A Circulating Current Suppression Method for Modular Multilevel Converter Based on Virtual Reactance

Abstract

The invention belongs to the technical field of DC power transmission, specifically a modular multilevel converter circulating current inhibition method based on virtual reactance. An inhibition modelcorresponding to a bridge arm circulating current is built through the analysis of the generation mechanism of an MMC circulating current, and the method achieves the correction of the upper and lower bridge arm voltages of an MMC through virtual reactance, so as to achieve the purpose of circulating current inhibition. The method achieves the virtualization of a hardware equivalent reactor through a software algorithm to change the equivalent output reactance of a bridge arm at each phase, thereby effectively inhibiting the circulating current of a modular multilevel converter.

Description

technical field [0001] The invention belongs to the technical field of direct current transmission, in particular to a method for suppressing circulating current of a modular multilevel converter based on virtual reactance. Background technique [0002] Modular multilevel converter (MMC) has attracted extensive attention from scholars all over the world since it was first proposed in 2002. Compared with traditional multilevel converters, modular multilevel converters rely on highly modular The design makes it have good voltage expansion capability and power expansion capability and the output voltage waveform quality is good, but the biggest advantage of the modular multilevel converter is that it has a common DC bus, so that the energy can be transferred between the three-phase bridge arms. exchange, which is beneficial to three-phase balance control. [0003] The energy of the modular multilevel converter is distributed and stored in the capacitors of each sub-module. The...

AI Technical Summary

Problems solved by technology

[0006] It can be seen from the first-order differential equation that the voltage drop of the internal circulating current of the x-phase on the bridge arm reactance and the bridge arm equivalent resistance is caused by the difference between the voltage sum of the upper and lower bridge arms and the DC side voltage. And the circulating current only exists in the internal circuit of the MMC. Although it has no effect on the power supply or load connected to the outside of the MMC, its existence will distort the current of the bridge arm, cause loss of switching devices, and even affect the safe operation of the compensation system.

[0007] The current suppression method for the circulating current is to reduce the circulating current between the three-phase bridge arms by increasing the reactance value of the bridge arm reactance on the hardware. However, if the actual bridge arm reactance value is too large, it will affect the tracking ability of the MMC system. Large inductance will bring problems such as increased volume and cost

Images