Sorting-Selecting Modulation Technique for Double-Capacitor Sub-Modules based Modular Multi-Level Converters

Sub-Modules (SM) are basic building blocks in Modular Multi-level converters (MMC). Multiple Sub-Module topologies with two capacitors per SM are reported in the literature. These SMs are termed as Double-Capacitor Sub-Modules (DCSM) in this paper. The motivation behind the evolution of these DCSMs includes the introduction of DC-side short circuit withstanding capability and reduction in the effective number of devices per SM. Some DCSMs has the feature to have a parallel connection of its capacitors. This helps in reducing the SM capacitor voltage ripple at lower switching frequencies. In this paper, a sorting-selecting based modulation scheme for different DCSMs is presented. The traditional sorting-selecting modulation technique either inserts or bypasses SMs depending on the arm current direction. However, the sorting-selecting algorithm used for DCSMs needs to decide the operating mode of the inserted SM as well. The algorithm to select the SMs is improvised for DCSMs with capacitor paralleling feature. This proposed algorithm helps in the decision making of the series/parallel capacitor connection in the inserted DCSM. A detailed analysis of the proposed sorting-selecting algorithm is performed along with the simulation studies to validate the scheme.