The global transition to renewable energy has positioned wind power as a cornerstone in the decarbonisation of modern power systems. Among various technologies, Type-IV wind turbine generators (WTGs), based on permanent magnet synchronous generators and interfaced via full-scale back-to-back converters, are widely adopted for their efficiency, controllability, and suitability in weak-grid environments. However, the increasing share of these inverter-based resources presents stability challenges, as conventional grid-following (GFL) controls cannot independently regulate grid voltage and frequency.

To address this, grid-forming (GFM) control has emerged as a promising alternative, enabling autonomous frequency and voltage regulation by emulating synchronous generator behaviour. While GFM is well established in battery energy storage systems, its application to WTGs introduces unique complexities due to the tight coupling of mechanical, electrical, and aerodynamic subsystems. In particular, GFM-WTG implementations, where the machine-side converter regulates the DC-link voltage, require rapid torque modulation, fundamentally altering the interaction between the control system and drivetrain.

This heightened coupling amplifies lightly damped torsional modes in the low-frequency range (0.1–10 Hz), accelerating mechanical fatigue and stressing drivetrain components. This paper investigates and compares the electromechanical behaviour of GFM and GFL WTGs using a real-time power hardware-in-the-loop (PHIL) platform. Experimental results, under identical mechanical and grid conditions, show that GFM control exacerbates torsional oscillations due to a stiffer electromagnetic torque response. Furthermore, conventional machine-side damping strategies, effective in GFL operation, are shown to be ineffective in GFM WTGs due to the control demands of DC-link voltage regulation.

By identifying and demonstrating the root cause of this behaviour, the study underscores the critical role of converter control architecture in shaping drivetrain resonance. These findings form a foundation for developing more effective damping strategies for future GFM-enabled wind energy systems.

Key contributions of this work are:

• Experimental validation, via a real-time PHIL platform, of the increased severity of torsional oscillations in GFM WTGs compared to GFL WTGs under identical operating conditions.
• Practical identification of the root cause of torsional resonance amplification in GFM WTGs, highlighting the limitations of conventional machine-side damping approaches.

These insights provide a vital basis for future research addressing the electromechanical challenges of converting existing WTGs to operate reliably in GFM mode.