Wind Turbine Models for Power System Stability Studies

Licentiatavhandling, 2006

The purpose of this thesis is to develop dynamic models of wind turbines for power system stability studies. More specifically, the wind turbine models are mainly intended for voltage and frequency stability studies. In developing the wind turbine models, each part of the wind turbines are examined to define relevant behaviors that significantly influence the power system response. Correspondingly, mathematical models of these parts are then presented with various possible levels of detail. Simplified models for each part of the wind turbines are evaluated against more detailed models to provide a clear understanding on how model simplifications may influence result validity and simulation efficiency. In order to obtain confident results, the wind turbine models are then validated against field measurement data. Two different cases of validation are then presented. Based on the measurement data of two different wind turbines, most typical behaviors of the wind turbines are discussed. Finally, both conformity and nonsimilarity between simulation results of the wind turbine models and the field measurement data are elaborated. Two different methods of predicting stator transient current of a wind turbine generator following a fault are presented. The first method implements a modified fifth-order model of an induction generator which is developed to be compatible with the fundamental frequency network model. The second method utilizes an analytical method in combination with the third-order model of an induction generator. A solution for the implementation of wind turbine models that require a simulation time step smaller than the standard simulation time step is also proposed in the thesis. In order to comprehend behaviors of wind turbines subject to different power system stability phenomena, a number of simulations are performed in the power system simulation tool PSS/E with the standard simulation time step of 10 ms. Each stability phenomenon are simulated using different wind turbine models. The simulation results are evaluated to determine the most appropriate wind turbinemodel for each particular power system stability study. It is concluded that a fixed-speed wind turbine model consisting of the third-order model of an induction generator and the two-mass model of a drive train is a compromised solution to provide a single wind turbine model for different types of power system stability studies. The thesis also presents aggregated models of a wind farm with fixed-speed wind turbines. The result of the simulations are validated against field measurement data.