The structural dynamics of wind turbines have not been sufficiently represented for the evaluation of grid-forming operation and model-based controller design. In contrast to the conventional operation of wind turbines with power-optimized partial and power-limiting full load region, the power output of grid-forming turbines with the ability to deliver ancillary services to the grid is determined by grid events like phase jumps and frequency changes. Therefore, an analytically tractable structural dynamic wind turbine model [1] in combination with a converter model in grid-forming mode presented in [2] is proposed here. In particular, the tower degrees of freedom in fore-aft and side-to-side direction as well as the torsion angle of the drive train are taken into account.

The aim of investigation is to determine the resulting mechanical loads on wind turbines that occur due to dynamic phase jumps and frequency changes in the grid induced by the grid-forming framework. As a result, it is therefore possible to determine how much control power can be made available without overloading related to fatigue and extreme loads on the mechanical structure to an unacceptable amount. For assessment of the occurring loads, the damage equivalent load (DEL) is used, which has been successfully utilized in [1] for validation of an advanced model-based wind turbine controller. Further, the induced oscillations, through sub-harmonic power system oscillations on the tower side-to-side movement are investigated.

In order to better understand the aforementioned interactions between mechanical turbines and electrical grids, a generic wind turbine model is developed in this study. The model includes the two tower degrees of freedom mentioned above, two degrees of freedom for the drive train, and one degree of freedom for the blades. The grid-forming converter control scheme includes a fast feed-forward phase intervention with droop control for the inverter voltage angle generation. The coupled converter and wind turbine model is used in case studies to investigate behavior during angle jumps, power oscillations, and shows the implication for the turbine loads resulting from grid-forming operation.

[1] Pöschke, Florian; Gauterin, Eckhard; Kühn, Martin; Fortmann, Jens; Schulte, Horst: Load Mitigation and Power Tracking Capability for Wind Turbines using Linear Matrix Inequality‐based Control Design. In: Wind Energy Vol. 23, Issue 9. (2020), pp. 1792-1809, doi: 10.1002/we.2516

[2] Klaes, Norbert; Goldschmidt, Nico; Fortmann, Jens: Voltage Fed Control of Distributed Power Generation Inverters with Inherent Service to Grid Stability. In: Energies 2020 13(10). (2020), 2579; doi:10.3390/en13102579