Circulating-current restraining method for three-phase modular multilevel convertor

Abstract

The invention discloses a circulating-current restraining method for a three-phase modular multilevel convertor. The method comprises the following steps of: processing detected bridge current by utilizing fundamental frequency-doubled rotating coordinates transform based on negative sequences, and then performing signal processing and feedforward compensation link so as to acquire an additional control signal used for restraining circulating current; subtracting the additional control signal from a half of a DC voltage value, superposing an output voltage reference signal on the AC side of the existing converter to acquire a bridge voltage reference signal, and performing pulse width modulation to acquire trigger pulses of various bridges of the converter to ensure that the converter outputs corresponding voltage. The circulating-current restraining method can restrain the circulating current in the converter in a low level under the condition of not increasing the inductance of the bridges so as to basically eliminate the distortion of the bridge current to make the bridge current restored into sinusoidal waveform.

Description

technical field [0001] The invention relates to the technical fields of flexible power transmission and distribution of power systems, power electronics and user power, and in particular to a method for suppressing circulation of three-phase modular multilevel converters. Background technique [0002] Modular multilevel converter (MMC) is a multilevel converter topology that has attracted much attention in recent years. It uses multiple submodules (submodules, SM) cascaded, and by controlling the input and disconnection states of each submodule separately, it can make the AC voltage waveform output by the converter contain many voltage steps to approach the sine wave to the maximum extent. Thereby reducing the harmonic content in the output voltage and reducing the waveform distortion to meet the harmonic requirements of the power grid. [0003] The basic structure of a three-phase modular multilevel converter (such as figure 1 Shown) is composed of three-phase six bridge ...

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Problems solved by technology

[0005] However, the above-mentioned method of only increasing the value of the bridge arm inductance to suppress the circulating current only passively increases the circulating current impedance, and it is impossible to completely eliminate the circulating current, and this method of increasing the bridge arm inductance requires an increase in equipment investment. In actual engineering, Especially in high-voltage and large-capacity applications, increasing the value of the inductance in the high-voltage reactor requires a very high cost

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