Method of Open-Loop Modal Analysis for Fxamining the Subsynchronous Interactions Introduced by VSC Control in an MTDC/AC System

This paper proposes a method of open-loop modal analysis to examine the subsynchronous interactions (SSIs) introduced by the converter control in an ac power system integrated with a multiterminal dc network. A control loop of the voltage source converters (VSCs) is selected to derive a closed-loop interconnected model, wherein the selected control loop and the rest of the power system are modeled as two interconnected open-loop subsystems. The analysis in this paper indicates that under the condition of open-loop modal proximity, strong SSIs may be introduced by the selected VSC control loop. It is very likely that the strong SSIs may degrade the damping of power system sub synchronous oscillations (SSOs). It is proposed that the damping degradation be estimated by calculating the open-loop SSO residues. Hence, the modal analysis for the open-loop subsystems can identify the open-loop modal proximity and estimate the degree of stability degradation under the condition of open-loop modal proximity. In addition, the VSC control loops and the synchronous generators contributing to the stability degradation can be identified. The application of the proposed method of open-loop modal analysis is demonstrated through example power systems in this paper.