Efficient modeling of DFIG- and FSC-based wind turbines for frequency stability analysis

The contribution of wind turbines (WTs) to enhance the frequency stability of power systems is traditionally analyzed using commonly applied root mean square (RMS) models. RMS WT models require smaller simulation time steps compared to conventional active devices (i.e., synchronous generators and dynamic loads) due to the comparatively smaller time constants of the converter controllers. Such small time steps become relevant in simulations of large-scale power systems with a high level of WT penetration and lead to high computational time and effort. This paper presents simplified simulation models of a doubly-fed induction generator-based WT and a full-scale converter-based WT, which enable higher simulation time steps due to the negligence of very small time constants with no relevant effects in the time frame of interest of frequency stability analysis. The models are derived from detailed RMS WT models based on fundamental machine and converter equations. In order to verify the validity of the underlying simplifications, the simplified models are compared to the detailed RMS models with a focus on their general behavior in case of step responses and their frequency responses in the event of a frequency drop in a 220 kV test system. For this purpose, both the detailed RMS WT models as well as the simplified WT models are extended with a droop-based fast frequency response controller and implemented in a MATLAB-based RMS simulation tool. The results of the case studies show feasible and comparable general behavior of the WT models as well as plausible frequency responses.