Impedance modeling and analysis of type-III wind turbines

Abstract

Small-signal interaction between the power grid and converter-based generators is an issue that will become ever more present with the introduction of additional renewable generation year after year. A proven tool to predict, analyze and help mitigate these interactions is the use of impedance-based stability through the use of small-signal models obtained through the use of harmonic linearization. The derivation of these models is complex and considerable effort must be made to obtain clean models that represent all the dynamics of interest. This theory has been used in the past to model and analyze voltage source converters (VSC) in a wide variety of configurations, both on paper and in actual hardware.

These results are then validated and used to propose impedance-based mitigation strategies. The most successful, and promising, damping method emulates a virtual impedance on the power circuit by introducing an additional current control loop into the current controller. This current loop is filtered so that it doesn’t react to the fundamental or switching frequency. The additional damping loop is tuned based on the desired phase margin between the turbine and grid impedance.

This thesis presents the continuation of these modeling efforts by deriving the impedance models for type-III wind turbines and using the results to analyze and mitigate subsynchronous resonance between the turbine and a series-compensated grid. The modeling approach leverages the fact that the dc bus voltage will hardly react to a small-signal voltage perturbation on the ac terminals to separate the turbine impedance into two parallel terms. One of these terms is modeled as a VSC behind an inductor, but the other converter is represented as a VSC behind the induction machine. Modeling the effects of the induction machine on this second VSC by means of harmonic linearization is the main contribution of this thesis.