When the PV array is working under normal conditions, each
PV string is generating current. The current flowing out of the
i
th string is I
i+, where i = 1 … n. If PV strings are all electrically
identical and have the same environmental working condition,
then I
1+=I
2+=… =I
n+. The total current flowing out of the array is
I
pv+= I
1++I
2++…+I
n+. Similarly, the current coming back to each
string is I
1-, I
2-, … I
n-. Thus, the total current coming back to
the array is I
pv- which should be equal to I
pv+. Since no external
ground point is involved, the current flowing through the GFPD
(I
g
) should be zero. Notice that the PV array is supplying power,
while the PV inverter absorbs the power and feeds it into the
utility grid.