An Improved Pre-charging Strategy for Asymmetrical Mixed-Cell Submodule (SM) Based Modular Multilevel Converter

The start-up procedure of the modular multilevel converter (MMC) exhibits a heightened level of intricacy when compared to that of two-level or multilevel converters. This increase in complexity stems from the substantial presence of numerous self-isolated SM capacitors within the MMC. As a result, the MMC poses a greater degree of unpredictability during its start-up phase. During the starting phase of the modular multilevel converter (MMC), it is imperative to have all capacitors within each submodule charged uniformly to a specific voltage level before transitioning into regular operation. Failure to achieve this equilibrium could lead to the occurrence of a considerable inrush current the moment the MMC is started up, posing a significant risk to the reliable functioning of both the insulated gate bipolar transistors (IGBTs) and the capacitors. In this research work, asymmetrical mixed-cell submodule (A-SM)-based modular multilevel converter topology is presented. The proposed topology requires a novel pre-charging technique to stabilize each asymmetric mixed-cell submodule at the rated voltage before getting into operation. Here in this research article, considering asymmetric mixed-cell SM circuits, a modified pre-charging strategy is proposed for the MMC, which can implement soft start-up from the DC side. Separate detailed algorithms are introduced to pre-charge the capacitors for binary GP ratio-based MMC and ternary GP ratio-based MMC. The feasibility of the proposed method was verified through MATLAB simulations for a scaled three-phase, 9 kV system with 2 SMs per arm.