The MOV is a nonlinear resistive device, which starts to
conduct at specific instantaneous voltage and ceases to
conduct when the voltage falls below the same voltage at each
half cycle of the power frequency. The result is that there is a
non-linearly time-varying degree of series compensation
during a fault, due to the non-linear impedance characteristics
of the parallel MOV-series capacitor combination.
This non-linear voltage-current characteristic of the MOV
allows it to provide overvoltage protection across the capacitor
when connected in parallel with it. The MOV holds the voltage
across the capacitor within the permissible range of the
capacitor by allowing a self-regulating amount of current
through itself automatically. This non-linear relationship
between the voltage and current is shown in figure 5.
The MOV protective voltage is the instantaneous voltage
across the series capacitor at a specified current when the
MOV starts conducting. The protective voltage is typically
chosen above normal operating conditions, power system
swing, and overloads as illustrated in figure 5 [13], [15], [16].