The fault changes the configuration of the PV array and causes
subsequent fault currents. After the fault, String 1 only has two
modules left operating, since the rest of modules (Module 3 ~
Module n) are short circuited by two ground points F and G. As
a result, String 1 is significantly mismatched with other normal
strings. Meanwhile, the operating voltage of the PV array might
be even larger than the open-circuit voltage of faulted String 1.
Therefore, instead of supplying power, String 1 may be forced
to work as a load in the 4th quadrant of its I-V curve (see Fig.
3). Now String 1 has a negative current backfeeding from other
normal strings. This current is often called backfed current
(Iback, or reverse current). Iback will flow into the fault point F and
become a part of Ig. The other part of Ig is I1-, which is the current
coming from other (n-2) modules in String 1. Since Module
3 ~ Module n in String 1 are short-circuited by ground points,
I1- will be equal to the short-circuit current of each PV module
(Isc) under standard test conditions. Finally, the backfed current
(Iback) and the current from other modules(I1-) will merge as the
ground-fault current (Ig) at fault point F.