Shadow effect

[nidalls]

Hi,

If i have 6 strings of panels, each string consists of 17 panels (total 102 panels).
every two strings were installed on one mounting structure as the picture attached in the post.

And i use ABB inverter 27.6 TL-OUTD.
Connect three strings (2 from the same mounting structure and the third one from another mounting structure) to the same mppt.
totally i will use 3 mounting structures.

now if i have mutual shading, and the lower strings get shaded (all the panels of the lower string).

In this situation does the fuses in the ABB inverter will burn? due to reverse current from unshaded panels to the shaded panels (because i have different string voltages voltages in parallel) ?

[bcroe]

Your shaded strings will not be damaged. They will continue to maintain voltage, but the current output will
be reduced to 10% or less, still holding off any current from unshaded strings. Bruce Roe

[Mike90250]

I have a different opinion on panel shading and damage, Occasional shading, early or late in the day, or a stray leaf or twig not a big deal.

But an array with constant shading over part of the panel, will activate the bypass diode(s) for hours on end, for days on end, The bypass diode eventually will burn out, disabling the entire panel.
Fix the racking so the panels are not shaded, or don't bother to install the shaded panels.

[nidalls]

Your shaded strings will not be damaged. They will continue to maintain voltage, but the current output will
be reduced to 10% or less, still holding off any current from unshaded strings. Bruce Roe

The lower string will be shaded, and because they are portrait not landscape all the by bass diodes in this string will be activated.
V lower string= number of diodes in each module * Vdiode * number of panels

and the upper string will have a higher voltage something like 700v

the difference in voltage between the strings in the same mppt will be high. Does this will make a reverse current? from the higher voltage string to the lower voltage string?

[nidalls]

I have a different opinion on panel shading and damage, Occasional shading, early or late in the day, or a stray leaf or twig not a big deal.

But an array with constant shading over part of the panel, will activate the bypass diode(s) for hours on end, for days on end, The bypass diode eventually will burn out, disabling the entire panel.
Fix the racking so the panels are not shaded, or don't bother to install the shaded panels.

if there is a limited area, and the shading time is too small in winter.
in this case, in winter when the there is 3 strings on one mppt, two of them have full radiation and the third one fully shaded.
how this will effect the other strings? Vstring #1= 700 Vstring #2= 700 Vstring #3= number of by pass diodes in each module * Vdiode * number of panels which will be a very small amount!


the difference in voltage between the strings in the same mppt will be high. Does this will make a reverse current? from the higher voltage string to the lower voltage string?

[bcroe]

The only way the bypass diodes will be activated, is that the MPPT controller drops its operating point significantly
so that the non shaded cells in a string can operate at full power. In fact single string arrays do that but are supposed
to be tolerant of this condition.

HOWEVER with 3 strings in parallel the MPPT is going to run at full voltage from the 2 unshaded strings. The shaded
string sections DO NOT particularly put out greatly reduced voltage, but rather lose most of their current capacity. So
in this situation the total system voltage WILL NOT drop down to where the the unshaded cells can force full current in
that string. Rather the shaded cells will remain in operation at greatly reduced current to attain system voltage, no
problem.

If only a very few cells are shaded, the unshaded potion might shift up its V-I curve and supply enough voltage
to match system voltage, but in the process they are in a REDUCED current mode which flows through the few
bypassed cells. I would agree this is the most stressful situation, as the current will keep decreasing if more cells
are shaded. So small area shade for long periods is most stressful on bypass diodes; very long strings will be
worst. Small shade might be caused by something on the panel, but won't remain long with a moving sun.

Note, even at near zero current, shaded cells will manage enough voltage to hold off any current from
unshaded string cells. Bruce Roe