Modified series compensation for voltage sag / swell mitigation

A voltage source inverter (VSI) is employed to implement a variable frequency drive (VFD) for obtaining desired speed-torque characteristics of an AC motor which is normally fed by a diode rectifier at the front end. These uncontrolled converters inject excessive harmonic currents in to AC mains resulting in detoriated power quality in terms of AC voltage waveform distortionat the point of common coupling (PCC) and increase losses in the distribution system. Because of voltage sag and swells, the DC link voltage is reduced and the drive needs to be derated. An active series compensator is proposed here which is capable of reducing harmonic currents at the PCC and regulating the DC link voltage at the input of the VSI, thus offering dual functionality.