Optimization of Cell Voltage and Circulating Current With Zero-Mean Current Command Injection in Modular Multilevel Converters.

One of the modular multilevel converter (MMC) applications with scalability is renewable energy generation. Nowadays, renewable energy is especially the source of momentum, which stimulates the energy transition and reduction of greenhouse gas emissions. However, its existing control schemes may not precisely reveal the inherent mannerism of MMCs. The aim of this article is to optimize the cell voltage, arm current, and circulating current of converter arms in MMCs based on the Lagrange multiplier of an energy cost function and arm current tracking control law. Meanwhile, determination of the capacitors and switch ratings in each cell still has a big challenge, and it can result in cell-voltage fluctuations under unbalanced grid-connection conditions. This article presents unique approaches to derive cell-voltage expression in terms of arm currents, which reduces cell-voltage fluctuations by injecting zero-mean current, and the duty-ratio control laws. Moreover, tradeoff between cell voltage and circulating current can be also conducted. Experimental and simulated results from a 3-kW single-phase MMC have verified the functionality of the proposed control method, analysis, and comprehensive discussion. [ABSTRACT FROM AUTHOR]