Research On Fault Current Limiting And Protection Method Of Flexible DC Power Network Based On Half-Bridge Converter

With its high flexibility and strong controllability flexible DC power grid has been widely used in the absorption of new energy.However,with its low damping and low inertia.the current will rise rapidly once the bipolar short-circuit fault occurs.At the same time.the rated parameters of the converter are low and the current resistance is poor.If the fault current is not limited,the converter will be locked out of operation in a very short time.Therefore.based on the fault characteristics,this paper studies the current limiting control strategy to make the converter delay and lock,so as to obtain sufficient operation time for protection.At the same time,it studies the main protection and backup protection methods of the line,so as to realize fault identification and removal,and ensure the safe and reliable operation of the flexible DC system.Firstly,from the perspective of improving converter control strategy,an active DC fault current limiting control technology based on hierarchical switch submodule is proposed.By reducing the input number of submodules during the fault period,the capacitor discharge current is suppressed,and the fault current amplitude and rising speed are limited.After analyzing the change of the current of the rear axle arm after the fault,two boundary conditions of gold short circuit at the outlet of the converter and high resistance short circuit at the end of the line were selected to set the number of submodules that should be put in during the fault.On this basis,the bridge arm current is used as the characteristic quantity,and the submodule switching criterion is designed according to its overcurrent degree and rising rate.The submodule is controlled to perform switching according to the setting value,which can delay the converter locking and reduce the power fluctuation of the system.Then,a new principle of differential protection based on current accumulative value is proposed on the basis of the above-mentioned current limiting control strategies.The change of fault component after adopting current limiting control measures is analyzed,the shortest and longest zero crossing time of receiving end current is derived,and the influence of polarity reversal delay of receiving end current on traditional current protection is measured.Based on this,the differential protection criterion is constructed by using the cumulative value of the current at both ends of the line to expand the difference between internal and external faults.and the adaptive coefficient is designed to further improve the sensitivity of protection action in the fault area,to complete the fault line identification and removal.Finally.considering the risk of main protection rejection,a backup protection method of flexible DC power grid based on the sixth harmonic component is proposed.By analyzing the fault characteristics of the uncontrolled rectifier phase after the converter is locked,the fault characteristics of the constant sixth harmonic component under different bridge arm conduction state are obtained.The sixth harmonic reactance is calculated by using this characteristic component,and the short-delay action reactance and long-delay action reactance are constructed.The fault area is identified by combining with the limit current resistance time of power electronic equipment to ensure the safe operation of the system.