question

Josep morancho avatar image
Josep morancho asked

2 strings in parallel with diferent shading

My roof has a chimney and I have space for 6 solar panels of 540W (50V open circuit and 14A max).

I can not connect all 6 panels in serial as the bigest SmartSolar supports 250V only, Then I have to create 2 serials strings of 3 panels and connect in parallel.

The problem is that the shadow of the chimney will affect some hours of the day one of the strings. Then the voltage of the string will decrease.and will have a voltage lower than the other string.

Can 2 parallel strings have diferent voltages?

In electrical circuits this situation would produce a non-desired current recirculation between the circuits and an increase of intensity in one of them. How does the solar panels deal with this situation? intensity increase could be higher than max solar panel intensity.

On the other hand, Can a SmartSolar MPPT deal with parallel strings with diferent voltages when shading appears?

Thanks






MPPT Controllers
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3 Answers
pwfarnell avatar image
pwfarnell answered ·

Solar panels typically have blocking diodes to prevent reversed current flow. Because the panels are connected together they will always have the same voltage, which means that only one of the parallel strings will be able to operate at its maximum power point, the other will produce less power. The controller will attempt to operate the combined panels at the total maximum power point. This will be either

  1. at the maximum power voltage of the shaded string, which will reduce the power of the unshaded string so you will get partial output from both strings
  2. at the maximum power voltage of the unshaded string, which will then be too high for the shaded string to produce power, so you get half power

Ending up at either 1 or 2 will depend on the amount of shading and the panel design.

In this situation for optimal performance you would have 2 controllers so the unshaded panel is always running at maximum power and the shaded panel is running at a lower power but the maximum that can be achieved with the given level of shading.

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usernamepasswordbs avatar image
usernamepasswordbs answered ·

I have a similar roof situation and i have bought a RS450 with two MPPT inputs to solve the same problem that you have.

Having said that I still need to know which panels to connect on each MPPT input. I wonder whether I should connect most of the panels on one string and just the 2 panels that might suffer from shading to the other input. It would mean very unequal sized strings but maybe would produce more power over all.

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paragon avatar image
paragon answered ·

Hello,
My PV constellation:
4x strings connected in parallel to two strings each, shaded differently in time and feeding a Victron RS 450/100-tr.


It is important that the parallel connected strings have the same cross-section and cable length to the Y-connector and the number of modules in the parallel connected strings is the same.


As I understand it, this would mean that the internal resistance of the shaded string connected in parallel would have to be higher impedance than the input of the MPPT. What prevents a reverse current from the not shaded string into the shaded string. The rest should be clarified by the installed diodes in the PV modules.


Out of "fear" I additionally installed 8x PV fuses in the Y-distributors in each string, which is probably overkill.


In this constellation my plant works since one year without problems.

Translated with www.DeepL.com/Translator (free version)

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