A Low Modulation Control Method for Modular Multilevel Converter

Abstract

The invention discloses a method for controlling the low-modulation degree of a modularized multi-level converter. The modulation degree is divided into two intervals of the high-modulation degree and the low-modulation degree. In different intervals, the amplitude, angular frequency and overlapping ratio of the carrier wave different. According to the reference voltage of the upper bridge arm and the lower bridge arm, the modulation degree interval is judged, and the amplitude, angular frequency and overlap ratio of the carrier are obtained, and the number of sub-modules that need to be invested in the upper bridge arm and the lower bridge arm is further obtained, and then according to the capacitor voltage The balance control method realizes the balance control of the capacitor voltage. There is no need to sort the sub-modules of the bridge arm, only the maximum or minimum value of the capacitor voltage of the sub-modules in operation or not in operation needs to be obtained, which reduces the amount of calculation. The method of the present invention has better line voltage harmonic characteristics at low modulation levels than carrier in-phase cascaded modulation, and is the same as carrier in-phase cascaded modulation at high modulation levels, and realizes the line voltage harmonic characteristics of the modular multilevel converter in the entire modulation interval Optimized to improve system performance.

Description

technical field [0001] The invention relates to a modular multilevel converter, in particular to a method for controlling low-modulation degree of the modular multilevel converter. Background technique [0002] Modular Multilevel Converters (MMC) have the characteristics of high modularity, easy expansion, good harmonic performance, high efficiency, and common DC side, and have become the most promising multilevel topology. One of the structures has been popularized and applied in the field of flexible direct current transmission engineering, and has good application prospects in the fields of medium and high voltage motor drives and power quality control. [0003] When MMC is applied to occasions such as flexible direct current transmission, motor drive, static var generator and active power filter, in some working conditions, MMC will work in the area of ​​low modulation degree, for example, when MMC is applied to transmit wind power and photovoltaic In the flexible direc...

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Problems solved by technology

[0004] Existing modular multilevel converter modulation techniques mainly include nearest level approximation modulation, carrier phase shift modulation and carrier in-phase cascading modulation, etc. When the MMC works under low modulation conditions, these three modulation methods are There is a problem: the number of output levels is reduced, the harmonic characteristics are poor, and it is difficult to ensure that the MMC maintains efficient and stable operation under low modulation conditions

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