Study on Calculation Method for Steady-State Short-Circuit Current of MMC During a DC Pole-to-Pole Fault

The modular multilevel converter based high-voltage direct current (MMC-HVDC) has drawn great attention in the field of HVDC transmission. Research on short-circuit fault current calculation method is of great significance for equipment protection and key MMC components selection. In common cases, IGBT will be blocked within a short period of time after the occurrence of a bipolar fault, and the short-circuit current will be suppressed. However, there are also cases where the fault progresses to stable stage. Therefore, research on the steady-state short-circuit current calculation method is of high importance for protection design and failure analysis. Taking AC-side reactance and DC-side short-circuit impedance into consideration, a steady-state short-circuit current calculation method based on solving the circuit initial value is proposed. Current calculation is discussed in three situations divided based on different fault parameters after defining the commutation overlap angle are respectively analyzed. The short-circuit current can be solved by formulating the equations based on the conduction state of the diode in the converter, and then solving the electrical quantities through the current boundary conditions of each state. Finally, a fault circuit model is built in PSCAD/EMTDC for simulation, and the accuracy of the calculation method is verified under different fault parameters.