The block diagram of basic SVC Controller incorporating voltage regulator is shown in Fig.3.11. This shows that both voltage (VSV C) and current (ISV C) signals are obtained from potential and current transformers and then rectified. The AC filter is basically a notch filter to eliminate the signal component of frequency corresponding to the parallel resonance in the system viewed from the SVC bus. The line capacitance (in parallel with SVC capacitance) can result in parallel resonance with the line inductance. The SVC voltage regulator has a tendency to destabilize this resonant mode of oscillation and the notch filter is aimed at overcoming this problem. As a matter of fact, any parallel resonance mode (of frequency below second harmonic) can have adverse interaction with SVC voltage regulator. If series capacitors are used along with SVC, then they can cause parallel resonance with a neighbouring shunt reactor. If the second (parallel resonance) mode has a lower frequency (say below 20 Hz), a high pass filter in addition to the notch filter has been suggested [23].