Analysis of Subsynchronous Oscillations Caused by Interactions of DFIG-Based Farms and SVGs

The static var generators (SVGs) installed at the point of common coupling (PCC) of wind farms can significantly impact the sub-synchronous oscillation (SSO) performance of the power system with grid-connected wind farms. For example, in 2017 multiple SSO events were caused by interactions among a doubly fed induction generator (DFIG) based farm, constant reactive power mode (CRPM) based SVGs, and the weak grid in Xinjiang, China. This paper reveals the mechanisms of the SSO caused by the interaction among DFIG-based farms, SVGs with different control modes, and weak grids. It is discovered that CRPM-based SVGs with low current controller bandwidths (CCBWs) can suppress SSOs in the power system with grid-connected DFIG-based farms, but that constant voltage mode (CVM) based SVGs and constant power factor mode (CPFM) based SVGs can worsen the SSO performance of the system. Furthermore, it is revealed why the amplitude of the super-synchronous component is larger than the sub-synchronous component in these events by deriving their amplitude ratio equation. Finally, the theoretical analysis results are verified by the small-signal method and electromagnetic transient (EMT) simulation.