Research on fast open-loop phase locking scheme for three-phase unbalanced grid

Fast and accurate grid synchronization is essential for grid-connected converter control. The existing phase locking schemes are divided into two categories: closed-loop phase locking (CLPL) and open-loop phase locking (OLPL). OLPL methods show good steady-state performance and strong robustness to the variations and imbalance of the grid, however, the dynamic response is not fast enough due to their PID-based controller. Usually, more than one grid cycle is required for CLPL methods to lock completely the new phase. On the contrary, OLPL methods are characterized by fast grid synchronization, which has attracted a lot of attention in recent years. This paper proposes a novel fast open-loop phase locking scheme based on the synchronous reference frame (SRF-OLPL) to synchronize the true phase angle of the grid, which possesses well adaptive disturbance elimination properties and remarkable performance of dynamic responses at the same time.