Asymmetrical Low-Voltage Ride-Through Control Strategy Based-on PBC With Virtual Voltage Compensation for Voltage-Controlled VSG

Grid-forming (GFM) inverters have the advantages of voltage and frequency support in weak grid. However, unlike grid-following inverters, GFM inverters present a more challenging picture during asymmetrical voltage sag. The voltage-controlled virtual synchronous generator (VVSG) is the typical control method of GFM inverter. However, the problem of large inrush, slow response speed, and negative-sequence current control need to be carefully considered. In the existing low-voltage ride-through (LVRT) control strategies, these issues cannot be decently coped with while maintaining the ability to support the grid. In this situation, system instability and deterioration of power quality may occur. To overcome these limitations, an asymmetrical LVRT control strategy is proposed. First, the positive and negative-sequence models are established and the asymmetrical instantaneous currents are calculated. Based on this, the influences of the VVSG control method on the instantaneous negative-sequence currents are analyzed, which reveals the role of negative-sequence virtual voltage amplitude, grid phase angle, and response speed. Then, the double closed-loop control based on VVSG control method is further improved. The negative-sequence virtual voltage is compensated in the outer-loop to achieve control of negative-sequence current and inrush current. A novel passivity-based control (PBC) strategy is extended into innerloop which has fast transient response and strong robustness. Furthermore, through the transient power angle stability analysis, the improvement of the transient margin is proved. Finally, simulation and experimental results verify the effectiveness of the proposed method.