Switching function based analysis of the modular multilevel converter for low/medium voltage applications

The advantages of the Modular Multilevel Converter (MMC) as a Voltage Source Converter (VSC) is well known. These includes scalability, increased reliability, lower Total Harmonic Distortion (THD) in output variables, fault tolerance and so forth. The MMC has found its most application in High Voltage DC (HVDC) systems again because of the stated advantages. In the analysis of such systems, because of the high number of Sub-Modules, several assumptions and approximations including the components of the arm current and form of each component are made. In reality however, the component of the arm current and their form could vary from one circuit configuration to another. In this paper, a switching function based analysis of the MMC is performed using a generalized model of the MMC. The model is developed in the QD reference frame and uses the sum and differences of upper and lower arm sub-module capacitor voltages. Simulation results have been included to show the form of the model variables.