A control method for improving ac input current waveform of current-type PWM rectifiers

Abstract

Recently, PWM rectifiers which realize high-power factor and almost sinusoidal ac input current have been proposed and some of them have already been put into practice. In many cases the PWM rectifiers are of current type, that is, they have a smoothing reactor on the dc side. In such rectifiers, an LC filter has to be inserted on the ac side to reduce current harmonics due to the PWM operation. However, the LC filter may cause waveform distortion and transient oscillation of the ac side current. As a solution to this problem, a new control method is proposed introducing ac side current control by means of state feedback of the LC filter. In this case, both the dc output current and the ac input current controls should be achieved by the PWM operation of the rectifier. Furthermore, to obtain sufficient controllability regardless of the operating condition, the dc output and ac input current controls should be done independently. To meet these requirements, existing control methods for PWM rectifiers are not directly applicable. In this paper, a novel control circuit configuration and a method of generating a PWM pulse pattern suitable for the proposed control method are described in detail. From the viewpoint of stability, feedback coefficients and circuit parameters are investigated on the basis of root locus analysis. Some experimental results also are shown to confirm the effectiveness of the proposed control method and the validity of the analytical results. Improvement of the ac side current waveform and the stability of the transient response are demonstrated by these experimental results.