Analysis on Sub-Synchronous Oscillation Associated with Wind Farms Based on Direct-Driven Wind Generators

Recently, several literatures reported that sub-synchronous oscillation (SSO) events occurred in wind farms based on direct-driven wind generators (DDWGs). This paper focuses on whether a wind farm consists of only DDWGs would bring an SSO. First, a small-signal model of the DDWG-based wind farm is proposed. Eigen-analysis indicates that there are natural modes with sub-synchronous frequency; however, their damping ratios are significantly large under specific control parameters. Further, the relationships between the sub-synchronous modes and the number of grid-connected DDWGs, the connecting reactance and the circuit topologies of the wind farm are analyzed. Taguchi's orthogonal array testing is also introduced to analyze the impact of the wind farm's stochastic operating conditions on the modes. Results show that if the DDWGs are well-tuned, the dynamics of the wind farm are similar to one DDWG, and the sub-synchronous modes have little correlation with those factors, thus SSO can hardly arise when the model of wind farm only includes DDWGs. Electromagnetic transient simulations are also carried out to prove the accuracy and efficiency of the results.