The wind turbine converter phase lock loop(PLL)loss of synchronization may happen when ac fault occurs in modular multilevel converter-based wind farms.The state-of-art studies are focused on mechanisms and improvements of synchronization stability at the fault stage and lacks a synchronization stability study at the fault recovery stage after the fault is clear.This study focuses on PLL synchronization stability at the fault recovery stage.The PLL frequency limita-tion because of loss of synchronization at the fault stage and oscillation overshoot at the recovery stage are taken into considerations,and the system transient model considering PLL frequency limitation is established.Furthermore,the sta-bility region at the fault recovery stage is introduced to investigate the stability influences including PLL frequency limits,the output current of wind turbine converter at the fault recovery stage,system voltage(voltage recovery speed),and the PI parameters of the PLL frequency controller.Based on the stability region,recovery stage stability enhancement meth-ods are proposed,including parameter optimization and software reset of PLL.Simulations verify the rationality of theoretical analysis.
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