Differential Mode and Common Mode Coupling Analysis and Robust Design of Three Phase Four Bridge

The three-phase four-bridge (3P4B) inverter has been widely used in unbalanced load compensation and emergency power supply. However, unlike the widely studied three-phase three-bridge (3P3B) inverter, the 3P4B suffers from common-mode stability problems under different grid impendence due to the existence of the fourth bridge. To address this issue, this paper firstly establishes the differential-mode (DM) and common-mode (CM) equivalent circuit models of the 3P4B, revealing the essential reasons for the CM stability problem. Then, starting from the maximum and minimum resonant frequency under wide varying grid impedance, a CM circuit design scheme is proposed. This scheme can keep the 3P4B stable under arbitrary grid impedance. Finally, this paper demonstrates the correctness of the design scheme through simulation.