Comparing AC Dynamic Transients Propagated Through VSC HVDC Connection With Master-Slave Control Versus DC Voltage Droop Control

This paper investigates the propagation of ac dynamic transients through a voltage-source converter-based high-voltage direct current (VSC HVdc) connection, comparing master-slave control and dc voltage droop control, and the impact of the propagation of ac dynamic transients on the small-signal stability of an ac/dc power system is also examined. In this paper, an interconnected model of the ac/dc power system is derived. Analysis using the derived model indicates that under master-slave control, the ac dynamic transients can propagate only unidirectionally through the VSC HVdc connection, and instability from a local ac system may not spread throughout the entire ac/dc power system through the VSC HVdc connection. However, under dc voltage droop control, the propagation becomes bidirectional between the VSCs that participate in the dc voltage droop control, and instability from the local ac system will spread throughout the ac/dc power system. In addition, even if the local ac subsystems are stable, the bidirectional propagation of ac dynamic transients through the VSC HVdc connection may still cause instability. Hence, implementation of dc voltage droop control increases the ac/dc power system's instability risk.