The voltage difference, ΔU, is related to the short circuit
impedance, the real and reactive power output of the wind
power generation unit. It is clear that the variations of the
generated power will result in the variations of the voltage at
PCC. If the impedance Zk is small then the voltage variations
will be small (the grid is strong). On the other hand, if Zk is
large, then the voltage variations will be large (the grid is
weak). However, strong or weak are relative concepts. For a
given wind power capacity P the ratio RSC = SSC / P is a
measure of the strength. The grid may be considered as strong
with respect to the wind farm installation if RSC is above 20