Mechanical Stresses on a Wind Turbine Shaft due to Voltage Source Converters

This paper proposes an analysis of the generator output currents and airgap pulsating torque components to understand the root causes of early failures of wind turbine drivetrains. It highlights the mechanical stresses caused by a machine-side converter when supplied by a permanent magnet synchronous generator (PMSG). Such a study enables comprehension of the vibrations occasionally experienced on the mechanical shaft, which may cause production downtime. The mechanical-induced stresses are evaluated in normal and in resonance modes. The turbine shaft is a nodal interconnection of five moments of inertia linked with stiffness constants and damping factors. The analytical expression of the harmonics generated by the two-level back-to-back voltage source converter (VSC) and three-level back-to-back neutral-point-clamped (NPC) are derived and discussed. Numerical simulations are performed to support the theoretical developments. The precise location of the mechanical harmonics is quantified and evaluated in the frequency domain. The possible excitation frequencies in the wind turbine shaft’s resonance modes are presented in a Campbell diagram.