RDFT based control of a 3-phase 4-leg shunt active power filter under unbalanced load and non-ideal mains voltage conditions

In this paper, a control algorithm based on Recursive Discrete Fourier Transform (RDFT) is proposed for 3-phase, 4-leg shunt active power filter (APF) to prevent harmonic currents, compensate reactive power and neutral line current and balance the load currents under unbalanced non-linear load and nonideal mains voltage conditions. The APF is composed of a 4-leg voltage source inverter (VSI) with a common DC-link capacitor and hysteresis band PWM current controller. The control algorithm takes advantage of a computationally efficient RDFT and it is based on extraction of the positive sequence component from the measured current with the Fortescue decomposition. The extraction of the positive sequence component guarantees the accurate computation of current components under both balanced and unbalanced line current conditions. The advantage of the proposed method to the common control techniques of the active filters falls in the fact that the proposed method extracts the harmonic and the fundamental components faster and with more accuracy and has a faster dynamic response. Simulation results indicates that the 4-leg APF with RDFT control algorithm, compensates harmonics and neutral currents and balances load currents under all non-ideal mains voltage conditions adequately.