Impact of LVRT Control on Transient Synchronizing Stability of PLL-Based Wind Turbine Converter Connected to High Impedance AC Grid

With large scale of wind turbines connected to high impedance AC grid, transient synchronizing stability of PLL (phase locked loop) based WTC (wind turbine converter) during LVRT (low voltage ride through) is increasing concerned. However, the existing research is usually carried out at the assumption of constant current injection. The influence of dynamic current injection adjusted by LVRT control is neglected. Yet, in high impedance AC grid, the dynamic current injection significantly affects the terminal voltage tracked by PLL. Then LVRT control has much impact on the transient synchronizing stability, which is concerned especially in this paper. First, an novel equivalent motion model is developed, in which PLL is modelled in the form of rotor motion and LVRT control adjusts its equivalent driving force similar as governor in synchronous generator. Then based on the developed model, accelerating-decelerating area method is employed to reveal the transient stability mechanism from energy's transformation and dissipation. Transient stability region method is employed to present the visualized stability region. Furtherly, the influence of K-factor, grid impedance and grid voltage sag are studied from the influence of them on accelerating-decelerating area and stability region. Finally, simulated results are presented to verify the analytical results.