A repetitive control approach for three-phase shunt active power filters with real-time wavelet transform

Active power filter (APF) has been widely recognized as a substitute for classical passive L-C filter for eliminating the power line harmonics/reactive current due to nonlinear loads and switching devices. The tracking capability of the current controller dominates the performance of the APF. Since the required harmonic compensating currents are of periodic, the repetitive control may be considered as an effective means to achieve accurate current tracking. However, the use of low-pass or bandpass filter in the control loop may introduce phase lag and deteroriate the tracking performance. This paper presents a novel repetitive current control scheme, which utilizes a real-time delayless digital filter based on wavelet transform, to command the APF deliver desired compensating currents. The filtering capability of the wavelet transform is elaborated and its real-time version is also developed for incorporating with the repetitive control scheme. For maintaining a constant dc-capacitor voltage despite of inverter conduction loss, a simple proportional-integral algorithm is employed in the voltage loop. A 2-kVA DSP-based controlled prototype has been built to justify the enhanced tracking accuracy and robustness of the proposed wavelet-based repetitive controller.