Enhancing Common-Mode Loop Stability of Three-Phase Four-Bridge Grid-Connected Inverters via Control Delay

As the three-phase four-bridge inverter (3P4B) can effectively compensate for the unbalanced three-phase loads in the grid, it is an important converter option for distributed generation grid connection. As in a three-phase three-bridge inverter (3P3B), the wide variation in grid impedance also poses instability issues for 3P4B. This issue has been well-addressed for 3P3B, which can be seen as a differential-mode circuit. However, 3P4B has an extra common-mode circuit, and the solution to the instability problem has not been investigated so far. To address this issue, this paper first analyzes the mechanism of 3P4B common-mode circuit instability and discovers its stability range difference from its differential-mode circuit. Then, an equivalent control delay compensator is independently introduced into the common-mode loop, which extends its stable range. This paper also conducts a detailed analysis of the control delay compensator’s impacts on the common-mode control loop and proposes a quantitative design method for the compensator accordingly. Experimental results validate that the proposed method effectively mitigates common-mode loop instability even under a wide range of grid impedance variations.