I. Introduction
When the gravity energy storage is connected to the grid, the terminal voltage of the generator-motor must be consistent with the grid voltage in phase sequence, phase, amplitude and frequency to ensure the safe and stable operation of the whole system after the motor is connected to the grid[1]. However, in the actual gravity energy storage system, there is always a certain error in the voltage index of the generator-motor terminal and the grid terminal, and then the transient impulse current is generated. In the actual gravity energy storage system, there is always a certain error in the voltage index of the generator-motor terminal and the grid terminal, and then the transient impulse current is generated [2]–[4]. In addition, because the generator-motor group of the gravity energy storage system is different from the conventional generator set, it needs to switch frequently between the charging and discharging operating conditions according to the requirements of the power grid, and each switch needs to complete the start and grid connection of the generator-motor. In the case of frequent grid connection at this time, if there is always a large error in the voltage index of the machine end and the grid end, a stable transient impulse current will be generated. If there is no human intervention and control, long-term operation will bring hidden dangers to the safety of the grid-connected system, causing a series of problems such as equipment aging or even damage [5].