Method for reducing voltage fluctuation of sub-module of MMC under unbalanced grid voltage

Abstract

The invention provides a method for reducing capacitance voltage fluctuation of a sub-module of a modular multilevel converter (MMC) under unbalanced grid voltage. When the grid voltage is asymmetrical, physical quantities such as positive and negative sequence modulation ratios after unbalance are determined, then amplitude values of third harmonic injection voltage and phase angles of three phases are separately calculated and obtained, and finally calculated third harmonic voltage is injected to the three phases through an injection controller, thereby reducing the fluctuation degree of capacitance and voltage of the sub-module of the MMC under the unbalanced grid voltage. The technical scheme provided by the method introduces the injection amount of the third harmonic voltage, and solves the problems that when the grid voltage is unbalanced, the fluctuation of the capacitance and the voltage of the sub-module increases and a conventional method of reducing the voltage fluctuation of the sub-module is not applicable to a situation of the unbalance of the grid voltage. The method combines the characteristics of economy and control flexibility, can make the fluctuation degree of the capacitance and the voltage of the sub-module greatly reduce under the premise of maintaining direct current bus voltage and conveying power unchanged, thereby reducing a requirement for a capacitance value of a capacitor of the sub-module, the volume and the cost of the capacitor of the sub-module are reduced, and the method is beneficial to achieve light weight of the modular multilevel converter.

Description

technical field [0001] The invention relates to the technical field of flexible direct current transmission, in particular to a method for reducing voltage fluctuations of MMC sub-module capacitors under unbalanced grid voltage. Background technique [0002] Flexible direct current transmission (VSC-HVDC) is a new generation of direct current transmission technology after alternating current and conventional direct current transmission. Flexible DC transmission technology has the characteristics of independent adjustment of active power and reactive power, strong weak grid access and low voltage ride-through capabilities, low AC filtering and reactive power compensation requirements, etc. Flexible DC transmission is an important piece of equipment for building smart grids. Compared with traditional methods, flexible DC transmission has stronger technologies in island power supply, urban distribution network capacity expansion, AC system interconnection, and large-scale wind ...

AI Technical Summary

Problems solved by technology

[0006] In order to overcome the deficiency that the above-mentioned existing sub-module voltage fluctuation suppression method is not applicable to the sub-module voltage fluctuation under the condition of unbalanced grid voltage, the present invention provides a method for reducing voltage fluctuation of MMC sub-module under unbalanced grid voltage, when When the AC grid voltage is asymmetrical, determine the third harmonic voltage correction amount of the three phases respectively; determine the modulation wave voltage of MMC under unbalanced grid voltage through the third harmonic voltage correction amount; determine the increase valve side according to the modulation wave voltage peak value of MMC The ratio of the AC voltage amplitude; finally, by injecting the third harmonic voltage correction amount and correspondingly increasing the AC voltage on the valve side to reduce the voltage fluctuation of the sub-module capacitor of the MMC under unbalanced grid voltage

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