A Method of Open-Loop Modal Analysis to Examine the SSOs in a Multi-Machine Power System With Multiple Variable-Speed Wind Generators

We propose a method of open-loop modal analysis to examine the sub-synchronous oscillations (SSOs) in a multi-machine power system with multiple variable-speed wind generators (VSWGs). The proposed method of open-loop modal analysis applies the modal analysis to open-loop state-space models of the VSWGs and the power system. The method identifies any pair of open-loop SSO modes separately from the open-loop VSWG systems and the open-loop power system, which are close to each other on the complex plane. The identified pair of open-loop SSO modes is referred to as the condition of open-loop modal resonance (OLMR). Our analysis indicates that under the condition of the OLMR, it is likely that the SSO stability of the multimachine power system is degraded by the grid connection of the VSWGs. Therefore, a modal resonance index (MRI) is proposed to detect the SSO instability risk introduced by the VSWGs. In addition, the problematic SGs and VSWGs in the multi-machine power system can be identified; this identification can help develop measures to effectively eliminate the detected SSO instability risk introduced by the VSWGs.