Research On Circulating Current Suppression Technology Of MMC Applied To DC Distribution Network

The hybrid sub-module MMC is a new inverter topology combining half bridge sub-modules and full-bridge sub-modules,which can not only use the full-bridge submodule to isolate the DC-side fault current but also enable economy of the halfbridge sub-module be reflected.This dissertation mainly studies the hybrid submodule MMC topology and control strategy,and at the same time combines with the recent level-approximation modulation strategy to propose an adaptive sub-module capacitor voltage sequencing algorithm.Based on this,the circulation of mixed submodule MMC and its restraining strategy are studied.The main contents are as follows:First of all,this paper compares the two most basic sub-modules of MMC,the half-bridge sub-module and the full-bridge sub-module.Based on a detailed analysis of the operation modes and working principles of the two sub-modules and their respective advantages and disadvantages,this paper proposes a hybrid sub-module MMC topology.The structure is composed of a half-bridge sub-module and a fullbridge sub-module.While retaining the full-bridge sub-module DC fault traversing capability,it also takes into account the economic value of the half-bridge sub-module.Then,the mathematical model of hybrid submodule MMC is deduced.Based on this,the inner loop controller and outer loop controller of hybrid submodule MMC are designed.Second,this paper completes the selection of main circuit parameters and modulation strategies.It includes: According to the different types of faults on the DC side,calculated the proportion of half-bridge and full-bridge sub-modules;The capacitance value is calculated according to the fluctuation of capacitor voltage;From the perspective of avoiding resonance with the MMC double frequency circulation,deduced the hybrid sub-module MMC arm inductor;Combining with the recent levelapproximation modulation strategy,a capacitor voltage sequencing algorithm suitable for the hybrid sub-module is proposed.Then,based on the establishment of the bridge arm current equation and the analysis of the causes of the formation of the bridge arm circulation from different aspects,the double-frequency component in the circulation is obtained from the perspective of energy fluctuation.In the dq rotation coordinate system,the doublefrequency circulation component is suppressed.Based on this,a control strategy based on improved dq synchronous rotating coordinate system is used.This strategy can simplify the mathematical model of circulation in the dq coordinate system and make it more consistent with the mathematical model and physical characteristics of MMC.Finally,the simulation model of the 2-terminal hybrid sub-module MMC system is built on the PSCAD/EMTDC simulation platform.The simulation is divided into two parts: In the first part,verify the effectiveness of the proposed modulation strategy and sub-module capacitor voltage sequencing and the proposed hybrid submodule MMC has the ability to resist low-power disturbances and to isolate the DCside fault current.In the second part,the effects of two kinds of circulation suppression strategies are compared from the viewpoint of the peak value of the circulation,the amplitude of the harmonics in the circulation,the fluctuation of the capacitance voltage of the sub-module and the output voltage and current of the AC side.And verify that when the flow of MMC is reversed,the circulation suppression strategy used has the same inhibitory effect.