A small-signal impedance method for analyzing the SSR of series-compensated DFIG-based wind farms

Interaction between fixed series compensation and doubly-fed induction generator (DFIG)-based wind farms can cause a new type of subsynchronous resonance (SSR) problem, which is also named as subsynchronous control interaction (SSCI). To explain its basic mechanism as well as to evaluate the risk, this paper proposes a small-signal impedance method, based on which, a practical series-compensated power system including DFIGs can be simplified into a second-order series RLC circuit around the operating point concerned and then the frequency and stability of SSR can be intuitively represented by the equivalent parameters of the circuit. The derivation of the circuit parameters is given and the asymmetry phenomenon of equivalent impedance matrix in dq0 reference frame is observed. Both Eigenvalue analysis and time-domain simulation have verified the effectiveness of the proposed method and its advantage in accuracy over the existing impedance-based method. The impacts on SSR due to wind speed, number of online DFIGs and their control parameters are also investigated.