Circulating current control for modular multilevel converter based on selective harmonic elimination with ultra-low switching frequency

Abstract

Multilevel converters (MCs) are utilized in medium voltage (MV) high power applications due to its higher efficiency than two level converters. On the other hand, modular multilevel converters (MMCs) provide several advantages with regard to other MCs, such as higher scalability, reliability and no requirement of a common DC capacitor. Particularly, low switching frequency modulations, such as (2N+1) selective harmonic elimination (SHE) - pulse width modulation (PWM), may improve the efficiency of MMCs when they are utilized in MV and high power applications, where the number of sub-modules is not high. This work presents a new circulating current control for MMC when (2N+1) SHE-PWM is utilized. Therefore, it is possible to operate the converter simultaneously with low switching frequency and low capacitor voltage ripple at every sub-module besides a correct energy balance between arms. In addition, a new method to implement (2N+1) SHE-PWM for MMCs, which is also valid to implement standard SHE-PWM for any MC, is provided. Using this method, different equation systems are not required for every switching pattern. In this way, this technique provides simultaneously both the switching patterns and the firing angles which solve the SHE problem, simplifying the searching task. Simulation results which have been obtained from a MMC with 5 sub-modules at every arm, have validated the novel proposed circulating current control. Furthermore, the spectrum of the simulated line to line voltage waveform has proved the correct performance of the proposed (2N+1) SHE-PWM implementation method. Several sets of angles have been provided throughout the ma range, where 17 harmonics have been controlled.