Does string 1 pull down string 2 if string 1 is in a poorer shaded spot?

DIY
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1

I have 2 strings in parallel. Both on the same MPPT.

String one is only 300W while string 2 is 890W so it would not be a huge loss to unplug string 1 completely for this time of year but on the other hand if it is not harming string 2 then ‘every little helps’.

I was testing with string 1 plugged/unplugged just now but too many variables today with clouds coming and going and also was just moving from morning into peak output window so could not tell with confidence of any effect either way since the output was bouncing all over the place for those other reasons.

Until a couple of days ago they were both in this none ideal location. It is behind a thick treeline of mainly mature Oaks so partially shaded nearly all of the day.

This happened progressively in the last few weeks due to the sun getting lower and lower in the sky. I said to myself I would test them out in this position and if shading was an issue by the trees then I would run a cable up the hill to position the ground string (string 2) up the hill to clear the trees. Well it was and so I did.

Now the ground mounted panels are up the hill and unshaded, by trees at least, still clouds when they are about, while the 300W on my van are still behind the trees on top of my van.

So will the different situations of the strings affect the main one up the hill to any considerable degree or will it be negligible and thus better to still keep both plugged in to squeeze what little I can get out of the 300W string?

2

Yes. You can avoid this by using separate MPPTs for each string

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3

You can also use string diodes to decouple the two strings. But think that two mppt will bring the better results.

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4

Wattage aside, is the voc of both strings the same or different? That is what matters more in tracking both the strings on the mppt.

But yes a seond mppt is the most ideal move.

5

Of course. I covered all that in the previous post.

Seems people are not getting the question. I am not asking about string compatibility, I already know they work together. I am asking about shading effects of one string, if it has any meaningful effect on the other none shaded string.

6

It shouldn’t if the reverse blocking diodes are working correctly.

Possibly the wording here didnt help. It sounds like you are trying to conbine two different powered strings rather than the same power but affected by operating conditions.

7

Huh? No that’s still not right lol.

They are different power but also different operating conditions.

The different power is besides the point. I am only asking about the different operating conditions.

Forget my particular setup if it is easier.

If there were two identical model and power strings, one shaded almost always, one unshaded almost always, both on the same MPPT, will the shading of one string affect the output of the none shaded one all other things being equal? There will be many real world examples of this for housing setups where they are fixed on different sides of a roof.

What I am wondering is, will the voltage dropping on the shaded string pull down the voltage of all strings, including the unshaded one on the MPPT to meet the average?

Ok I just found this thread, which indicates that so long as they are in parallel, which they are, then it should not matter.

The OP asks exactly what I have been asking:

Won’t the shaded strings have a lower voltage causing the MPPT to underperform for the unshaded strings?

The answer then follows:

Shading doesn’t affect parallel performance like it affects series performance. So, shading on part of one string will cause that string to perform poorly, but the other strings should be OK. The best performing string will more or less set the output voltage, and the under-performing string will rise to that voltage but may not put out anywhere near as much current as the other strings, thus reduced total output, but not dragging down the other strings’ outputs.

Having been monitoring the output voltage of the MPPT that does seem to be the case.

8

If the voltage of both strings, in nominal conditions, is the same, then a shaded string will not reduce the voltage of the unshaded string.

The point is, that the “working” voltage of a pv module is reached very early with very little light, then, with increasing solar radiation, the current will rise.

That’s why a east/west configuration of identical modules/strings works perfectly fine on the same MPPT.

But that’s also the reason why everyone asks you for the modules you use and if you have the same modules, and especially, the same working voltages in both strings. :wink:

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9

Yes and I linked to an earlier thread if anyone wanted details of all the numbers of the strings as it was covered thoroughly there.

While not the same, I had already done all the number crunching on that on the other thread and in practice I knew they worked fine when they were both situated in the same place so that is not in question. It was just about shading vs. none shading.

10

Sometimes the unshaded string is nearly unaffected, sometimes it is noticeably penalized It depends on how the shaded string’s I–V curve interacts with the unshaded one. The only real test would be to measure the output combined with shaded and non shaded strings then remove the shaded string and see if the output power changes.

11

I can tell you from my own experience, that the voltage of shaded and unshaded identical modules in pure sunshine is almost the same. I have 8 modules (2s4p) connected to a MPPT 150/70 and have this shading every winter and I measured this a few times to be sure there is no problem. It is a hard shading from the neighbors building, so not some branches from trees or so, - I mean there is a real hard cut between those modules. :wink:

And when the voltage is the same, then the shaded modules will not pull the unshaded modules down. This works fine. But in my opinion it must be the same modules, to avoid different behaviour and especially different voltages.

12

No, the main one, 890W, unshaded, will not be affected by the permanently shaded 300W one, because the MPP of the 300W will be way below the MPP of the 890W one on the IV curve. Of course, if the MPPT is smart enough to lock and keep the correct MPP.

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13

Yes, since I posted this I have had some better solar days and I am not noticing any particular difference on the none-shaded ones. What I am noticing is a huge increase in output on the none-shaded due to no longer being shaded. :slight_smile: That is when it is not too cloudy of course which took me several days to see as we had a string of storms lately.

The voltage is staying up at the higher level which it would have been before.

You speak above of your own experience, but I guess you haven’t tried this to speak from experience. Mixing panels can be fine if the vmp is the same, from what I read and in practice this has been my experience.

14

Yes so in this case I guess the 300W will produce almost or totally no output which is a moot point since the other option would be to unplug it which would also produce no output. :smile:

15

Well, my own experience is not only based on what I see in my system, it is founded on more than basic electronic understanding and a lot of other sources of information.

And yes, you can mix your modules as you want, but still this different modules can react different to external parameters. Can you guarantee, that modules with same Vmp (at 25°C) have the same temperature coefficient? Can this different modules provide and passthrough the same currents over all conditions? This might not be visible in one particular operating point, but it might change significant in other conditions.

So yes, for you this might be ok, but you can’t argue that this is always fine. Always fine this would be with identical modules.

I think we can close this discission here now, as you confirmed the setup is working fine for you. :wink:

16

I never claimed in all combinations of mixed panels would work fine just that the way I set it up worked for me and no reason why it wouldn’t work for others if they followed the same principles. That is the nature of science, that it is repeatable. :slight_smile: I only followed what others advised before I tried and it worked.

I am sure there are edge cases where even the same modules don’t work. Maybe you got a faulty panel or two in the array. :smile:

Yes it is besides the point and the discussion as run its course.

17

My setup was once: 5×280W panels in one string, paralleled with 10×255W panels in two parallel 5× strings, ie. 3 parallel strings of 5 panels, with one string of 280W panels (1400Wp), and two of 255W panels (1275Wp). Voltages were close enough for me to be very confident of the setup.

When the sun emerged from the east, above the trees, I clearly saw (in VRM) each string, one at a time, contributing to the overall power: generally slight increase in power, then a sudden step (and voltage hesitations as the MPPT found the new maximum power point), then continuing slight increase (as the general lighting increased), then another big step, and the third one. This means that the shaded strings didn’t impair the non-shaded ones.

A few years later, I re-introduced 2×255W panel, which were put on the side to have 5× paralleled strings, and added a new MTTP. I now have the 5×280W panel on one MTTP, and 2 parallel 6×255W on another MPPT. I didn’t want to parallel 5× and 6× panels, due to too different voltages in my opinion, hence the second MTTP. But the resulting system displays the exact same behaviour in VRM when the sun emerges from the trees: regular increase, and 3 big steps when each string get fully exposed.