Study on the impact characteristics of inrush currents on flexible HVDC converter and transient impulse suppression methods

Abstract

[Objective] In order to address the issue of shutdowns caused by converter protection actions due to inrush currents and transient impulses in high-voltage flexible direct current(HVDC) transmission systems, [Methods] first, the generation mechanism of inrush currents was explained. Then, the impact characteristics of inrush currents on the operation of modular multilevel converter(MMC) under different grid voltages and current collection control method were investigated. Based on this, a transient impulse suppression method for MMC under inrush currents was proposed, which involved switching between the grid-side and valve-side control modes. Finally, an electromagnetic transient model of the actual engineering system was built in MATLAB/Simulink for simulation verification. [Results] The simulation result demonstrated that different collection control method had a significant impact on the transient electrical characteristics of MMC under inrush currents. [Conclusion] Compared to the grid-side control method, the proposed valve-side collection control method significantly reduces the disturbance impact of inrush currents on the converter. The peak current in the converter bridge arm and the voltage fluctuation of the submodule capacitor decrease by 40% and 67%, respectively, which can ensure the safe and stable operation of the flexible HVDC converter.

Keywords

flexible HVDC transmission / inrush current / control mode switching / transient impulse suppression / influence factor