A typical model of doubly-fed wind farm connected to the grid through series compensation is provided, which can be used to study subsynchronous oscillation(SSO). LC resonance may occur between the stator inductance and the capacitance in the 500kV line. At some frequencies, the equivalent resistance of the rotor is negative, and the proportional factor of the inner current loop of the rotor-side converter(RSC) will amplify the negative resistance, resulting in unstable subsynchronous control interaction(SSCI) in the system.