Impact of Unbalance on Electrical and Torsional Resonances in Power Electronic Interfaced Wind Energy Systems

Type 3 wind generators in series compensated networks could lead to subsynchronous resonances (SSR). Previous research focuses on balanced operation only. In this paper, impact of unbalance on Type 3 wind energy systems is investigated in two aspects: 1) impact on electric resonances and 2) impact on torsional resonances. In the first aspect, impedance models of the system in DQ domain and phase domain are developed. Particularly, negative sequence impedances are examined. Nyquist stability criterion is applied to detect possible resonances. In the second aspect, transfer functions of the electromagnetic torque versus the rotating speed due to negative sequence components are developed. The impact of negative sequence components on electromechanical damping is then analyzed. The analysis results are verified by time-domain simulation results.