Stability Investigation of Bidirectional AC-DC Converter Considering Operating Conditions

This paper investigates the stability performances of bidirectional ac-dc converters considering operating conditions based on the small-signal impedance model. It finds that the converter has different stability margins when working in different operating conditions (including inverter mode and rectifier mode, as well as generating/absorbing reactive power), which should be taken into account in the design of the converter for bidirectional energy conversions. To have an intuitive understanding of the instability mechanisms of bidirectional converters in different operating conditions, the concept of positive feedback is used. It finds that with phase-lock loop (PLL) and dc-link voltage control loop (VCL), positive feedback effects appear when the converter operating in different conditions; and the influencing mechanisms of the parameters, including control parameters of PLL, inner current and outer voltage controllers and power ratings, on the converter's stability can be explicitly identified by the positive feedback gains. And it indicates that reducing the values of the parameters along the positive feedback paths will be a practical way to enhance the converter's stability. The theoretical stability analysis results are verified by simulations and experiments.