Design of a Three-Phase Three-Level Back-to-Back Bridge Interconnection-Based Filter Scheme

Abstract

This article proposes a systematic design methodology for a back-to-back bridge structure to be used with three-phase bidirectional AC-AC converters (also called an active front end) feeding a motor with long unshielded cables. The bridge provides a path for circulating common-mode (CM) harmonics, resulting in low motor-bearing voltage, suppression of motor winding over-voltage, superior low-frequency conducted emissions attenuation, and wide band performance. The proposed filter structure is designed using simplified sets of derived analytical attenuation pertaining to different groups of harmonics. Consequently, each filter component is optimized for low weight using specific core materials depending on current-ripple content. Following the physical design, parasitic models for the converter, filter, motor, and long cables are developed in the time domain to allow prediction of CM and differential-mode (DM) voltages as well as reducing the filter’s impact on high-frequency emissions. The overall design methodology is validated through emissions characterization using a 12-kW rated three-level neutral point clamped (NPC) converter in a back-to-back configuration.