Adjustable Carrier Phase Shift Operation of Switching Cycle Control for Modular Multilevel Converters

Abstract

Power-density improvement in Modulator Multilevel Converters (MMCs) has been a major recent research focus, with submodule (SM) capacitor size/capacitance being an important consideration in any MMC design. Almost all earlier SM capacitance reduction techniques are derived from the average/line-cycle model of MMC, which ignores the critical switching behavior of the converter, leaving scope for improvement. The recently proposed switching cycle control (SCC), on the contrary, analyses MMC behavior in one switching cycle and aims to balance SM capacitors within a single switching cycle. The ability to balance capacitor voltages in one switching cycle paves the way for MMC to be used in DC-AC and DC-DC applications. SCC provides a drastic reduction in capacitance and significantly increases power density, but still faces some challenges. SCC requires MMCs to traverse between different operating modes to avoid circulating current singularity. The carrier phase between modules of the same arm varies under these different operating conditions. This transition between different modes can easily interrupt the load current. This paper proposes an adaptive carrier phase control scheme to ensure a seamless control under different operating conditions. The analysis is supported by simulations and 6kV experimental results.