Analytical averaged loss model of half-bridge Modular Multilevel Converters

Abstract

Modular Multilevel Converter (MMC) is one of the most promising topology in medium- and high voltage high power application such as high-voltage dc (HVDC) transmission systems and medium voltage drives (MVD). In high power application, the semiconductor loss modelling is essential for system efficiency evaluation, thermal system design. Power loss information is also indispensable in device thermal stress and system reliability assessment. This paper proposed an averaged loss modelling method for MMC. Similar to the classical averaged model of a converter, the loss model computes the loss on a sufficiently small time interval. It averages the semiconductor conduction loss and switching loss over a switching period to get a closed-form loss expression. Compared to the equivalent current based loss model designed for HVDC in literature, the proposed method is more flexible for different submodule and different modulation method. The analytical steady-state loss model can be used to analyze MMC system loss and device thermal stress under various operation conditions. The analytical model is compared with a lookup table method in literature and verified by a low voltage MMC experiment setup. The predicted loss matches well with the simulation and experiment measurement.