Torsional Vibration Analysis of Virtual-Synchronous-Controlled DFIG-Based Wind Turbines

Abstract

The deployment of inertia control in wind turbines (WTs) is required by grid operators to support system frequency stability, while its addition also strengthens the dynamic coupling of WT's mechanical system with the electrical gird, potentially exacerbating torsional vibrations in drivetrains. Such side effect has been reported for the supplementary inertia control in the current grid-following (GFL) control frame, while for its alternative, i.e., the so-called virtual inertia control, this effect has rarely been concerned. To fill this gap, this article conducts a comprehensive study on the torsional stability for virtual-synchronous-controlled (VSynC) DFIG-based WTs. A reduced-order small-signal model considering different wind speed operating regions is firstly proposed. Then, how the paths of different controls, such as virtual inertia control, pitching control, etc., affecting the torsional mechanical/electrical torque are sorted out, and correlated loop shaping on torsional damping are investigated in detail. Through a comparative analysis with GFL scheme, we confirm that VSynC-DFIG suffers more sever torsional stability challenges due to its curtailed electrical damping contribution. Furthermore, we identify the operating conditions that are prone to instability and reveal the mechanisms by which critical control factors impact stability. Finally, hardware-in-the-loop simulations are conducted to validate the correctness of the analyses.