A Proposal of a 3.3-kV/6.6-kV Transformerless Hybrid Filter, and Experimental Verification Based on a 200-V Laboratory System

This paper proposes a transformerless shunt hybrid filter for harmonic compensation of a three-phase six-pulse diode rectifier, where the ac line-to-line voltage is 3.3kV or 6.6kV. The hybrid filter consists of a single tuned LC filter per phase and an active filter with a dc capacitor voltage as low as 300V or 600V. The two filters are directly connected in series with each other without any transformer. The passive filter absorbs harmonic currents produced by the rectifier, whereas the active filter improves the filtering characteristics of the passive filter. The required rating of the active filter is much smaller than that of a conventional shunt active filter used alone. In addition, no additional switching-ripple filter is required for the active filter, because the LC filter acts not only as a tuned LC filter around the 7th-harmonic frequency but also as a switching-ripple filter around 10kHz. A feedforward control scheme is also proposed to improve the active filter performance. 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. The hybrid filter gives satisfactory compensation performance, thus allowing us to put it into practical use.