Abstract — In this paper the cascade output voltage control of a unidirectional three-phase three-switch buck-type unity power factor PWM rectifier is discussed. Two control structures, i.e. an inner DC side inductor current control in combination with active input filter damping and an inner AC side input current and input filter capacitor voltage control are discussed. Guidelines for the control design which is based on a dynamically equivalent DC-to-DC converter model are given. Small signal transfer functions are derived by state space averaging. Finally, a comparative evaluation of the dynamics and of the realization effort of the control schemes is given and the continuation of the research is discussed. Index Terms — Three-phase three-switch buck-type PWM rectifier, cascade control, dynamically equivalent DC-to-DC converter model, control design and evaluation. I. INTRODUCTION T hree phase buck-type PWM rectifier systems (also known as current source rectifiers) are frequently employed as front- end converters in utility interfaced systems such as, e.g., power supplies in telecommunications and process technology, and AC drives. As compared to boost-type (voltage-source) topologies, the control design for current source topologies in general is more involved due to the higher system order which is caused by second order filtering on the AC and DC side. On the other hand, current source rectifiers do provide a wide output voltage control range and do allow current limitation also in case of an output short circuit. ~ ~ + L F u N,i i LF,i i rec,i i C F ,i C F D i,N− D i,N+ D i+ D i− S i D F L i i C i 0 u u C F ,i ~ u 0 C R Fig. 1: Structure of the power circuit of the investigated three-phase three-switch AC-to-DC buck-type PWM rectifier. In order to reduce the number of power transistors as compared to bidirectional current source systems a three- phase three-switch buck-type PWM rectifier topology has been developed in [1] (cf. Fig. 1). The control of this system to the knowledge of the authors so far has been treated in more detail only in [2] and [3] where an output voltage control in combination with an off-line PWM pattern generation scheme has been proposed. Apart from the fact, that the dynamics and the phase shift resulting from the input filter have been neglected, operation with off-line PWM patterns in general results in slow transient response of the output voltage control and in a potentially low input current quality and/or in input filter resonance problems [4]. This does give the motivation for a more detailed analysis of the system control. In this paper, cascade output voltage control schemes which have been employed successfully for the bidirectional current source PWM converter and for the VIENNA Rectifier III [5] are adapted and comparatively evaluated for the three-switch rectifier. For the bidirectional current source topology, usually an outer DC voltage or current control loop is employed to keep the output quantity at a constant value [6], [7], [8]. In order to form sinusoidal input currents two different concepts for the inner closed-loop control have been proposed: 1. Inner AC Current Control When employing an AC current control (cf. Fig. 2(a)) the three input phase currents are controlled with high dynamics [9]. The templates for the current reference values are synchronized with the mains phase voltages in order to achieve ohmic fundamental mains behavior and/or unity power factor. Due to the high order of the system to be controlled, the current loop bandwidth has to be selected well below the input filter resonance frequency, i.e. current control is not effective at the filter resonance. Furthermore, additional active or passive damping is required in order to effectively suppress input line current oscillations. Therefore, in this paper an AC current control in combination with an underlying filter capacitor voltage control [6] is provided. Thomas Nussbaumer and Johann W. Kolar Swiss Federal Institute of Technology (ETH) Zurich Power Electronic Systems Laboratory ETH Zentrum, ETL/H23 CH-8092 Zurich, Switzerland Tel.: +41-1-632-6973 Tel.: +41-1-632-2834 email: nussbaumer@lem.ee.ethz.ch email: kolar@lem.ee.ethz.ch Comparative Evaluation of Control Techniques for a Three-Phase Three-Switch Buck-Type AC-to-DC PWM Converter System