Suppression of Sub-synchronous Oscillation of Offshore Wind Farm Integrated by AC-DC Parallel System

With the development of long-distance offshore wind farms, the AC-DC parallel system has become one of the important transmission ways for offshore wind farm integration. However, the Doubly fed Induction Generator (DFIG) and the flexible DC rectifier side controller sub-synchronous oscillation (SSO) may occur between AC series compensation systems. In this paper, a hybrid impedance model of a doubly-fed wind farm via AC-DC parallel grid-connected system is established. Based on the stability criterion of the impedance characteristics of the cascade system, the principle and parameter characteristics of sub-synchronous oscillation are analyzed, and virtual resistance control is proposed to suppress sub-synchronous oscillation. The simulation results show that under certain conditions, the doubly-fed wind turbine controller exhibits a negative resistance effect in the sub-synchronous frequency band, causing sub-synchronous oscillation.