", sorry I missed it with your other question.

 

[Kramerica Industries]

Yes.
Can you buy another panel? Odd numbers (15) are odd in many situations.

Maybe, would need to get with the wholesaler and see if they have another 1.

Yes, that would be best.
I guess this is your "original question", sorry I missed it with your other question.

No worries, I shouldn’t have confused the issue by adding the info for the SCC.

My PV runs are around 50’ each to my 4 string combiner. I would need another loaded combiner for the 5th string, Plus wire to combine them together.

As far as the max Voc for the SCC, there’s another 50’ of #6 wire to where it’s mounted, there will be some voltage drop so being a bit close to the Max ins’t a concern for me Right now.

If it does became a problem, then I can spend the money for the extra equipment and wire the panels back to 3 per string.

Thanks for the help!

 

[snoobler]

In a xSyP array, all series strings need to be very nearly the same voltage. If you pair a notably different string in with others, the operating voltage of the array will not be near ANY of the series strings' Vmp.

 

[Kramerica Industries]

In a xSyP array, all series strings need to be very nearly the same voltage. If you pair a notably different string in with others, the operating voltage of the array will not be near ANY of the series strings' Vmp.

That’s what I was fearing. Thanks for replying.

 

[snoobler]

When you're talking about using different brands/sizes, if you get the series strings Vmp within about 10% of each other, you'll likely not notice much difference, but if you put 3 of one panel on one string and 4 on another, it's going to be a shitshow. You'll get reduced performance from both strings by a notable amount.

EDIT: I don't know this for certain, but I expect the 3 panel string might be ignored completely because the MPPT will see the 4S peak voltage and pull it down until power drops, so it might run them at the 4 panel Vmp. Since this is higher than the Voc of the 3 panels, it may never see them, and they just sit there resisting the back flow of the current from the higher voltage paralleled array.

 

[Kramerica Industries]

When you're talking about using different brands/sizes, if you get the series strings Vmp within about 10% of each other, you'll likely not notice much difference, but if you put 3 of one panel on one string and 4 on another, it's going to be a shitshow. You'll get reduced performance from both strings by a notable amount.

EDIT: I don't know this for certain, but I expect the 3 panel string might be ignored completely because the MPPT will see the 4S peak voltage and pull it down until power drops, so it might run them at the 4 panel Vmp. Since this is higher than the Voc of the 3 panels, it may never see them, and they just sit there resisting the back flow of the current from the higher voltage paralleled array.

Definitely the info I was looking for. Thanks for adding.

I will need to find a panel similar to the ones I have now.

 

[gnubie]

EDIT: I don't know this for certain, but I expect the 3 panel string might be ignored completely because the MPPT will see the 4S peak voltage and pull it down until power drops, so it might run them at the 4 panel Vmp. Since this is higher than the Voc of the 3 panels, it may never see them, and they just sit there resisting the back flow of the current from the higher voltage paralleled array.

That's about it. If there's enough mismatch and the higher voltage string produces more power it's quite possible the controller will sit close to it's Vmp slashing power from the lower. That's even more likely to be the case if there are 3 strings and only one is at the lower voltage.

 

[snoobler]

3S5P vs 4S4P means one less pair of conductors and 1 less breaker for the cost of a panel. If you just want 4S4P for MOAR POWA, then fine.

I'm also the lazy bastard who bought a 250/100 SCC because I didn't want to rewire my 3S6P array to 2S9P (cold weather issues) with a second charge controller, so I can't throw stones when one talks about spending money to solve a trivial problem.

 

[Dzl]

To understand/answer your original question see this post (contains pictures and example diagrams). [Edit: learning it possibly contains some errors or oversimplifications]

 

[snoobler]

To understand/answer your original question see this post (contains pictures and example diagrams)

The portion of the quoted article pertinent to this discussion is not correct.

"the total voltage of the array is determined by the solar panel of the lowest voltage rating"

MPPT will find the optimal Vmp for the combined array if the Vmp/Voc overlap significantly. You will get a Vmp that's somewhere between the Vmp of the paralleled strings, not the lowest of the 4 as in the example in the article. Is it optimal? No. Is is a guarantee that you'll bring the higher Vmp panels down to the Vmp of the lowest panel? Only if that happens to be where you get the best power, which isn't the case.

For something as gross a 4S vs. 3S of the same type of panel, you may very well see the 3S portion of the array completely ignored. MPPT sweeps through the voltage range to find Vmp. If the 4 panels Vmp is above the Voc of the 3 panels, the MPPT will see the power drop as it pushes the 4S voltage lower, so it may never lower the Vmp to below the Voc of the 3S panels.

If the MPPT keeps lowering the voltage, and it sees more current coming in, from the 3S array while under its VoC, it will keep lowering it until the 4S array current drops faster than the rise of the 3S, so the end voltage will likely be between the Vmps of the 3S and 4S arrays, and neither array will perform near max power.

 

[JoeHam]

It’s not “wrong” to mix different panels but there are consequences.

Here is the best explanation I have found:

Mixing solar panels – Dos and Don’ts • SOLAR POWER SECRETS

Mixing Solar Panels: Discover Now Dos and Dont's of Mixing the Same Types or Different Types of Solar Panels. Act Now to Save Your Money!

solarpanelsvenue.com

 

[Kramerica Industries]

I think we are missing the point here a bit guys, All panels are exactly the same. Same model, voltage and amperage. I’m just asking about using 1 less panel on the last string, that’s all. Forget the SCC, wiring, breaker and so on.

I look at it as a series string in a array getting partially shaded, but in my example the shading is forever. The total wattage and amps doesn’t change, just the total voltage on, lets say string 4 of 4.

I guess I can change the orientation of those 3 panels, like facing them North so that they provide power during the morning hours, but I would like to the know the science behind it. Snobbler is on the right path.

Thanks to all for providing feedback, it’s all very helpful.