Subsynchronous Oscillations Caused by Open-Loop Modal Coupling Between VSC-Based HVDC Line and Power System

A closed-loop interconnected model of a power system with a voltage source converter (VSC)-based HVDC line (VSC-HVDC line) is established, wherein, a selected control system of the VSC-HVDC line and rest of the power system are modeled as two separate open-loop subsystems. Based on the established model, a special condition of open-loop modal coupling is studied, when a complex pole of the open-loop selected control subsystem of the VSC-HVDC line is close to a subsynchronous oscillation (SSO) mode of open-loop power subsystem on the complex plane. Theoretical analysis indicates that when the open-loop modal coupling occurs, subsynchronous interactions (SSIs) between the VSC-HVDC line and the rest of power system may become strong. It is very likely that the open-loop modal coupling may degrade the damping of power system SSOs. Hence, the mechanism that the VSC-HVDC line may cause the SSOs in the power system is revealed from the perspective of open-loop modal coupling. In this paper, study cases of example power systems with the VSC-HVDC line are presented to demonstrate and evaluate the analysis and conclusions made.