Torsional mode amplification in grid-forming type-IV wind turbines

Grid-forming (GFM) control in Type-IV wind turbine generators (WTGs) enhances voltage and frequency support but fundamentally alters drivetrain dynamics compared with grid-following (GFL) operation. A frequency-domain modelling framework and phasor-domain analysis reveal that machine-side DC-link voltage control in GFM mode introduces an anti-phase torque component, which provides negative damping to the drivetrain and amplifies torsional oscillations. Comparative evaluations show that, while GFL operation naturally decouples drivetrain dynamics from grid disturbances and maintains sufficient positive damping, GFM control actions consistently weaken damping and excite sustained oscillations. To validate these findings, a 2 kW experimental platform is developed, which reproduces a 2.5 Hz torsional mode and demonstrates strong agreement with theoretical predictions. The results confirm the root mechanism of torsional amplification in GFM-WTGs and establish the practical relevance of the theoretical analysis. These insights provide a foundation for developing tailored mitigation strategies to ensure reliable operation.