The global energy transition aims to expand renewable energy while conventional power plants are gradually decommissioned. Voltage and frequency stability, previously ensured by synchronous generators, must in the future be provided by converter-based generation and storage. Grid-forming (GFM) control concepts, designed to replicate voltage source behaviour of synchronous machines, are widely considered as a key technology for fully converter-based power systems. While these concepts have been developed since the 1990s, accepted validation and testing procedures for GFM wind turbines are still largely missing. This is particularly relevant for turbines with doubly-fed induction generators, where the direct electrical connection between stator and grid creates electromechanical coupling effects similar to conventional generators. In this context, the question arises how well established simulation tools and test environments can replicate these interactions.

This work presents a co-simulation approach to investigate coupled electromechanical dynamics in a fully digital environment. The framework was developed as part of the PRAKTISCH research project, in which Fraunhofer IWES, together with the University of Applied Sciences Duesseldorf and Nordex Energy SE & Co. KG, is working on characterizing methods for GFM properties of wind turbines on hardware-in-the-loop (HIL) test benches. The co-simulation couples a detailed aeroelastic model, usually used for load certification, with a detailed Electromagnetic Transient (EMT) model of the same turbine. The aeroelastic side is implemented in MoWiT, developed by Fraunhofer IWES. The EMT side is realised in PSCAD, which also acts as the co-simulation driver. Turbine manufacturers can integrate their real controllers via standard DLL interfaces. To orchestrate both environments, a custom interface DLL and a Windows Service were developed at IWES.

To demonstrate applicability, exemplary results are presented based on a Nordex wind turbine. The aeroelastic model has been derived from and verified against a DNV Bladed model provided by the manufacturer. The EMT model contains the turbine’s power electronics including transformer and grid. Originally, the electrical part is connected to a simplified aerodynamic representation of the rotor and the drivetrain; in this study, it is replaced by the detailed full turbine MoWiT model while the interface signal specification remains unchanged.

To investigate the influence of increased mechanical fidelity on electromagnetic transients, grid fault cases such as Fault Ride-Through events are simulated. The coupled system response is compared between the simplified and detailed approach in both time and frequency domain. Particular focus is laid on the effect of the reduced mechanical model sampling rate introduced by the larger step size of the detailed aeroelastic model and whether relevant dynamics are lost or distorted through the coupling.

This study provides a first overview of coupled electromechanical dynamics achievable with the presented approach. On this basis, the coupling can be further developed by extending the signal interface and by integrating simulation models of real test bench components for a complete digital representation of the test bench dynamics.