Analysis and design of enhanced DFT-based controller for selective harmonic compensation in active power filters

Abstract

Traditional Discrete Fourier Transform(DFT)-based repetitive controller is widely used in active power filters(APF) thanks to its unique merits such as excellent selectivity and simplicity. However, the structure of the only one feedback path results in the same steps of leading angles for all harmonic frequencies. Since the phase response may not be proportional to frequency and the gain of the plant varies with frequency, identical leading angles and gain coefficients can't compensate the phase lag and the gain attenuation effectively at different harmonics. All those facts imply constraints to obtain the best stability margins and performance. The proposed controller provides feedback path and gain coefficient for each harmonic according to the characteristics of the plant respectively. In addition, the correction term is embedded in the forward channel to provide overall phase compensation and correct the plant for better characteristics. As a result, the novel control structure incorporated in sliding DFT can improve the performance greatly. Detailed design consideration is given with focus on stability analysis and transient behavior of the APF. Experimental results validate the effectiveness of the theoretical analysis.