The aim of this paper is to present a review of recently used current control techniques for three-phase voltagesource pulsewidth modulated converters. Various techniques, different in concept, have been described in two main groups: linear and nonlinear. The first includes proportional integral stationary and synchronous) and state feedback controllers, and predictive techniques with constant switching frequency. The second comprises bang-bang (hysteresis, delta modulation) controllers and predictive controllers with on-line optimization. New trends in the current control-neural networks and fuzzy-logicbased controllers-are discussed, as well. Selected oscillograms accompany the presentation in order to illustrate properties of the described controller groups.

Abstract-This paper presents the comparative evaluation of the performance of three state-of-the-art current control techniques for active filters. The linear rotating frame current controller, the fixed-frequency hysteresis controller, and the digital deadbeat controller are considered. The main control innovations, determined by industrial applications, are presented, suitable criteria for the comparison are identified, and the differences in the performance of the three controllers in a typical parallel active filter setup are investigated by simulations.

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The paper introduces the family of quasi-direct converters, i.e., forced-commutated adddac converters including small energy storage devices in the dc link. In particular, the case of three-phase to three-phase quasi-direct converter is considered. Since energy storage minimization calls for instantaneous inputloutput power balance, a proper control strategy is needed. The paper describes a simple and effective control technique which also provides high-power factor and small distortion of the supply currents. After a discussion of the general properties of quasi-direct converters, design criteria of both power and control sections are given, and experimental results of a 2-kVA prototype are reported.

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Abstract-An improved implementation of the constantfrequency hysteresis current control of three-phase voltage-source inverters is presented. A simple, self-adjusting analog prediction of the hysteresis band is added to the phase-locked-loop control to ensure constant switching frequency, even at a high rate of output voltage change, such as required in active filters, fast drives, and other highly demanding applications. This provision also improves the relative position of phase modulation pulses, thus reducing the current ripple. The prediction method is robust and uses a small number of inexpensive components. It does not require trimming or tunings, giving the whole system the capability to adjust itself to different load conditions. Thus, the control becomes suitable to hybrid or monolithic integration. In this paper, the basic principles are described, and a detailed stability analysis is done. The control performance is illustrated, both by simulated and experimental results.Index Terms-Constant switching frequency, hysteresis current control, voltage-source inverters.

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