Selective Harmonic Compensation Using Active Power Filter with Enhanced Double-Loop Controller

In this paper, a novel double-loop current controller is proposed to enhance the performance of shunt APFs. In contrast to the conventional multi-loop current control strategies where the inner loop and the outer loop are highly coupled, the proposed method realizes two independent control objectives at different frequency ranges by properly reconstructing the controllers of the inner and the outer loops. First, an accurate line harmonic current tracking is realized by an outer control loop with high loop gain at harmonic frequencies. Second, the APF power flow is regulated by the inner loop control that has a high loop gain at the fundamental frequency, but limited loop gain the selected harmonic frequencies. Due to the coordinated effects of the proposed inner and outer control loops, a controllable reactive power generation without using any phase-locked loops (PLLs) can be achieved even when grid voltage is distorted. In addition, the proposed double-loop controller can be easily used to achieve an accurate load harmonic current compensation without any load harmonic current detection process. Finally, to reject the impact of unsymmetrical grid fault, the inner loop control can be further enhanced by using resonant controllers with cross couplings in the stationary reference frames.