Research On LCL-Type Four-Leg APF Based On Repetitive Control

With the increasing demand of electricity,more and more non-linear loads are connected to grid,leading to increasing pollution of harmonic.Four-leg active power filter(APF)is an effective means to solve the problems of harmonic and unbalance in low-voltage power system which has received extensive attention.The turning on and off of power device in APF system generate a large number of high-frequency harmonics related to switching frequency,so output filter is required.Compared with L-type filter,LCL-type filter can provide better filtering effect with less inductance and strong attenuation ability in high frequency.Therefore,parameter design of LCL filter,active damping method and repetitive control of LCL-type four-leg APF were studied in this thesis.The mathematical model of LCL-type four-leg APF in three-phase static coordinate system has coupling relation because of unknowns.Zero sequence component and non-zero sequence component separation method was adopted to eliminate the common unknowns and establish no-coupling mathematical model in ??? coordinate system.Constraint conditions of four-branch LCL filter for four-leg converter were analysed,based on which LCL-filter parameters were selected with graphic method.The filtering-performance of four-branch LCL-filter was validated by simulation.Intrinsic resonant of LCL filter might drive the system to an unstable condition.In order to solve this problem without additional sensors,the active damping method of notch filter was studied in this thesis.Mechanism of resonant suppression and intrinsic relation among notch filter,passive damping and virtual resistor were analyzed through deduction of control block diagram.In allusion to the flaw of notch filter's robustness,detecting actual resonant frequency with goertzel algorithm and tuning notch filter's parameters on line were taken to enhance adaptive capacity of notch filter.The parameters were tuned based on discrete-pole allocation to solve the problem of frequency offset in discretization process and reduce the number of parameters tuning on-line.The simulation verified that self-adaptice notch filter can restrain resonance effectively,and maintain the stability of system when grid impedance change.Repetitive control can track harmonic command signal without static error,but it has slow dynamic response.The compound control strategy consisting of proportional control and repetitive control was studied to improve repetitive control's dynamic performance.Compound controller's structure,tracing property and coupling effect between proportional controller and repetitive controller were analysed in details.The internal model filter was selected according to the requirements of system stability and steady-state tracking accuracy,and the compensator was designed according to the frequency characteristic of equivalent controlled object with amplitude and phase being compensated separately.Modified zero-phase-shifting notch filter,which can solve the problem that traditional zero-phase-shifted notch filter's notch frequency is fixed,was taken to eliminate the amplitude spikes when compensating controlled object's amplitude.The forward link was set by the geometrical characteristic of system's stability condition.The simulation results showed that the composite controller has good steady-state tracking accuracy and dynamic response performance.Experiment was carried out based on the experimental platform of LCL-type four-leg APF.The result showed that the four-branch LCL-filter designed in this thesis can significantly reduce the high frequency components in grid-side current,the notch filter can achieve the resonant-suppression effect similar to passive damping,and the composite controller has good steady and dynamic performance.