An Improved Time Domain Method Based Fault Current Calculation for Multi Terminal MMC-HVDC Transmission System

In view of the insufficient analytic calculation accuracy of the fault current in modular multilevel converter based flexible multi-terminal high voltage direct current (MMC-HVDC) transmission system, an improved time-domain solving method taking the influence of the converter stations located at far-end into account was proposed. Firstly, on the basis of analyzing the discharging path of the capacitor in the post-fault submodule, the systemic parameters such as the value of the equivalent capacitance in converter station and so on were derived, and an equivalent network model of the post-fault MMC-HVDC system was built. Secondly, decoupling the converter stations in DC network and taking the approximate solutions of post-fault currents in all branches as the initial values, the equivalent resistances and equivalent inductances of the lines in multi-terminal MMC-HVDC network were successively revised while the influences of far-end converter stations were taken into account, and then the values of fault currents of the lines in MMC-HVDC system were obtained. Finally, based on RT-LAB simulation platform a four-terminal flexible DC network model was constructed, by which the calculated values of fault currents and the simulation results of detailed electromagnetic transient were contrasted. Contrast results show that by use of the proposed solving method for fault currents, the values of currents after the short circuit fault in all branches of multi-terminal MMC-HVDC network can be effectively calculated, and the maximum error is less than 5%.