A Dc-Link Startup Pre-charging Control Strategy for Modular Multilevel Converter

Startup pre-charging is essential to the operation of modular multilevel converter (MMC). The startup procedure ofMMCincludes uncontrolled stage and controlled stage. The conventional startup pre-charging methods mainly focus on the controlled stage, while the performance of uncontrolled stage is deteriorated by the inrush currents. To achieve a soft-startup performance of MMC, this paper presents a dc-link startup pre-charging control strategywhich contains three stages. The first stage is pre-charging with current-limiting resistors based on the principle of half-bridge MMC. The second stage is bypassing the current-limiting resistors sequentially. The mathematical model of bypassing resistors sequentially is established, and the optimal bypassing times are achieved tominimize the inrush current during the uncontrolled startup stage. In the last controllable charging stage, constant current charging is adopted to reduce the inrush current effectively. Finally, the performance of the proposed method is verified by simulations.