Switching Pattern Optimisation Algorithm for Modular Multilevel Converters

The modular multilevel converters (MMCs) are widely used in different applications in recent years. Due to lowloss operation, compactness and high modularity, MMC is greatly attractive for high voltage direct current (HVDC) applications. The HVDC station costs and losses are highly related to the switching frequency, capacitor voltage ripple and power quality; which are influenced by modulation algorithm and cell selection methods. This paper formulates the modulation algorithm in an advanced mathematical approach in order to initiate a versatile optimisation problem for switching pattern generation. The most optimised patterns can be obtained through employing various optimisation constraints for different applications. To overcome the calculation complexity for the optimisation problem, a permissive implementable algorithm is proposed and studied in a time-domain simulation platform. The method utilizes the current level for switching purposes in order to obtain the lossless switching at zero-current crossing. Although this paper focuses on HVDC, the mathematical model is widely available for any MMC application and, consequently, can enhance the converter performance by cutting down the losses.