I I zyxwvutsrqponmlkjihgfedcbaZYXW 1 zyxwvutsrqponmlkjih i I ! I I i zyxwvutsrqponmlk IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 9, NO. 2, MARCH 1994 173 A New Control Algorithm for Three- Phase PWM Buck Rectifier with Input Displacement Factor Compensation Silva Hiti, zyxwvutsrqp Student Member, zyxwvutsrq IEEE, Vlatko VlatkoviC, Student Member, IEEE, DuSan BorojeviC, Member, IEEE, and Fred C. Y. Lee, Fellow, IEEE Abstruct- A new control algorithm for the three-phase buck rectifier with an input filter is developed. The algorithm employs a separate control loop for compensation of the input current displacement factor in steady-state, in addition to the standard output voltage regulation loop. The algorithm allows separate design of the input filter and of closed-loop output voltage control. The design procedure is explained and illustrated with an example. The algorithm is verified experimentally on a 1 kW, 100 kHz, three-phase isolated buck converter. I. INTRODUCTION HREE-PHASE PWM converters are increasingly being T considered for use as front-end power processing units with power factor correction. The three-phase buck converter with six-step PWM [l], 121 provides dc output voltage and sinusoidal input currents with no low-frequency harmonics. However, the switching frequency harmonics contained in the input currents must be suppressed by the input filter, which produces a phase shift zyxwvutsr 9 between the line frequency components of the input currents and input voltages. The phase shift varies with the load and with the magnitude of the input phase voltage. It can result in unacceptably low input displacement factor (IDF = COS@) under some operating conditions. The solution proposed originally in [3] and more recently in [4] and [5] is to control the phase shift of the input currents in a closed loop. In that case, the input filter is included in the control loops together with the converter. The full state feedback of input filter states is usually employed [3]-[5], and extensive digital hardware is needed for the controller imple- mentation zyxwvutsrqp [5]. The presented algorithms, where all filter states have to be measured and controlled, are mostly impractical for high power circuits in which required attenuation can be met only by multistage filters. This paper presents a new simple control algorithm which provides output voltage regulation and IDF compensation without resorting to the control of input filter states. The phase shift of the input currents is compensated through a low-bandwidth output current loop which assures that the Manuscript received July 19, 1993; revised January 5, 1994. The authors are with the Virginia Power Electronics Center, The Bradley Department of Electrical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-01 11 USA. IEEE Log Number 9400089. Fig. 1. Three-phase buck rectifier. converter in steady-state draws the desired reactive current. A fast control loop, designed by considering only output filter dynamics, is employed for output voltage regulation. Stability of the integrated system, consisting of the converter with out- put voltage regulation and an input filter, is assured by proper input filter design. The algorithm is verified experimentally. 11. CONVERTER MODELING A circuit diagram of the three-phase buck converter is shown in Fig. 1. At any given instant, only one switch in the upper half and one switch in the bottom half of the bridge conducts, so that the input voltages are never shorted, and a freewheeling path for the inductor current is always provided. If the switching function of a switch swjk in Fig. 1 is defined as 1, SWjk closed Sjk(t) = { 0, SWjk open ' zyx j E {a, 4 cl, /c E {P, (1) sak + Sbk + sck = 1, E {p,n}. (2) these conditions can be expressed as Then, from Fig. 1, the input currents are given by while the output voltage vpn is 0885-8993/94$04.00 0 1994 IEEE (3) (4)