New Current Control Structure for Shunt Active Power Filters

The paper describes a new current control structure for active power filters. The proposed current controller uses an array of resonant controllers, one for the fundamental, and one for each harmonic, to achieve zero phase-shift and unity gain in the close-loop transfer function for selected harmonics. The complete current controller is the superposition of all individual harmonic controllers. The novelty consists on the controller structure and design, which is based on the pole-zero cancellation technique, taking into account the load transfer function at each harmonic frequency. An important feature of the proposed control is that the cross-coupling between d- and q-axis is considered and compensated for within the transfer function of each resonant controller. This gives stable operation at higher frequencies for the active filter compared to similar existing controllers. A comparative theoretical and experimental analysis of the proposed control with an existing resonant controller is provided. The results are supported by measurement on a 7 kVA laboratory setup, indicating a reduction in total harmonic current distortion factor from 27 % to less than 2 %.