Study On One-cycle Control Three-phase Four-wire Active Power Filter With Four-leg

Active power filter (APF) is a new power electronics device used to restraining harmonics and compensating reactive power. By realtime detecting harmonics and reactive current in the power line, then generate the compensate current which is equal to the harmful current in amplitude and opposite in direction, realtime compensation is achieved. APF can achieve dynamic compensation for harmonics and reactive current, and the compensate performance is not affected by the impedance of the electric network, it has advantages as follows, fast response, high controllability and adaptability, etc.Analysis is performed to existing main circuit topology, harmful current detecting method and current tracking control of APF in this paper. The compensate performance of APF is determined by detecting the harmful currents accurately and real-timely and the current tracking control strategy of the converter. At present, the typic harmful current detection methods are the Fast Fourier Transform Algorithm in frequency domain and methods based on the instantaneous reactive power theory, these methods all require some transform and quick, real-time calculating, so high precision analog multipliers or high speed DSP chip with fast A/D are needed, this results in complex circuit and high cost,which have restricted the development of APF. The typical control methods for APF such as triangular wave modulate, hysteresis control and timing control, etc, these methods all have some demerits. Because of these demerits, the generalization and application of APF is restrained to a certian extent.One-cycle control method is a nonlinear control method. By controlling the switch duty-ratio, it can make the average value of a switched variable equal to or proportional to the control reference in each cycle, so the stationary and transient errors are eliminated. One-cycle control response fast, it can reject power perturbations and the control circuit is simple and reliable.Based on the research on one-cycle control theory, one-cycle control is applied to three-phase four-wire APF with four-leg in this paper. Analysis for the average model of the APF in switching period is performed and the control equation is derived. Based the simplest solutions of control equations, one-cycle controller for the three-phase four-wire APF with four-leg is presented. Three-phase four-wire APF with one-cycle control need not sense the three phase load currents or the three phase voltages, need not calculate the harmonics and reactive current components, as well as the use of any multipliers. Thecontroller is composed of several integrators with reset along with several flip-flops, comparators and some linear components. The control circuit is simple and reliable, which make it be realized easily. The switch frequency of the main circuit is constant, which is desirable for industrial applications. Based on the controller, simulation is carry out for several different loads using PSIM simulation software, the simulation results verify that the three-phase four-wire APF with one-cycle control can dynamically and effectually compensate the harmonics, reactive current and zero sequence current and they have excellent compensating capability.