Complementing AC and DC Terminal Stability Analyses of MMC With Circulating Current Inner-Loop Impedance Model

Learning from two-level voltage-source converters, the existing impedance-based stability analyses of modular multilevel converters (MMCs) focus on system modes with finite closed-loop transfer functions, which consider perturbations of the current flowing into the interconnected ac/dc terminal as the input. However, this approach is insufficient for MMCs due to the actively controlled circulating current circuits, resulting from the distributed modulation of each arm and the circulating current control (CCC). To address this limitation, two cases that are not covered by the ac/dc terminal stability analysis are initially presented to support the conjecture. Subsequently, an inner-loop impedance involving the circulating circuit is established, which considers the dynamics of interconnected terminals and divides the injected voltage perturbation by the corresponding current perturbation. To avoid the right-half-plane pole check of the Nyquist criterion and improve the accuracy of mode identification, the optimization technique is integrated with the logarithmic derivative-based criterion. By utilizing the circulating current inner-loop impedance, it becomes possible to achieve CCC parameter tuning with stability constraints and conduct an internal stability analysis of MMC-based systems. In a nutshell, this work supportsthe integration of converter-dominated systems from the perspective of classical control theories.