Subsynchronous oscillation (SSO) is a critical issue for the direct-drive permanent magnet synchronous generator (D-PMSG) based wind farm integrated to a weak onshore AC grid. To analyze the mechanism of the SSO phenomenon of D-PMSG based wind farm, widely used impedance-based stability analysis method is utilized in this paper. First, the impedance model based on the harmonic linearization theory of grid-connected D-PMSG is proposed, and the mechanism of sub/supsynchronous currents coupling is analyzed quantitatively for the first time. Then, based on the impedance model and relative stability criterion, the influence of wind farm operating parameters and grid impedance on stability is discussed. Simulations are carried out to verify the correctness of theoretical analysis.

This paper proposes a simplified model of PWM converter, which is proved to be effective in saving simulation time. Take doubly-fed wind power system as an example, the PWM converter is simplified in two aspects: a) the AC side of converter precisely and flows the output voltage reference; b) ignore the power loss of converter, which means the AC side power equals to DC side. Based on the two assumptions, the voltage of AC side could be replaced by a voltage-controlled voltage source, and the DC side could be replaced a controlled current source. By this way, the complex IGBTs converter is simplified to an equivalent circuit. Simulation with specific model and simplified model then performed in PSCAD/EMTDC. By comparing their waveforms and simulation speed, the correctness and benefits of the proposed model are verified.