DC Fault Analysis And Fault Of Current Limiting For MMC-HVDC System

Modular Multilevel Converter(MMC)is getting more and more favor in the field of high voltage direct current(HVDC)transmission,depending on its obvious advantages such as high modularization,low harmonic distortion rate,low switching loss,easy to improve the voltage level and so on.However,the traditional half-bridge MMC structure can not effectively block the fault current of the DC side,which seriously threatens the steady state operation of the system and restricts the development of MMC.Therefore,the timely treatment of MMC-HVDC DC side faults is one of the key problems that need to be dealt with in the actual power grid engineering.In this paper,the significance of vigorously developing MMC in the efficient utilization of global resources and the stage achievements in the field of MMC flexible direct current transmission in China in recent years are summarized,and the current research status of MMC DC side faults at home and abroad and the advantages and disadvantages of various fault current limiting solutions are analyzed.The topological structure and working principle of MMC are briefly described,and the two-terminal MMC-HVDC control system is built and the corresponding control strategy is given.Secondly,in view of the MMC-the dc side of the typical faults in the HVDC system,namely the monopolar grounding,bipolar single-line breakdown and short circuit fault features,the MMC under different fault conditions is deduced after atresia and disconnect the ac breaker before and after the fault current expression,three failure respectively through MATLAB/SIMULINK simulation,delving into the failure of failure characteristics and the effects on the system stable running,a more suitable for the design of below the dc side of fault current limiting method laid a theoretical basis.Finally,in the case of bipolar short-circuit fault,a self-adjusting current limiting control strategy suitable for the semi-bridge MMC system is proposed,aiming at the inherent defect that the semi-bridge MMC system cannot block the fault current in time.In the case of DC side faults,the DC voltage of the converter can be reduced rapidly by rapidly changing the input and number of submodules,and the fault current can be suppressed by the control strategy.Based on half bridge type double end MMC flexible dc power grid to MMC self-adjusting control strategy of current limiting the validity of the simulation and experimental results proved that the MMC self-adjusting control strategy can effectively reduce the MMC dc voltage and then suppress the fault current,rapid response speed,no need to change or increase the other inverter structure between fault current limiting device can reduce the bridge arm current peak,and make the inverter for overcurrent and atresia of risk to a minimum.