Linear Phase-Locked Loop Based Hybrid Grid Synchronization for Grid-Forming Inverters

The next generation grid-forming (GFM) inverter-based resources (IBRs) suffer from grid synchronization issues during their grid integration to a stiff grid or in the event of severe grid faults. Thus, this paper introduces an improved hybrid power and voltage synchronization scheme for a three-phase droop-controlled GFM inverter. During normal grid operation, Active Power Loop (APL) based synchronization is used. However, during grid connection (breaker operation), or transients such as voltage dip because of grid fault, linear phase-locked loop (LPLL) based voltage synchronization scheme is enabled to avoid high active and reactive power swings. To ensure smooth transition between the synchronization strategies, two frameworks are proposed. The first scheme is enabled by moni-toring only voltage magnitude and the second one rely on both voltage magnitude and phase-angle error during faults. The proposed schemes leverage the linearity of the PLL implementation and avoid the tradeoff-based bandwidth design of the classical synchronous referenced frame-based PLL (SRFPLL) depending upon grid conditions. The improved hybrid grid synchronization along with the two transition schemes is modelled, simulated and compared with the state-of-the-art hybrid synchronization scheme. It is observed that the GFM inverter achieves faster re-synchronization during both the inception and post grid fault event, facilitated by the proposed smooth transition-based hybrid grid synchronization.