Understanding Subsynchronous Oscillations in DFIG-Based Wind Farms Without Series Compensation

Subsynchronous oscillation (SSO) phenomena caused by interactions between controllers of Doubly-fed induction generators (DFIGs) and series capacitor compensators has been well recognized. However, SSO can also happen on a DFIG-based wind farm without series compensation as reported in a recent real event. This paper firstly analyzes the recorded voltage waveform during this SSO event. Then an equivalent model of the system including the wind farm, static synchronous compensator (STATCOM) and the power grid was established. The paper derives the subsynchronous frequency response of the DFIG converter together with the STATCOM in detail and gives the criterion for SSO to occur, which is verified by both time domain simulation and spectrum analysis. The methodology adopted in this paper can be generalized to analyze the causes of a type of SSO that involves DFIGs and STATCOM but not series compensation. The paper also provides practical suggestions on adjustments of control parameters with the DFIG and STATCOM to prevent this type of SSO.