Improving Stability and Power Sharing of Bidirectional Power Converters by Relaxing DC Capacitor Voltage

This article presents a new control structure for bidirectional power electronic converters. Commonly, a proportional-integrating (PI) controller is used in the dc capacitor voltage loop, to generate the set point for the current controller, which tightly regulates this voltage to a given value. This article shows that such an approach causes stability issues that are aggravated during reverse power flow operation and intensify during weak grid conditions and uncertainties. The proposed controller allows small variations of dc capacitor voltage in proportion to the power flowing through the converter. This improves the stability of the converter irrespective of the power direction and in the presence of uncertainties and weak grid conditions. The proposed control structure also simplifies the design process, and it can be applied to dc/dc and dc/ac (single-phase and three-phase) converters. Moreover, the proposed method grants an inherent power-sharing capability when multiple converters share the same dc bus. The article also adopts full state feedback and an optimal control approach, i.e., the linear-quadratic tracker (LQT), for systematic design of the current control loop and enhanced robustness. Detailed analyses, simulations, comparisons, and experimental results are included to demonstrate the effectiveness of the proposed approach.