Improved Active Current Control Scheme of Wind Energy Conversion Systems With PLL Synchronization During Grid Faults

Wind energy conversion systems (WECSs) depend upon phase-locked loops (PLLs) for synchronization with the power grid. As the voltage dip during grid faults lowers the transfer capacity between WECSs and the grid, PLLs may suffer loss of synchronization (LOS) if the active current reference of WECSs exceeds the transfer capacity. To analyze the PLL synchronization stability, a U - I_d plane analysis method is presented. The transfer capacity constraint and the WECS active current control scheme are mapped to the U ( I_d ) curve and control curve in a U - I_d plane, respectively, describing the relationship between the terminal voltage and the WECS active current. Hence, the influence of grid parameters and the WECS control scheme can be analyzed separately. Accordingly, to prevent LOS, an improved active current control scheme of WECSs is proposed based on the short circuit ratio. The scheme corresponds to a control curve in the U - I_d plane that ensures the existence of stable equilibrium points and is quite easy to apply in practice. Finally, both the simulations using DIgSILENT PowerFactory and the experiments based on RT-lab and RTDS validate the performance of the proposed control scheme.