Research On The Novel MMC Sub-modile Topology With DC Fault Clearance Capability

In recent years,in the field of VSC-HVDC based on fully controlled devices,modular multi-level converters(MMC)are widely used Converter which has become one of the most potential voltage source converters because of its high waveform quality,low switching loss,easy to upgrade voltage level and many other advantages.However,in the process of long-distance HVDC transmission,there is a high probability of DC fault.When the MMC,which is composed of the most widely used half bridge sub-modules,has DC fault current clearing when the factor module has DC fault,the continuous current diode at the lower end of the factor module and the converter can not achieve DC fault current clearing through the blocking of the sub-modules,which restricts the industrial promotion of MMC-HVDC.Therefore,how to improve the half bridge module and make it have the ability of clearing DC fault is the main research focus of this paper.In this paper,the topology and working principle of MMC are analyzed,the control strategies of each level are studied,and the over-current of traditional HBSM-MMC before and after the double pole short circuit is analyzed in detail.Then several typical sub-modules with DC fault clearing ability are introduced and analyzed.In order to improve the defects of these sub-modules,such as difficult control and complex structure,a half bridge with bidirectional switch sub-module(HBBSSM)is proposed in this paper topology,which changes the bottom of half bridge sub-module into bi-directional switch which can be bi-directional turned off.After the fault is locked,the fault current circuit can be blocked directly,and the number of IGBTs in it is the same as that in half bridge sub-module,the structure and control are simple,the encapsulation is difficult,and it has good DC fault clearing ability under the condition of meeting the same pulse trigger.Secondly,due to the risk of switching device overvoltage during DC fault clearing when hbbssm is not locked at the same time,this paper improves it and then proposes a topology of diode clamp with bidirectional switch sub-module(DCBSSM).This topology can solve the over-voltage problem and reduce the difficulty of the control system by introducing bypass absorption circuit.Taking the most serious DC bipolar short circuit as an example,this paper analyzes and studies the fault clearing principle of these two sub-modules,and then uses Matlab/Simulink platform to establish a simulation model to verify the DC fault clearing ability of the two sub-module topologies proposed in this paper,and compares them under the condition of non simultaneous triggering to verify the effectiveness of the devices in dcbssm without over-voltage.Finally,the two sub-modules are compared horizontally,and their respective economic advantages are found.Compared with other sub-modules with fault clearing ability which are more difficult to control,the two sub-modules have simple control,and their working principles are completely consistent with the half bridge sub-module.The mature half bridge sub-module control system can be completely transplanted,saving the use cost.