A medium-voltage transformerless AC/DC power conversion system consisting of a diode rectifier and a shunt hybrid filter

This paper proposes a 3.3-kV transformerless AC-to-DC power conversion system consisting of a three-phase six-pulse diode rectifier and a shunt hybrid filter. The hybrid filter is formed by a single tuned LC filter per phase and a small-rated three-phase active filter, which are directly connected in series without any matching transformer. The required rating of the active filter is much smaller than that of a conventional standalone active filter be. In addition, no additional switching-ripple filter is required for the hybrid filter because the LC filter functions not only as a harmonic filter tuned at the seventh harmonic frequency but also as a switching-ripple filter around 10 kHz. Experimental results obtained from a 200-V 5-kW laboratory system and simulation results of a 3.3-kV 300-kW system confirm the validity and effectiveness of the system.

I. Introduction

Voltage and current harmonics in power transmission and distribution systems have become a serious problem. Therefore, harmonic current-free rectifiers capable of operating at unity power factor are required as utility interfaces for adjustable-speed drives in a range of several hundreds of kilowatts. However, the size and weight of a conventional medium-voltage diode or pulsewidth-modulated (PWM) rectifier are dominated by step-down and step-up transformers [1]. Fortunately, the 3.3-kV/4.5-kV insulated gate bipolar transistors (IGBT) has already been available, and the 6.5-kV IGBT is expected to be on the market in the near future. This makes it possible to construct a medium-voltage adjustable-speed drive system characterized by direct connection to industrial/utility grids without any transformer.