Stability of Small-Gain Closed-Loop System and Its Applications in VSC-Based DC/AC Power Systems

Abstract

In a VSC-based HVDC/AC power system, dynamic interactions introduced by the HVDC system are usually weak. Hence, the small-signal stability of the HVDC/AC system is not affected significantly by the HVDC system. This paper presents a description of a weakly coupled VSC-based HVDC/AC power system by a small-gain closed-loop model that consists of two open-loop subsystems interconnected by a small gain. A special condition of open-loop modal proximity of the HVDC/AC system is examined, i.e., in the small-gain closed-loop model, an oscillation mode of one open-loop subsystem is close to an oscillation mode of the other open-loop subsystem on the complex plane. Our analysis indicates that when the open-loop modal proximity occurs, the corresponding closed-loop oscillation modes repulse each other. The closed-loop modal repulsion results in strong dynamic interconnection of the open-loop subsystems, thereby degrading the closed-loop system stability. Two examples are presented to demonstrate the applications of the proposed small-gain closed-loop model. Finally, the utility and accuracy of the closed-loop modal repulsion are evaluated from the viewpoint of the stability of HVDC/AC power systems.