Grid voltage measurement-based phase-locked loop (PLL), is one of the most widely used synchronisation techniques for grid-interfaced inverters. However, this PLL easily propagates the harmonics from the ac grid voltage into the control loops of inverters, due to the direct voltage measurement, potentially leading to stability issues, particularly at weak grid conditions. The self-synchronising inverter can synchronise with ac grids without ac grid voltage measurement, avoiding harmonic propagation and increasing stable margin. However, due to no grid voltage measurement, the ac grid angle tracking accuracy of the self-synchronisation becomes a concern. Thus, this paper proposes an improved self-synchronising strategy, taking into account the impacts of ac filter and unbalanced three-phase ac grid voltage. The simulation results carried out in Matlab/Plecs and experimental results based on a 4kW self-synchronising inverter both clearly validate the correctness and effectiveness of the proposed solutions.
