Modulation method for reducing low-order harmonics of alternating-current voltage of modular multilevel converter

Abstract

The invention discloses a modulation method for reducing low-order harmonics of alternating-current voltage of a modular multilevel converter. Based on a carrier wave stacked pulse width modulation (PD-PWM) method, in each carrier wave period, the amplitude range of the carrier waves is calculated and changed in real time according to the actual measurement value of the capacitor voltage of a bridge arm, so that the modulation result of the bridge arm is corrected, and MMC AC voltage low-frequency harmonic waves caused by capacitor voltage fluctuation are reduced. Compared with an existing modulation method, the modulation method provided by the invention considers the influence of module capacitor voltage fluctuation on the alternating current output voltage, and a bridge arm switching function is equivalently corrected by changing the carrier wave amplitude, so that the problem of low-order harmonics of the alternating current voltage caused by the capacitor voltage fluctuation can be remarkably reduced.

Description

technical field [0001] The invention relates to a low-order harmonic modulation method for reducing the AC voltage of a modularized multilevel converter, and belongs to the technical field of high-power multilevel power electronic converters. Background technique [0002] The modular multilevel converter (MMC) adopts a cascaded structure of multiple modules, which is easy to expand, overcomes the limitations of the voltage level of the current fully-controlled power switching devices and the difficulty of connecting devices in series, and has high efficiency , so it is very suitable for the field of medium-high voltage and large-capacity power electronic conversion, and has great application prospects in medium-voltage occasions such as medium-voltage motor drives and medium-voltage distribution networks. [0003] The improvement of the quality of AC voltage waveform is one of the important research contents of MMC. According to the different spectrum distribution, the harm...

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Problems solved by technology

However, when this method is used in practice, additional upper and lower bridge arm energy balance controllers must be configured, otherwise the system will easily diverge.

The energy balance controller of the upper and lower bridge arms requires complex PI parameter design

In addition, some scholars started from the sub-module level, combined with the carrier phase-shift pulse width modulation method, assigned a carrier to each sub-module, and changed the carrier amplitude of the module in real time through the capacitor voltage of the module, thus correcting the module in each sub-module. The input duty cycle within a carrier cycle, but this method is only applicable to the situation where each module is attached with a dedicated capacitor voltage equalization controller, and cannot be used in the more general case of capacitor voltage equalization control based on bridge arm capacitor voltage sorting. application

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