Resonant current regulation for transformerless hybrid active filter to suppress harmonic resonances in industrial power systems

Severe harmonic distortion, due to unintentional series or parallel resonance of the passive filters or power factor correction capacitors, is a significant issue in the industrial power system. The transformerless hybrid active filter, which operates at reduced kVA rating and switching ripples, is a promising filtering solution in high-power applications. This paper presents resonant current regulation for the transformerless hybrid active filter to enhance its suppressing capability of harmonic resonances. The proposed current regulator is composed of various band-pass filters in parallel connection to resonate at harmonic frequencies for accurately controlling the active filter as variable harmonic conductance. The current tracking capability of the active filter and the associated damping performance can be improved without increasing overall gain of controller. Therefore, the harmonic resonance can be definitely avoided and the damping conductance can be dynamically adjusted to conform with the harmonics limitation. In addition, the required damping conductance to maintain the harmonic voltage at an allowable level is reduced, compared with proportional control only. Operational principles are detailed and computer simulations are provided to validate the effectiveness of the proposed approach.