Overcurrent Analysis And Protection Measures Of Bridge-Arm Current For AC/DC Fault In DC Grid

With China’s commitment to "carbon peak" and "carbon neutrality" at the United Nations,the total installed capacity of new energy generation,such as wind power and photovoltaics,has increased rapidly,and promoting the clean and low-carbon energy has become an important goal of China’s 14th Five-Year Plan.Meanwhile,Large-scale and long-distance transmission of new energy is driving the development of DC grid based on Modular Multilevel Converter(MMC).The overcurrent of the bridge arm under the AC/DC fault seriously endangers safety and reliability of power electronic devices because of its weak overcurrent capacity.Besides,the fault current of bridge arm is affected by AC/DC system and MMC control,which greatly aggravates the difficulty of its analysis and calculation.At first,the uneven distribution of the three-phase current component is the main reason of MMC bridge arm’s overcurrent under asymmetric AC system.In order to reduce the maximum fault current,a current dynamic regulation control strategy based on the injection of zero-sequence voltage is proposed which takes the optimal power device loss as a control goal.PSCAD/EMTDC simulation verified its excellent performance,reducing maximum DC flow by 51.5%,maximum peak current by 14.4%,and maximum loss by 20.9%.Then,an accurate fault current calculation method for DC fault is proposed,which fully considers the current feed of the AC/DC system.The optimal parameters of overhead lines are extracted to eliminate the coupling effect of positive and negative lines.And the expression of fault current complex frequency domain is obtained by using the network equivalent model.Compared with other fault current calculation methods,the proposed method can accurately describe the change trend of the fault current within 20ms after the fault.The maximum error is controlled within 3.8%,and it is applicable for different fault positions and different transition resistors of the line.Finally,this paper builds the physical experiment platform on the basis of DC grid simulation,designs its circuit structure and control frame,and carries out experiments such as normal operation,power step and fault characteristics,and compares the fault current calculation method with the short-circuit experimental results.