Is shading of the whole panel harmless to the panel?

[Guy from Glasgow]

People say here that it is completely OK to have panels/string max power current quit a bit greater than mppts max current. They say Mppt will only take what it can handle.
I.e. mppt will limit current from the panels on this situation and everything will be fine. Correct?
Similar situation arises when one of the panels in a string is shaded and limits current in the string. The shaded panel limits current of the string.
Should I conclude that this full shade on one panel is completely harmless to both shaded panel and the rest sunny panels?
Or is shaded panel going to overheat and will die prematurely because of that? In a couple of years for example?
I think it is supposed to be bad for individual cells to be shaded and overheated as a result. Hence the question.
Ta!

 

[sunshine_eggo]

People say here that it is completely OK to have panels/string max power current quit a bit greater than mppts max current. They say Mppt will only take what it can handle.

Generally correct.

I.e. mppt will limit current from the panels on this situation and everything will be fine. Correct?

MPPT is both output and input limited. It will output its maximum rated current and draw whatever that requires from the array.

Similar situation arises when one of the panels in a string is shaded and limits current in the string. The shaded panel limits current of the string.
Should I conclude that this full shade on one panel is completely harmless to both shaded panel and the rest sunny panels?

Yes. Shaded panels still produce voltage, so if you had two strings in parallel with one shaded, it's voltage would be sufficiently high to prevent the unshaded array from discharging into the shaded array.

Or is shaded panel going to overheat and will die prematurely because of that? In a couple of years for example?

No. 80%+ of the energy hitting the panels is going to heat anyway. Arrays sitting in the sun will still degrade if disconnected, but likely not as quickly as an operational array.

I think it is supposed to be bad for individual cells to be shaded and overheated as a result. Hence the question.

The thing is when a single cell is shaded, the entire string of cells is reduced to that current. The bypass diodes exist to isolate the shaded portions of the array and work to prevent the potential for reverse bias to overheat a shaded cell. Remember... when a cell is shaded, it is only receiving a small amount of energy, so 80-100% of a smaller number is notably less energy than 100% of the unshaded energy.

All that said... shading a devastating on panel performance. Shading a single cell will typically kill about 1/3 of a panel's output. Shade a single row of cells in a panel when in portrait orientation will kill near 100% of the output.

Avoid shade at all costs.

 

[Hedges]

Shaded or in full sun, shorted or open circuit, is OK.
Shaded but in series with other panels getting full sun may overheat diode, causing it to fail and damage panel, at least for some brands. Shade on one while sun is at an angle is OK.