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

This article proposes a systematic design methodology for a back-to-back (BtB) 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 overvoltage, superior low-frequency conducted emissions attenuation, and wideband 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 the prediction of CM and differential-mode (DM) voltages as well as reduce 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 BtB configuration.