A control strategy of a hybrid active filter for operation with harmonically unbalanced voltages and currents

This paper presents a hybrid active filter to suppress harmonic resonances and to reduce harmonic distortion under unbalanced conditions in medium voltage industrial networks. The proposed hybrid filter is a combination of a three-phase, two-level, voltage-source converter connected in parallel with the inductor of a shunt, single-tuned, passive filter. This topological structure greatly decreases the voltage and current stress over the elements of the active filter. Since in the industrial networks the voltages and currents are usually unbalanced, the control algorithm also has to ensure sufficient filtering of unbalanced harmonics. We propose using a proportional-resonant (PR), multi-loop controller, that allows to half the number of resonators, compared with the conventional solution of using proportional-integral (PI) control principle. Theoretical analyses and design considerations are presented, and experimental results obtained with a Real-Time Digital Simulator (RTDS) and DSP coded control algorithm are provided to validate viability and effectiveness of the proposed hybrid filter.