Operation of DR–HVdc-Connected Grid-Forming Wind Turbine Converters Using Robust Loop-Shaping Controllers

Abstract

Off-shore wind power plants can be connected to the on-shore grid using diode rectifier HVdc links. As diode rectifiers are passive converters, off-shore WPPs require grid-forming capability. This paper shows how to improve the WTG dynamic response and the voltage and current harmonic rejection by using H∞-based controllers. The paper explains how to synthesise three different H∞ voltage controllers: the first is a single-loop H∞ controller, the second is a cascaded H∞ controller and the third is a proportional–resonant controller that is optimised using H∞ synthesis. The three H∞-based controllers improve the performance and the robustness obtained with a benchmark case PR controller tuned using the root locus technique. All the controllers are designed in continuous time and implemented in discrete time, applying bilinear discretisation with a sampling rate of 0.25 ms. Detailed PSCAD simulations validate the improvement of the performance and robustness, as well as an improvement in the harmonic rejection. The single H∞ controller shows the best combined characteristics of all tried controllers, at the expense of losing the separation between voltage and current control loops.