An asymmetrical half-bridge active DC filter for industrial drive systems using multipulse utility rectifiers with harmonic reducing input transformers

The operation of parallel-connected multi-pulse 3-phase rectifiers are described that use isolating and also non-isolating multi-winding input transformers. The performance of these rectifiers are analysed with unbalanced transformer output voltages and inductances. The rectifier operation is more sensitive to these imbalances the larger the number of parallel bridges used. Auto-transformers require inter-phase blocking reactors to balance the rectifier input currents. A multi-pulse PWM asymmetrical half-bridge DC filter is examined that can be used to balance the rectifier currents under unbalanced supply conditions and and when using an auto-transformer. The power electronics can be rated at less than 20% voltage and make the matching of the transformer output voltage and inductance non-critical. When using auto-transformers, interphase blocking inductors can also be eliminated. This paper describes the unipolar PWM switching control technique for the asymmetrical half-bridge in an 18-pulse diode rectifier and discusses the bridge voltage ratings.