The increasing deployment of grid-forming (GFM) control in Type-IV wind turbine generators (WTGs) enhances frequency and voltage support but fundamentally alters electromechanical behaviour compared with conventional grid-following (GFL) operation. A key challenge is the excitation of lightly damped torsional modes (0.1–10 Hz), where rapid torque variations imposed by DC-link voltage regulation can oppose natural damping and amplify drivetrain oscillations.

This paper establishes the root mechanism of torsional amplification in GFM-WTGs through a simplified frequency-domain modelling framework and complementary phasor-domain insights. The analysis shows that, unlike in GFL operation where machine-side action reinforces natural damping, the GFM control architecture introduces an anti-phase torque component that can lead to weak or negative damping.

Theoretical findings are validated on a 2 kW hardware testbed, which reproduces a 2.5 Hz torsional mode under both grid- and wind-side disturbances. Experimental results confirm that torsional oscillations, self-damped in GFL operation, are strongly amplified in GFM mode, underscoring the practical significance of the identified mechanism.

Key contributions of this work are:

• Simplified modelling framework: A frequency- and phasor-domain approach that shows how GFM control introduces negative damping compared with the naturally damped GFL case.
• Experimental validation: Real-time hardware tests validate the proposed theoretical findings, adding practical credibility beyond simulation-only studies.

By clarifying the fundamental role of converter control architecture in shaping drivetrain dynamics, this work highlights the limitations of directly transferring GFL-based damping strategies to GFM systems. These insights provide a foundation for developing tailored mitigation approaches that ensure reliable and mechanically robust operation of future GFM-enabled wind energy systems.