Enhanced DFT-Based Controller for Selective Harmonic Compensation in Active Power Filters

Among the various selective harmonic current controllers, the traditional repetitive-based discrete Fourier transform controller (DFTRC) is increasingly favored thanks to its unique merits of excellent selectivity and simplicity. However, the traditional approach suffers from disadvantages like slow dynamic responses, defective control structure, and sensitivity to frequency fluctuation. In order to alleviate aforementioned short-comings, an enhanced discrete Fourier transform-based controller is proposed. The proposed controller is based on the generalized discrete Fourier transform to improve the overall control performances of dynamic responses, structure flexibility, and control robustness. The proposed controller provides individual positive-feedback path and gain coefficient for each harmonic component. Therefore, it not only maintains the advantage of excellent selectivity but also realizes the individual control parameters tuning for each harmonic frequency, which is impossible in traditional DFTRC owing to the congenital defects of control structure. In addition, a correction function is embedded in the forward path to provide the overall phase compensation and correct the plant for better characteristic. The Lagrange interpolation method is adopted to realize the fractional-order delay and its adaption to frequency variations with fixed sampling frequency. Detailed mathematical model along with optimized design principle is given to make full use of the advantages of the enhanced control structure. Extensive experimental results validate the effectiveness of the theoretical analysis and the improvement achieved by the proposed controller.