A Fast Sorting Strategy Based on a Two-way Merge Sort for Balancing the Capacitor Voltages in Modular Multilevel Converters

Fangzhou Zhao; Guochun Xiao; Min Liu; Daoshu Yang

A Fast Sorting Strategy Based on a Two-way Merge Sort for Balancing the Capacitor Voltages in Modular Multilevel Converters

Vol. 17, No. 2, pp. 346-357, Mar. 2017

10.6113/JPE.2019.17.2.346

Capacitor voltages balancing

Fast sorting algorithms

Modular multilevel converter

Two-way merge sort

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Abstract

The Modular Multilevel Converter (MMC) is particularly attractive for medium and high power applications such as High-Voltage Direct Current (HVDC) systems. In order to reach a high voltage, the number of cascaded submodules (SMs) is generally very large. Thus, in the applications with hundreds or even thousands of SMs such as MMC-HVDCs, the sorting algorithm of the conventional voltage balancing strategy is extremely slow. This complicates the controller design and increases the hardware cost tremendously. This paper presents a Two-Way Merge Sort (TWMS) strategy based on the prediction of the capacitor voltages under ideal conditions. It also proposes an innovative Insertion Sort Correction for the TWMS (ISC-TWMS) to solve issues in practical engineering under non-ideal conditions. The proposed sorting methods are combined with the features of the MMC-HVDC control strategy, which significantly accelerates the sorting process and reduces the implementation efforts. In comparison with the commonly used quicksort algorithm, it saves at least two-thirds of the sorting execution time in one arm with 100 SMs, and saves more with a higher number of SMs. A 501-level MMC-HVDC simulation model in PSCAD/EMTDC has been built to verify the validity of the proposed strategies. The fast speed and high efficiency of the algorithms are demonstrated by experiments with a DSP controller (TMS320F28335).

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Cite this article

[IEEE Style]

F. Zhao, G. Xiao, M. Liu, D. Yang, "A Fast Sorting Strategy Based on a Two-way Merge Sort for Balancing the Capacitor Voltages in Modular Multilevel Converters," Journal of Power Electronics, vol. 17, no. 2, pp. 346-357, 2017. DOI: 10.6113/JPE.2019.17.2.346.

[ACM Style]

Fangzhou Zhao, Guochun Xiao, Min Liu, and Daoshu Yang. 2017. A Fast Sorting Strategy Based on a Two-way Merge Sort for Balancing the Capacitor Voltages in Modular Multilevel Converters. Journal of Power Electronics, 17, 2, (2017), 346-357. DOI: 10.6113/JPE.2019.17.2.346.