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

Abstract

[EN] 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-infinity -based controllers. The paper explains how to synthesise three different H-infinity voltage controllers: the first is a single-loop H-infinity controller, the second is a cascaded H(infinity )controller and the third is a proportional-resonant controller that is optimised using H-infinity synthesis. The three H-infinity -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(infinity )controller shows the best combined characteristics of all tried controllers, at the expense of losing the separation between voltage and current control loops.