A voltage sag is an abnormal condition in the grid voltage
v
, characterized by a short-term reduction in one or more
phases. Voltage at the point of common coupling
v
can be
affected by the fault, such as phase-to-ground or phase-to-
phase short circuit, occurred in distant parts of the transmission
or distribution system. Voltage sags can be roughly divided into
balanced and unbalanced sags. Balanced voltage sags imply
an equal voltage drop in all three phases while unbalanced
sags involve different voltage drops in different phases and
a possible phase shift. A widely accepted classification of
voltage sags is presented in [24].
The most usual duration of voltage sags is in the range of
a few tenths of a second. The stiffer the grid, the less influence
the voltage support has on the voltage on the PCC [25]. Voltage
support capabilities are limited with the stiffness of the grid.
The occurrence of a voltage sag at the terminals of a
power plant can cause severe problems for the operation of
the power plant if not controlled properly. One of the main
objectives of dealing with voltage sags is to prevent tripping
of protection devices in the power plant. Overcurrent in any
of the three phases of the grid side converter usually results in
the immediate disconnection of the power plant from the grid