Overview of emerging subsynchronous oscillations in practical wind power systems

Abstract

Emerging subsynchronous oscillation (SSO) issues in grid-connected wind farms have gained serious attention after several incidents occurred in the US and China. The temporal and spatial variation of wind speed, the diversified converter control structures, and the continuously varying number of in-service wind turbine generators (WTGs) greatly affect the equivalent impedance reflected by the wind farms. Such frequency-dependent equivalent impedance, as indicated by both theoretical analyses and practical experiences, has a huge impact on the mechanism and characteristics of SSO. In this regard, WTGs contribute to a more complicated type of SSO than those in conventional turbo-generators (CTGs). For instance, SSO in practical wind power systems can be triggered without the participation of series compensation in the transmission network. The emerging SSO phenomena observed in practical WTGs are relatively new, and cannot be properly interpreted by existing literature. This work will first review the historical development of SSO definitions and classification by IEEE, followed by discussions on recent incidents in practical wind farms due to the emerging SSO. Subsequently, new terminologies and classification of SSO are proposed with the aim to better understand SSO in large-scale wind integrated power systems. This paper offers a practical insight and a future perspective for the reader to properly deal with SSO associated with wind power systems.