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Electrical string definition
https://forum.pvsyst.com/viewtopic.php?f=25&t=4390
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Author:  Lockyy [ Fri May 03, 2019 2:39 pm ]
Post subject:  Electrical string definition

Hello,

I have a question. If I have rows of tables 4x20
More possibilities for the stringing:

1)Each row is one string:
_________
_________
_________
_________

2)Every two rows are one string:
_________ _________
_________| _________|
_________ _________
_________| _________|

3) Every four rows are one string

What are the number of strings in width and in height:
1)
width: 1
hight: 4

2)
width: 2
hight: 2

3)
width: 4
hight: 1

???

The simulation shows that 3) ist much worse than 1). But in reality in 3) for each string, only some module will be shadowed in the morning. But as I understood PVSyst, this is not how it works?
If the lower modules are shadowed in 3), the whole string is influenced!?

Can somebody enlight me?

Author:  dtarin [ Tue May 07, 2019 9:02 pm ]
Post subject:  Re: Electrical string definition

Quote:


What are the number of strings in width and in height:
1)
width: 1
hight: 4

2)
width: 2
hight: 2

3)
width: 4
hight: 1

???

Yes That is correct
1) 4 H x 1 W
2) 2 H x 2 W
3) 1 H x 4 L
Quote:
The simulation shows that 3) ist much worse than 1). But in reality in 3) for each string, only some module will be shadowed in the morning. But as I understood PVSyst, this is not how it works?
If the lower modules are shadowed in 3), the whole string is influenced!?

Can somebody enlight me?
Yes, the lowest current drives the current of the string, so in #3, when the bottom modules are shaded, the entire string has reduced output. The according to strings method of shading will capture this effect for c-Si modules (setting electrical effect to 100%).

Author:  Lockyy [ Thu May 09, 2019 11:54 am ]
Post subject:  Re: Electrical string definition

Quote:
Yes, the lowest current drives the current of the string, so in #3, when the bottom modules are shaded, the entire string has reduced output. The according to strings method of shading will capture this effect for c-Si modules (setting electrical effect to 100%).
For 3:
When the last module is shaded, the MPP-Tracker will finde same current at a different voltage and skip the modules. So IF the rest modules have enough voltage to bring it to the mpp-range, I lose 20 modules -> 1/4 of modules, the rest is normal.
For 1:
Same situation, I lose 1 string of 20 modules completly -> 1/4 of modules, the rest is normal.

It gets more complicated if I even consider the bypass-diods and only shading at the very bottom.

For 3: The string l lose 1/3 of 20 Modules (MPPT finds the MPP and skipps the last diods)
For 1: For the last string, IF 2/3 of the voltage is in the MPP-range, MPPT finds the MPP and skipps the last diods. -> I lose 1/3 of 20 modules

BUT: If 2/3 of the modules are shaded:

For 3: the string have the voltage of 15 full modules and 5 modules with only 1/3 of voltage, it is still possible to find the MPP and skip 2 bypass-diods each.
For 1: for the last string we only have 1/3 of voltage for 20 modules, so it will probably be outside the MPP-range -> I lose the string completely.

So in the end I have the MPPT and bypass-diods for finding the voltage with the "full" current. Or do I miss something?

In my opinion therefor it is better put only 5 shaded modules in one string each, so worst case you have 15 non-shaded modules with enough voltage. But I do not have english literature for this, just german one:
https://photovoltaikbuero.de/pv-know-ho ... n-teil-33/
Quote:
If you have a solar generator, the modules that are occasionally affected by partial shading (eg by a tree or a tree) all in a module string summarize, so that it then a string with shaded modules and a string (or more strings) with the unshaded Modules exist. This is a question that is asked again and again. The answer is no ... and why it goes like this is explained in the following article.

Author:  Lockyy [ Tue May 28, 2019 2:32 pm ]
Post subject:  Re: Electrical string definition

Anybody from the PVSyst can answer this? I tried the tool Electrical behaviour of PV arrays for th version 1) and it showed me for the shadow of 20 submodule (1 diod), that the U_Mpp is already close to the U_MPP-range. If 40 submodules are shaded (2 diods), the U_Mpp is already out of the U_Mpp_range.

If I have version 3), the maximum shaded submodules would be 15, so everything within the U_Mpp_range. So I think it is definitly the better version!

Can somebody from PVSyst answer me?

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