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%).
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.
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/
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.