The ±500 kV half-bridge modular multilevel converter-high-voltage DC (MMC-HVDC) grid project with overhead transmission line is under construction in China at present. For MMC-HVDC grid, only one electrical grounding point is allowed and thus, the metal return lines on the same towers with positive and negative polar lines are necessary. However, current studies mainly focused on the control methods of MMC-HVDC substation and few studies have been done on the transient overvoltage of overhead transmission line for the MMC-HVDC project. This study analysed the main transient overvoltage types of metal return lines of ±500 kV half-bridge MMC-HVDC grid which transmit electric power by overhead lines and clear the grounding fault by DC breakers. Based on a ±500 kV MMC-HVDC grid project, the simulation models were built using PSCAD and the possible transient overvoltage types were studied. The results showed that the most serious overvoltage was caused by the DC breaker clearing the grounding fault. Then the insulation coordination design for metal return lines was carried out. The recommended arcing horn clearances at different altitude were given. The research results provided support for the insulation design of the transmission line.
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