A new method to improve THD and reduce harmonics generated by a three phase diode rectifier type utility interface

In this paper, a new method to significantly improve the performance of a three phase diode rectifier type electric utility interface is proposed. The proposed method guarantees near sinusoidal input currents at unity power factor for varying load conditions. The method is effective in continuous as well as discontinuous operating modes of the diode rectifier. The scheme employs a zig-zag transformer along with a two switch PWM current controlled power converter to circulate a variable amplitude third harmonic current. The interconnecting zig-zag transformer virtually short circuits the zero sequence third harmonic current and also draws negligible 60 Hz currents from the input AC mains since there is no real power exchange. This method does not significantly alter the existing rectifier inverter hardware and the DC link voltage magnitude is unaltered. It is shown that the proposed approach is effective in discontinuous mode of the diode rectifier and in situations where the DC link inductor is absent such as in the front-end rectifier of an AC drive. Wide operating range and superior harmonic reduction ability are the main features of this scheme. Analysis, design, implementation, comparison and application aspects of this new active harmonic reduction scheme are discussed. Simulation and experimental results illustrate the effectiveness of the proposed method