Step-up MMC with staircase modulation: Analysis, control, and switching strategy

Abstract

Modular multilevel converters (MMC) provide a low-harmonic output voltage without needing several isolated power sources. This multilevel converter can use either half-bridge or full-bridge submodules with PWM or staircase modulation techniques. This paper analyzes the full-bridge MMC, which is called step-up MMC because of its boosting capability, using the staircase modulation. Different operational modes of the step-up MMC are studied, and a generalized control and switching algorithm is proposed. This algorithm ensures the capacitor charge balance while providing the required high-quality output voltage. Compared to a half-bridge MMC with a similar investment in capacitor and switches, the step-up MMC shows a superior performance as it has 1) low total harmonic distortion (THD) of the output waveform, 2) high output voltage with a low input DC voltage, and 3) the possibility to provide different achievable output voltages using submodules with negative voltage at their terminals. The performance of the proposed step-up MMC with staircase modulation and presented theories are validated by simulation case studies.