A Voltage Sag Ride-Through Control for Grid-Forming Converter in Urban Distribution Networks

Urban distribution networks contain a huge number of sensitive loads. Voltage sags severely disrupt their normal operation, leading to economic losses. To address this challenge, this paper proposes a fuzzy control-based pre-synchronization strategy to achieve seamless on-/off-grid transition for grid-forming inverters. First, the principle of the virtual synchronous generator (VSG) is derived by analogy with a synchronous generator, and its mathematical model is developed. Then, the mechanisms of active power–frequency(P-f) and reactive power–voltage(Q-V) control are discussed in detail, followed by the design of a voltage compensation controller and a dual-loop voltage–current controller. Next, a seamless transition strategy for switching between grid-connected and islanded modes is proposed. In particular, a pre-synchronization control strategy with fuzzy logic is employed before the transition from islanded to grid-connected mode, ensuring that the VSG output voltage is synchronized with the grid voltage. Finally, the effectiveness of the proposed strategy is validated through MATLAB/Simulink simulations.