Transient Model of DFIG-WT-dominated System for Nonlinear Oscillation Characteristic Analysis

Since doubly fed induction generator-based wind turbines (DFIG-WTs) has more complex structure than synchronous generator (SG), the DFIG-WTs has strong nonlinearity inherently. It leads the oscillation responses of DFIG-WT-dominated system to be significantly changed, especially after large disturbance. The oscillation characteristics could not be reflected by linear results any more. More seriously, the stability conclusion would also be contrary. However, there does not exist general mathematic method to solve the nonlinear problem in DFIG-based WT, thus an approximate model that balances accuracy and universality is needed. This paper proposes a novel model of DFIG-WT- dominated system, in which the nonlinear effects are appropriately reflected by the higher-order terms of Taylor series expansion. Firstly, the original nonlinear model and its Taylor series expansion forms are developed. Then, the nonlinear oscillation characteristics under different fault degrees are analyzed and the approximate degrees of model with different higher-order terms are evaluated. Finally, model application in multi-machine system is discussed. The proposed model could provide a model basis for modal series analysis, which is a common method for nonlinear oscillation characteristic analysis.