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Why does shading over one solar panel reduce the output of the whole string?

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Malvineous:
Hi all,

I've been reading up about solar, and one thing confuses me (well lots of things do, but you have to start somewhere!)

From what I have read, if you have a string of identical panels in series and one of the panels becomes shaded, the power output of the whole string drops to match the shaded panel.  In other words, if the panels are producing 8 A each and one becomes shaded and drops back to producing only 4 A, all the other panels in the string also drop back to supplying only 4 A each as well.  It's like each panel also acts as a current limiter, only passing up to whatever current it happens to be producing at that moment, and no more.

This confuses me because I was under the impression that the panels worked like unregulated power supplies, producing as high a voltage as possible (up to the Voc / open circuit rating), and then as power is drawn from them the voltage drops until the Isc / short circuit current is reached, by which time they are presumably at an extremely low voltage but at their full current rating.

If that's correct, then when one panel becomes shaded and it limits the current through the string of panels, shouldn't that make the voltage of the other unshaded panels increase accordingly from the reduction in current?  Shouldn't this result in the inverter seeing an increase in total voltage across the string, since the panels are all in series?  The inverter would presumably draw more current, dragging the voltage down low again but getting the extra power from the unshaded cells.

Apparently it doesn't work like this, but can anyone point out where my misunderstanding is?  If the unshaded panels suddenly can't deliver half the power they are generating, where does it go?

tszaboo:

One image is worth a thousand words.
mod: they remove the linked picture of course.

capt bullshot:
The drop in power output depends on how the load (inverter) handles the MPP algorithm.

Usual PV panels have one (or multiple) reverse diodes installed. Either one diode for the hole panel or e.g. 3 diodes for each third of the number of cells in the panel. The purpose of these diodes is: if one or more cells is shaded, the generated current drops for this cell. If other cells / panels are unshaded, they still can supply the full current.
The inverter can now try to still draw the maximum current from the non-shaded panels, this will drive the shaded panel into reversed voltage. The reverse diodes now start to conduct and reverse voltage drop across the shaded panel is limited to the diode drop. If the diodes wouldn't be there, the solar cells would see a quite high reverse voltage and may overheat due to this. So the diodes protect the shaded panel from overheating / damage.

Now, if one panel (or part of the panel) is shaded, these cells deliver less current. The inverter can now either draw this reduced current and keep the voltage across the shaded panel - this results in a gross reduction of delivered power, or just continue to draw the full current from the non-shaded panels. Due to the reverse diodes, the voltage across the shaded panel drops to zero (or a bit negative), reducing the total output of the string by just the amount of one panel.

So it's up to the MPP algorithm in the inverter to try lower voltages and see if there's more power available to maximize to strings power output. Some inverters do, others don't ...

capt bullshot:

--- Quote from: Malvineous on January 01, 2018, 07:54:34 am ---
Apparently it doesn't work like this, but can anyone point out where my misunderstanding is?  If the unshaded panels suddenly can't deliver half the power they are generating, where does it go?


--- End quote ---
It just heats up the panel, since the power isn't taken away from the panel.


--- Quote from: Malvineous on January 01, 2018, 07:54:34 am ---If that's correct, then when one panel becomes shaded and it limits the current through the string of panels, shouldn't that make the voltage of the other unshaded panels increase accordingly from the reduction in current? 

--- End quote ---
Yes, voltage rises, but just a little bit (see the MPP diagrams above)

Seekonk:
It actually doesn't take much to turn a panels output to crap.  MPPT of a big string is just about as bad an idea as a PWM controller.  The world is going to micro controllers for each panel.

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