Analysis and Calculation of Symmetrical Ground Fault Short Circuit Current Contributed by MMC-HVDC Converter Station at AC Side

With the development of flexible HVDC transmission systems, there impact on AC systems is growing. When the AC system fails, the short-circuit current contributed by the converter station is the core factor affecting the relay protection and the voltage level. However, there is no mature method for detailed analysis and accurate calculation of the transient process. To solve this problem, this paper takes an MMC-based DC grid project as the object to study the transient process of the fault current contributed by the converter station after a symmetrical ground fault occurs on the AC side. In addition, the equivalent model of internal current path and submodule of converter station is established, and a calculation method of the short-circuit current based on operation circuit is given. Finally, a simulation model is built based on PSCAD, which helps to verify the correctness of the proposed short-circuit current calculation method.