Series strings connected in parallel with different shading

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adrianc asked Apr 25, '19

Series strings connected in parallel with different shading

I am planning on installing 12 x 270W panels with 4 panels connected in series creating 3 strings that will be connected in parallel. I will have shade in the morning (winter) on the one string. The charge controller I will be using is the MPPT 250/100 (to allow for expansion). My questions are:

Each string will be producing different amount of power as one string will be in the shade and the other two in full sun. Does the controller handle this?
How many individual strings can be connected to this controller (assuming I am not exceeding the volts or amps of the controller?
What is the difference between the Tr and MC4? I assume the connections? The MC4 seems to allow for 3 separate connections, so is it limited to 3 strings?

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solgato commented · Apr 25, 2019 at 08:17 PM

If the shading is going to be different across the 3 strings you should consider running 3 smaller charge controllers in parallel, adding a BMV to the system and creating a network connection for all components.

In this scenario, you want your series wired panel strings to provide as much voltage as they can to feed the charger to reach its input spec. You can either leave some headroom for adding an additional panel or panels to each string, or the better choice would be just to create a new string with its own charge controller should you need to expand in the future since the panels may be older/newer or a slightly different spec.

The other nice thing about going this route is now you have sub-systems, so if one charger goes bad or one of your panel strings is faulty, the others won’t be affected.

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adrianc solgato commented · Apr 29, 2019 at 05:28 AM

Thank you. This helps

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Phil Gavin adrianc commented · Apr 29, 2019 at 06:04 AM

I disagree with this answer.

The voltage output of a shaded string changes minimally with shading. It's current output, and subsequently its power output changes drastically.

The shaded string might not contribute power but it doesn't conduct and drag things down either.

So being shaded it acts like it is not even present, and the MPPT behaves as if there is two strings instead of three.

Shading, is akin to difference in tilt and direction between like panels. It is a misconception that the whole array has to match in both tilt and direction for a single MPPT. The panels only need to match in tilt and direction within the same string, but differently tilted/directed strings of the same number of like panels can be paralleled to a single MPPT.

This can be used to your advantage, differences in string power production can be used to make a longer solar day at full production, say be combining over-paneled East and West facing strings.

In the morning, the East panels work, in the afternoon the West panels work and at noon they both half work, all the while never over-powering the MPPT.

Or alternatively, if you had fixed shaded (say by a chimney) that would definitely shade at least one string at a time as the sun traversed the sky all year, you could add an extra string, to top up production.

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adrianc Phil Gavin commented · Apr 30, 2019 at 10:10 AM

Thank you Phil. I have read that it battles to find the optimal MPP when one string is developing a very different amount of power due to shading.

What I have decided to do is get the SmartSolar MPPT 250/60 Tr as I can do 4 series x 3 parallel (12 panel array), which is the minimum size I can go whether 1 or 3 strings anyway and then see what happens in the real application. If the shading really causes a terrible imbalance and inefficiency then I can always purchase another MPPT (if the added efficiency justifies the cost).

I should have some real data to post here in about 6 weeks.

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Phil Gavin adrianc commented · May 01, 2019 at 06:34 AM

The secret is to have the same voltage per string, - don't mix different panels, or panel quantities between strings. Keep all the panels within a string at the same tilt and direction.

Please report back, although I am not sure what data would be conclusive.

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rainerb commented · May 01, 2019 at 06:50 AM

Hi,

I do not know if it was really necessary but for my MPPT 100/50 with two parallel strings (one shaded), I installed two MC4 solar diodes in order to "separate" the strings from each other.

At least this made some electrical sense for me and it works flawless for over a year.

Regards Rainer

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Phil Gavin rainerb commented · May 01, 2019 at 06:19 PM

Another string would have to exceed its Voc to push current in reverse through a shaded cell string.

But at Voc, a healthy has no current to be able to push by definition, not only that it will be functioning at the lower Vmpp anyway, so a shaded string is like an open circuit to a healthy string.

All you have succeeded in doing is creating .6V * Impp permanent power drain x 2, ( probably around 50W/string) = 100W. Averaging say 5 hours of production a day.

That's 0.5kWh/day trying to solve a problem you haven't got.

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rainerb Phil Gavin commented · May 01, 2019 at 07:30 PM

Hi Phil,

my Impp for one string is only max. 10A, so I have only a power loss of 6W per string. The 2x6W loss when the panels produce >1100W is acceptable for me.

The main reason why I installed the diodes is the winter. Sometimes one string is completely covered with snow and the other is operational. I do not know what happens in such a situation (without the diodes)?

Regards

Rainer

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Phil Gavin rainerb commented · May 01, 2019 at 08:20 PM

You are correct. Still an unnecessary loss though.

Do a bit of googling about the reverse voltages necessary to pass current through a PV panel in reverse.

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Answer 1 · 2019-04-26T06:49:22Z

JohnC answered · Apr 26, 2019 at 06:49 AM

Just adding to Sol's comment (which qualifies as an Answer).

The mppt's have one tracker. The terminals of either type combine within to a single pv bus. So any number of strings within the unit's capabilities - usually total pv Imp (edit to Isc)) is what you'd watch for doing that.

A single mppt would work, but not optimally. Markedly different orientation/shade of pv 'sections' ideally deserve their own mppt. Not necessarily one per string, but one per section would work fine. That may mean looking at those future 'expansion' plans and deciding the size of the mppt for each. You could even drop to the 150/ models, so likely a max of 3x panels/string, or even 2x then, as suits your batts and pv numbers..

The mppt's can be of mixed size and panel configuration.

Many people would design for 'now', and add another mppt at expansion time. Your choice..

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