After several instances of sub-synchronous oscillation (SSO) events occurred at wind farms (WFs) with Type-4 wind turbine generators (Type-4 WTGs)

the phenomenon has attracted wide attention. The emerging SSO events observed in Type-4 WTG-based WFs are more complex than those previously observed in thermal power plants or Type-3 WTG-based WFs. The SSO (for example

occurred in Hami Type-4 WTG-based WF

China)

could be triggered without the involvement of series compensation in the transmission system. Firstly

this paper discusses and summarizes the previous definitions and classifications

and an improved SSO classification is proposed by supplementing the conventional IEEE classification in the view of Type-4 WTG-related SSO. Secondly

an overview of identification methods for SSO sources and the applicable scope of these methods is evaluated. Then

mitigation and protection measures available for addressing the Type-4 WTG-related SSO are comprehensively reviewed. In addition

lessons learned from real-world SSO events

stakeholders' perspectives

as well as challenges and potential further study of the Type-4 WTG-related SSO are discussed. This review aims to enhance the understanding of the SSO associated with Type-4 WTGs and contribute to the development of effective solutions to mitigate the phenomenon in power systems.