Abstract: According to the data of the power grid operation department, the power system will experience power frequency overvoltage during the open-phase operation process after the single-phase transient earth fault. This kind of overvoltage is one of the constraints that the power grid operation department needs to consider when determining the transmission limit of the new energy gathering system. To analyze the overvoltage in the open-phase condition, we established a mathematical model of the three-phase voltages in the open-phase condition based on the parameters of a typical new energy gathering system. The influence of the power level of the new energy gathering system and the length of the transmission line on the power frequency overvoltage amplitude was studied. The severity of the power frequency overvoltage during the open-phase operation was also analyzed. Besides, the theoretical analysis results were verified through the hardware-in-the-loop simulation of the controller. The research results show that the active and reactive power output levels of the new energy gathering system are the key factors that affect the power frequency overvoltage amplitude. The increase in active power output will increase the highest phase voltage of the three-phase voltages and decrease the lowest phase voltage. The increase in capacitive reactive power output will increase the three-phase voltages at the same time. If the active power output level of the new energy gathering system does not reach the static voltage stability limit, and the new energy unit and the reactive power compensation device do not generate additional reactive current, then the overvoltage level will not reach the level to trigger line protection action.