Two-stage PV grid-connected control strategy based on adaptive virtual inertia and damping control for DC-link capacitor dynamics self-synchronization

Conventional DC-link voltage-controlled voltage source converter (VQ-VSC) controls DC-link capacitor voltage and reactive power output by using phase locked loop (PLL) for synchronous grid connection of new energy sources such as PV. However, the deterioration of dynamic performance in PLL can lead to instability in VQ-VSC control within a weak grid. To improve the stability of VQ-VSC grid connection, the DC-link capacitor dynamic self-synchronous grid-connected control strategy without using PLL is proposed in this paper. The converter virtual rotational inertia variability and virtual inertia damping frequency characteristics are analyzed based on the synchronous generator rotor rotation characteristics. The proposed control strategy provides adaptive virtual inertia and damping coefficients for DC-link capacitor self-synchronous units to dynamically match the inertia and damping requirements during frequency synchronization, and enables VQ-VSC to operate in grid-forming mode without changing the grid synchronization unit. Compared with constant virtual inertia-damping control and adaptive virtual inertia-damping control based on change rate of frequency, the simulation results demonstrate the effectiveness of the proposed control strategy, which significantly improves the grid connection stability of PV systems in weak grids.