So I need some insight into the bypass diodes installed in solar panels.

If you have a string of series connected solar panels that have in-built bypass diodes and one panel is fully shaded why does it bring down the output of the entire string?

Why do the bypass diodes on the panel thats fully shaded not activate and take that panel out of the series string so that the remaining panels in the string (that are not shaded) continue to produce decent output power?

Currently, what happens is that if one panel of a series string is fully or partially shaded it brings down the output power of the entire string, just wonder why the bypass diodes cant bypass this shaded panel. 

It should bypass the panel... What MPPT are you using, and does it have a decent shading algorithm?

37 minutes ago, Noobie said:

So I need some insight into the bypass diodes installed in solar panels.

If you have a string of series connected solar panels that have in-built bypass diodes and one panel is fully shaded why does it bring down the output of the entire string?

Why do the bypass diodes on the panel thats fully shaded not activate and take that panel out of the series string so that the remaining panels in the string (that are not shaded) continue to produce decent output power?

Currently, what happens is that if one panel of a series string is fully or partially shaded it brings down the output power of the entire string, just wonder why the bypass diodes cant bypass this shaded panel. 

https://www.electronics-tutorials.ws/diode/bypass-diodes.html

20 minutes ago, JustinSchoeman said:

It should bypass the panel... What MPPT are you using, and does it have a decent shading algorithm?

May be you can help on this one. When 1 of 6 panels in series get shade on one corner that is about 200x200m in size why would the current drop from 4.8A to 1.9A. At this time the voltage of the string only drops from 190 to 160V being about 15%. 

I take it that 1 of the 3 diodes in this shaded panel will have power through 1 diode. 

Edited June 21, 20233 yr by Scorp007

Eish.  These things are always tricky.

Firstly, each solar panel is actually an array of a number of parallel strings.  Most solar panels have limited (if any) internal bypass/blocking diodes, so shading even a small portion of a panel can potentially limit the entire output of the panel (depending on how many internal strings are affected by the shade).

Then you have the series string with one affected panel...  There should be an optimum power point where all the remaining panels are producing their normal current, at their normal voltage, but the one affected panel is bypassed (total panel voltage will be the forward voltage of the bypass diode).

(Shamelessly stolen from https://blog.spiritenergy.co.uk/contractor/solar-panel-shading-analysis )

Now MPPT stands for Maximum Power Point Tracker - and that is exactly what many do - they track the maximum power point. So as the one panel is shaded, they follow that one peak (labelled 'Local max') down, following the maximum power that the shaded panel can produce. But the MPPT never discovers that there is another, much better, power point that has formed at a higher current and lower voltage.

There are some MPPTs that have very good shading support, and will occasionally sample other possible maxima - but most will just track that one bogus power point.

2 hours ago, JustinSchoeman said:

Eish.  These things are always tricky.

Firstly, each solar panel is actually an array of a number of parallel strings.  Most solar panels have limited (if any) internal bypass/blocking diodes, so shading even a small portion of a panel can potentially limit the entire output of the panel (depending on how many internal strings are affected by the shade).

Then you have the series string with one affected panel...  There should be an optimum power point where all the remaining panels are producing their normal current, at their normal voltage, but the one affected panel is bypassed (total panel voltage will be the forward voltage of the bypass diode).

(Shamelessly stolen from https://blog.spiritenergy.co.uk/contractor/solar-panel-shading-analysis )

Now MPPT stands for Maximum Power Point Tracker - and that is exactly what many do - they track the maximum power point. So as the one panel is shaded, they follow that one peak (labelled 'Local max') down, following the maximum power that the shaded panel can produce. But the MPPT never discovers that there is another, much better, power point that has formed at a higher current and lower voltage.

There are some MPPTs that have very good shading support, and will occasionally sample other possible maxima - but most will just track that one bogus power point.

Thanks. I have this problem with 1 panels to close to the one next to it in winter. It is the top corner giving the shadow on the bottom corner. As you can see it goes not last long. Only a problem for a few weeks around the long shadows we get now. I have marked and drilled the holes to move the panel further away. 

As we know in a few weeks it's no longer a problem and we leave it to next winter. 😀😀

I would have thought my string inverter would look for another MPPT point as explained. 

Edited June 21, 20233 yr by Scorp007

15 minutes ago, Scorp007 said:

I would have thought my string inverter would look for another MPPT point as explained. 

Yeah. MPPT algorithm is a very important factor, but very few manufacturers specify what theirs is, so it is very difficult to know how well they will handle shading.

BUT, I just realised that this can not explain your problem.  The voltage should go UP for the false local maximum:

These are 2 of my strings, as the chimney shade moves over one of them. You can see how initially the current drops and the voltage goes up - this is the tracker following the false maximum. But then the current goes back up until it is almost the same as pv2 again, and the voltage drops - this is the tracker jumping back to the true maximum power point.

I can't think of anything that would cause both voltage and current to drop...

9 minutes ago, JustinSchoeman said:

Yeah. MPPT algorithm is a very important factor, but very few manufacturers specify what theirs is, so it is very difficult to know how well they will handle shading.

BUT, I just realised that this can not explain your problem.  The voltage should go UP for the false local maximum:

These are 2 of my strings, as the chimney shade moves over one of them. You can see how initially the current drops and the voltage goes up - this is the tracker following the false maximum. But then the current goes back up until it is almost the same as pv2 again, and the voltage drops - this is the tracker jumping back to the true maximum power point.

I can't think of anything that would cause both voltage and current to drop...

OK I also made a mistake. The voltage remained about the same. The 30V volt drop was 5min after the low point when the current bounced back to 4.8A from 1.9A.

10 hours ago, Noobie said:

Why do the bypass diodes on the panel that's fully shaded not activate and take that panel out of the series string so that the remaining panels in the string (that are not shaded) continue to produce decent output power?

As others have more or less pointed out, it's because the shaded panel is weak and drags the string voltage down as the MPPT draws more current. That's the local maxima mentioned. That situation will remain unless and until the MPPT drags the whole string voltage down a fair bit further, by attempting to draw more and more current, and it does so far enough that the shaded panels go down to zero volts (technically slightly negative). At that point, they go from weak panels to strong diodes, so as the MPPT attempts to draw more current, the shaded panels no longer sag lower in voltage, and the overall power goes up. That's the true maximum power point.

In practice, with say a toilet pipe or TV antenna shading multiple panels, there could me multiple peaks of different power levels, and the one that has the true maximum power point could change with time. Hence, a full scan every few minutes is best.

As far as I know, Axperts don't do proper MPPT scanning, and will only find the "false" maximum. @Georg594 has patched one Axpert firmware for better shade performance, but my understanding is that each patch has to be tuned for a particular number of panels. It would be best to fix the algorithm to perform true MPPT tracking, but that's pretty complex.

6 hours ago, Coulomb said:

... and drags the string voltage down as the MPPT draws more current.

Actually the MPPT will follow the reducing current capability of the shaded panel towards lower current and higher voltage - this is the local maximum (and also the true maximum until shading gets too bad).  To break out of this, it needs to draw more current again.

Thanks guys for explaining the changes taking place. The graphs I showed get the power down from about 780W to 300W with 6 panels and that for a small patch of shade. Luckily it does not go down to zero😀as we see mentioned a lot on forums.