Staircase Matrix Modulation for the Switched - Capacitor Modular Multilevel Converter with Sensor-less Voltage Balancing

Staircase modulation is a switching strategy widely used for multilevel inverters employing high number of output voltage levels. With tens of levels, a staircase modulated multilevel inverter produces a stepped, sinewave-like output voltage without requiring inductive or capacitive switching harmonics filters. Among the several multilevel inverter topologies, the modular multilevel converter (MMC) has proven its superiority due to its modular and easily scalable structure. However, voltage balancing of the submodule (SM) capacitors is a major challenge associated with MMC operation. In this paper, a staircase matrix modulation (SMM) technique, achieving sensor-less capacitor voltage balancing, is proposed for the switched capacitor MMC (SCMMC), which has no discrete arm inductor. The proposed SMM utilizes a symmetric switching matrix, where rotation between specific switching patterns of a certain voltage level keeps capacitor voltages balanced every switching cycle. The structure of the proposed symmetric matrix is described, and the proper choice of the pattern rotation frequency is quantified, both from the voltage balancing perspective. Time-domain simulations for an 11-level SCMMC under SMM are provided to illustrate the voltage balancing capability of the proposed SMM under steady state and transient conditions.