Staircase Matrix Modulation for the Switched-Capacitor Modular Multilevel Converter With Sensor-Less Capacitor Voltage Balancing

Abstract

Staircase modulation is a switching technique ubiquitous in multilevel inverters utilizing large number of output voltage levels. With tens of levels, the output of a multilevel inverter employing staircase modulation approaches a sinusoid without requiring switching harmonics filters. Out of various multilevel inverter topologies, the modular multilevel converter (MMC) became prominent due to its modularity, scalability, and efficiency. However, balancing the submodule (SM) capacitor voltages poses a significant challenge in MMC operation. In this work, a staircase matrix modulation (SMM) strategy, which achieves sensor-less capacitor voltage balancing, is proposed for the switched – capacitor MMC (SCMMC), an MMC topology with a very small arm inductor. The proposed SMM utilizes a full rank, symmetric switching matrix, where specific switching patterns are assigned for each voltage level. The structure of the proposed matrix, its unique features, and the process of populating its entries for any converter voltage level are described. Theoretical analysis on the operation of the proposed SMM, simulations for an 11-level SCMMC, and experimental results on a single-phase, 2 kW, 425 V, 4-level SCMMC prototype are presented to illustrate the voltage balancing capability of the proposed SMM. The resulting switching frequency of the SCMMC under SMM is also analyzed.

Keywords

• Capacitor voltage balancing
• low-frequency modulation
• staircase PWM
• switched-capacitor modular multilevel converter