There are few studies in the literature to solve the LOS problem
of the WTs. These methods are briefly reviewed here only for the
sake of coherency, though solving the LOS is not the main scope of
this paper. In [5–7], a voltage-dependent active current reduction
method during low-voltage faults is suggested to avoid LOS.
Assuming that the impedance between the WT terminal and the
faulted grid point around PoC is highly inductive (high X/R ratio),
it helps to avoid the LOS to reduce the active current reference as
voltage goes down. In [6], a novel method of freezing the
calculated phase angle when the voltage drops down to very low
values is suggested. Thus, the deviation of angle and frequency is
avoided artificially keeping the angle constant; in other words
renouncing the grid synchronisation. As a result, the active and
reactive current of the WT flows in accordance with the X/R of the
impedance. In [4], another novel method called PLL frequency
based active current injection is proposed to solve the LOS
problem in a closed-loop control manner, where the active current
is modified accordingly if the frequency of the WT terminal
voltage is detected to deviate from rated due to the LOS. In [8], the
LOS is tried to be avoided through the calculation of the phase
angle at the converter connection terminals and controlling the
power injection, where such an idea can be further improved using
phasor measurement unit (PMU) data from remote locations. In
addition to these, there might be additional methods to avoid LOS