Duty Cycle Decomposition-Based Common-Mode Voltage Elimination for Three-Phase Four-Leg Back-to-Back Converters

The non-isolated three-phase four-leg back-to-back (BTB) converters feature high power density, low losses, and low cost. It also exhibits high reliability, effective zero-sequence current control, and effectiveness in mitigating three-phase imbalance. It offers significant advantages for microgrid applications and renewable energy grid integration. However, the non-isolated structure allows high-frequency switching noise to propagate through parasitic capacitance, establishing a common-mode path that compromises equipment reliability. Therefore, common-mode voltage (CMV) suppression is a critical challenge for BTB converters. To significantly reduce CMV, this article proposes a duty cycle decomposition method. By decomposing the duty cycle of each leg into two virtual duty cycles with adjustable amplitudes and a constant difference, the number of upper-leg conduction on both sides of the converter can be matched in real time, theoretically eliminating CMV. While eliminating the CMV, the zero vector (0000) is symmetrically distributed in half a control cycle to improve the current quality. Firstly, zero vector adjustment is performed on the duty cycle calculated by the model prediction duty cycle control. Secondly, the duty cycle is decomposed into two virtual duty cycles by scattering and shifting the “on state” period within a single control cycle. The CMV is then eliminated by adjusting the virtual duty cycle magnitudes under the constraint that the difference between the two virtual duty cycles remains unchanged. Simulation and experimental results verify the effectiveness and feasibility of the proposed method.