On the Impedance Modeling and Equivalence of AC/DC Side Stability Analysis of a Grid-tied Type-IV Wind Turbine System

Abstract

Impedance modeling of a Type-IV wind turbine is usually associated with model reductions, e.g. the grid-side converter (GSC) is modeled in detail, whereas the machine-side converter (MSC) is simplified as a constant power load (CPL). Meanwhile, the Nyquist-based stability analyses are normally conducted on the ac side, where the evaluation of the stability margin can be difficult due to the presence of multiple eigen-loci. Although some similar analyses regarding the high-voltage dc systems are performed on the dc side, a justification of the consistency between the ac and dc side analysis is lacking. Therefore, this paper aims to address these issues by first developing a detailed impedance model of the Type-IV wind turbine, and then providing a formal proof of the equivalence between the ac and dc side analysis. The detailed Type-IV wind turbine model is verified by the measured frequency responses from simulations, as well as its correctness in Nyquist-based stability analysis. The MSC modeling effects are further discussed, for which a thorough comparison of the CPL-based model and the detailed model with respect to the stability margin is conducted. As a result, the feasibility of the CPL-based model for stability analysis is clarified.