Sliding mode observer of submodular capacitor voltages in Modular Multilevel Converter

Modular Multilevel Converter (MMC) has attracted the attention of researchers particularly those working in the area of transformer-less medium to high power conversion. This topology has adopted several interesting operational characteristics such as modularity, voltage scalability, multilevel waveform capability, safe failure management, redundancy and high reliability. MMC consists of cascaded connection of multiple, identical and individually controlled bidirectional half-bridge sub modules with floating DC capacitors. A significant aspect of MMC operation is maintaining the total sum of capacitor voltages of the sub modules per arm equal to the input DC bus voltage and ensuring equal voltage sharing among the submodules. In this paper, sliding mode observer is introduced to complement a Phase shifted Pulse Width Modulation based capacitor voltage balancing controller for a three-level MMC. Also, the mathematical model of MMC, voltage balancing control algorithm, and sliding mode submodular capacitor voltage observer are included in this paper. Finally, the proposed observer design is verified through computer simulation.