Low-modulation-index control method for modular multi-level current converter

Abstract

The invention discloses a low-modulation-index control method for a modular multi-level current converter. A modulation index is divided into two intervals of a high modulation index and a low modulation index; and in different intervals, carrier waves are different in amplitude, angular frequency and overlapping ratio. According to reference voltages of an upper bridge arm and a lower bridge arm, the modulation index interval in which the reference voltages are located is judged to obtain the amplitude, angular frequency and overlapping ratio of the carrier waves, the number of sub-modules required by the upper bridge arm and the lower bridge arm is further obtained, and then the balance control of capacitor voltages is realized according to a capacitor voltage balance control method. The sub-modules of the bridge arms do not need to be sorted and only a maximum value or a minimum value of the capacitor voltage of the sub-module put into operation or the sub-module not put into operation needs to be obtained, so that the calculation amount is reduced. According to the method, a linear voltage harmonic characteristic is superior to carrier wave in-phase stacking modulation under the low modulation index and is the same as the carrier wave in-phase stacking modulation under the high modulation index, an optimal harmonic characteristic of a linear voltage of the modular multi-level current converter in the whole modulation interval is realized, and the system performance is improved.

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 (ModularMultilevelConverters, MMC) have the characteristics of high modularity, easy expansion, good harmonic performance, high efficiency, and a common DC side, and have become one of the most promising multilevel topologies. First, it has been popularized and applied in the field of flexible direct current transmission projects, 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 photovolta...

AI Technical Summary

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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