EG4 18kpv MPPT with Mixed Voltages

[blueforce]

Hey all, I currently have an 18kpv with an array of 15 Aptos 440w bifacial panels (Voc 41.51) in 2 strings (8 and 7 panels each) and in a location that starts to get shadowed around 1pm and usually covered by 4pm. (These times are somewhat arbitrary for the purposes of the question). The panels have Tigo TS4-A-O optimizers connected.
8 panel string - 366.2 Voc max - 200-250 (typical voltage observed)
7 panel string - 320.4 Voc max - 150-225 (typical voltage observed)
I'm considering adding another array with different panels, orientation, and shadow times. Currently looking at large PV panels (>500w) which would mean the Voc max of this string would be >500 (but less than the 600v max of the 18kpv). This would get sunlight for about an hour or two after the first array is in shadow then partial shading will start to creep in. I could also make this array 2 strings to cut the voltage in half to more closely match the first two strings.

So here's my question: Given that the nominal voltage of the MPPT in the 18kpv is 360v and one string can "drag" another strings voltage up or down what can I expect to see for production once adding this third string? And should I add optimizers or is that an unnecessary expense?

 

[Quattrohead]

The voltage swing on your panels is very substantial, much more than I see on mine without optimizers.
My main string tops out around 450 volts with no load around midday and goes down to about 380 volts fully loaded.
I would give your other string a try without optimizers, as long as your are not needing the RSD safety aspect of them.

 

[blueforce]

I should clarify that the Voc max is what the strings are rated at and the "typical voltage observed" is what I've seen in Solar Assistant. Here are the voltages on a cold winter day:

And this is a hot August day:

 

[Quattrohead]

Your panels HATE the heat.
I have Panasonic 330W and do not see this much voltage drop in the Florida summer.

 

[rogerheflin]

Hey all, I currently have an 18kpv with an array of 15 Aptos 440w bifacial panels (Voc 41.51) in 2 strings (8 and 7 panels each) and in a location that starts to get shadowed around 1pm and usually covered by 4pm. (These times are somewhat arbitrary for the purposes of the question). The panels have Tigo TS4-A-O optimizers connected.
8 panel string - 366.2 Voc max - 200-250 (typical voltage observed)
7 panel string - 320.4 Voc max - 150-225 (typical voltage observed)
I'm considering adding another array with different panels, orientation, and shadow times. Currently looking at large PV panels (>500w) which would mean the Voc max of this string would be >500 (but less than the 600v max of the 18kpv). This would get sunlight for about an hour or two after the first array is in shadow then partial shading will start to creep in. I could also make this array 2 strings to cut the voltage in half to more closely match the first two strings.

So here's my question: Given that the nominal voltage of the MPPT in the 18kpv is 360v and one string can "drag" another strings voltage up or down what can I expect to see for production once adding this third string? And should I add optimizers or is that an unnecessary expense?

I have 3 strings on my 18kpv. All the same panels (37V VOC), each on its own MPPT input. One string is 11 panels, one is 8 panels and one is 7 panels and together they all seem to produce the overall power that I expect to get out of them, so they do not appear to be any dragging down from the spread of voltages.

Now if you get a string that is way too low then that string itself will may under-produce because it is making the MPPT work harder to boost the voltages.

 

[blueforce]

Your panels HATE the heat.
I have Panasonic 330W and do not see this much voltage drop in the Florida summer.

For MPPT2 those voltage drops around 1200 and 1300 are when shadows begin to cover 1 then 2 of the panels and it gets worse from there. Shadows are gonna be a factor for the other array as well which is why I'm considering optimizers and possibly making it 2 strings instead of 1.

Here's a mockup of the 2 arrays (proposed one is on the left) and the shadows they have to deal with:

 

[SplitUniverse]

Panels in a string should ideally be placed in such a way so they are all either in the shade or in the sun. Yes, they have bypass diodes and you can use optimizers but that is not a perfect solution. I can tell when the shadow from the vent pipe on my roof starts shading one of the panels just by looking at the output of the string that the panel is on. You want to place your string in such a way so all the panels experience as uniform of a sin exposure as possible.
On the subject of strings with different numbers of panels, I've had 3 strings that were all different and they all produced about the same per panel. Currently 2 strings are identical and one is "a panel short". Output per panel is pretty much identical so long as the sun exposure if the same. The voltage (including per panel) and amperage of the strings are different but each panel end up producing pretty much the same power. Please keep in mind that my strings are not paralleled but connected each to its own MPPT (Solark 15K).
If the panels in your string experience large differences in sun exposure I would recommend going with AC coupled Micros, those will manage each panel individually instead of dragging down the entire string. I use Hoymiles 4 panel inverters for that, I am honestly impressed with them.

 

[NotTooBright]

The 18kpv has 3 mppts, the first of which has 2 inputs. The performance of one mppt does not impact the others. As long as each of your strings is connected to its own mppt then you're good.

I have 3 strings on my 18kpv (no optimizers). 2 are on the same roof that experiences shading and 1 on another that gets no shading. They all perform as expected independently of one another.

 

[blueforce]

The 18kpv has 3 mppts, the first of which has 2 inputs. The performance of one mppt does not impact the others. As long as each of your strings is connected to its own mppt then you're good.

I have 3 strings on my 18kpv (no optimizers). 2 are on the same roof that experiences shading and 1 on another that gets no shading. They all perform as expected independently of one another.

This is the answer I've been looking for, someone with a similar situation.