Open-loop modal analysis to identify the SSO instability risk caused by grid-connected wind farms

Abstract

We proposed and applied the method of open-loop modal analysis previously for examining power system sub-synchronous oscillations (SSOs). However, the method was only applicable to grid-connected wind turbine generators, because we only theoretically proved the applicability of open-loop modal analysis for a single-input single-output (SISO) closed-loop interconnected model. In this study, we present the theoretical proof that in a multi-input multi-output closed-loop (MIMO) interconnected model, open-loop modal analysis is still applicable for examining power system SSOs by using the open-loop modal information. Hence, we propose to apply the open-loop modal analysis to detect the risk of growing SSOs caused by grid-connected wind farms, rather than wind turbine generators, in the power system. This is because in practice, dynamic interactions introduced by a grid-connected wind farm, in fact, constitute MIMO closed-loop interconnected model. Thus, the study has extended and enhanced the applicability of method of open-loop modal analysis. In the paper, the applicability is demonstrated and evaluated by example power systems with grid-connected wind farms. The SSO instability dangers brought about by the wind farms to the example power system were detected by the proposed method of open-loop modal analysis.