New Submodule Selection Algorithm for Low Device Switching Frequency Modulation of Medium-Voltage Modular Multilevel Converter

Low device switching frequency is preferred in high-power converters to achieve higher efficiency, higher device utilization, and feasible cooling system design. However, low device switching frequency operation leads to higher peak voltage in submodule (SM) capacitors of medium-voltage modular multilevel converters (MMC). The main reason is unequal distribution of energy among SMs in each fundamental period due to fewer switching commutations available for balancing. This results in increased converter cost due to higher voltage rating of SM capacitors and semiconductor devices. Therefore, the main objective of this paper is to propose a new SM selection algorithm to reduce the peak voltage of SM capacitors without significantly increasing the device switching frequency. The main idea is to avoid unnecessary switching commutations by ensuring balancing of SM capacitor voltages only at the end of charging period. The proposed algorithm has been verified with extensive simulation studies of a 5L-MMC and it has been observed that peak voltage is kept within 10 % of the nominal value while limiting the device switching frequency in the range of 350 to 400 Hz.

I. Introduction

Modular multilevel converter (MMC) is based on the series connection of submodules (SMs) that consists of semiconductor devices and dc capacitors. MMC was originally proposed for high-voltage direct-current (HVDC) transmission [1] and it has become an industry standard due to several advantages such as modularity, scalability, higher-efficiency, lower harmonics, and fault-tolerant capability [2]. Apart from HVDC transmission, it has become popular in many other industrial applications such as medium-voltage (MV) high-power induction motor drives, wind power integration, flexible alternating-current transmission systems, etc.