Fault Current Calculation of MMC-HVDC System Considering Mechanical DC Circuit Breaker

Direct current circuit breakers (DCCBs) are characterized by their fast response and limited fault propagation, making them widely applicable in the protection of modular multilevel converter (MMC) based high-voltage direct current (HVDC) and adaptive reclosing strategy. However, existing research lacks precise calculations of transient processes during the operation of DCCBs. To address this issue, this paper proposes a method for calculating short-circuit fault currents in mechanical DCCBs. First, the operating principles of MMC converter stations are analyzed and the fault isolation process of mechanical DCCBs. Next, the equivalent circuits for MMC converter stations, DCCBs, and transmission line faults are introduced, and perform step-bystep calculations based on the operational process of the mechanical DCCB. Finally, we establish a simulation model of a symmetrical bipolar direct current transmission line using the electromagnetic transient simulation software PSCAD to validate the proposed calculation method. The computed results align well with the simulation results, confirming the accuracy of the proposed calculations. This calculation method can be used for the parameter design of DC circuit breakers and DC grids, as well as for DC line protection.