Defining the exact number of sub-module transitions in fundamental frequency modulated modular multilevel converters

Abstract

The large number of voltage levels and high quality of a modular multilevel converter (MMC) output voltage and current makes modulation at fundamental frequency a viable alternative. However, a staircase output voltage does not necessarily translate to fundamental switching frequency of the MMC submodules (SMs). This is because the voltage balancing algorithm usually requires several SMs to switch at any sampling period. This paper proposes a voltage balancing algorithm capable of controlling the exact number of SM transitions that are allowed within the arms of an MMC enabling improved regulation of the SM capacitor voltages. The proposed implementation can be extended to any modulation and circulating current control method. It is also shown that enabling the minimum additional transitions within the arm offers the best compromise between SM capacitor voltage ripple and increase in the switching frequency. The proposed method is verified for all operating points with extensive simulation results and demonstrated in an experimental prototype with five SMs per arm.