## 1 Introduction  
When the transformer is switched on, due to the saturation characteristics of the internal core, the excitation inductance and winding impedance of the converter transformer are reduced, resulting in an excitation surge several times the rated current [1, 2]. The inrush current causes the converter winding to vibrate and force the oil flow inside the case and oil pipe to surge, pushing the gas relay plate to cause a heavy gas operation, which reduces the reliability of the power grid [3, 4, 5]. With the development of DC transmission technology, in recent years, a number of excitation surges caused by the gas relay baffle deflection have lead to converter transformer tripping accidents. Therefore, it is important to analyze the internal oil flow characteristics and the transient characteristics of the baffle plate of the converter gas relay to study the influence of the inrush current on the converter transformer and its related components [6; 7]. In the actual operation of the electrical system, the transformer usually undergoes multiple start-stop operations, and the inrush current is generated at each start-up. And because the inrush current has the characteristics of high amplitude, short time, and rapid attenuation, the gas relay cannot distinguish the difference between it and the real fault, so that the maloperation occurs. The effects of misoperation are as follows: (1) When the gas relay malfunctions due to inrush current, it may cause the transformer to disconnect the power grid and cause unnecessary outages. This not only affects the normal operation of the transformer, but also may lead to power supply interruption, affecting the user's electricity demand, especially in important power load areas. (2) Frequent malfunctions can lead to instability of the power system, especially in large-scale power grids. Misoperations may lead to a series of chain reactions, resulting in a wide range of power outages. (3) Too many misoperations have not been corrected in time. The system may be slow to respond to real faults, or over-protect some minor non-fault events, affecting the efficiency of the protection system. Existing research on gas relay mainly focuses on the causes of gas relay misoperation [8; 9; 10] and the measures to inhibit its malfunction [11; 12], and the current status of the research on the internal oil flow characteristics of gas relay and the transient characteristics of the baffle plate are as follows: [PERSON] [13] and others from North China Electric Power University proposed a method of non-electrical power protection. The electrical protection method of the initial angular acceleration of the baffle rotation can predict the fault energy and fault severity. [PERSON] of Xi'an Jiaotong University [14] and others used the Fluent platform to numerically simulate the internal flow field of the gas relay and the baffle plate action, and obtained that the inlet flow rate of the gas relay needs to be consistently greater than the threshold flow rate in order to push the baffle plate to rotate to the action angle. [PERSON] [15] and others from Kunming University of Science and Technology built a gas relay calibration calculation platform to simulate the heavy gas alarm process under the extreme working conditions that cannot be provided by the test bench, and quantitatively described the internal flow field of the gas relay. However, with the development of DC power transmission technology, the excitation inrush generated by the no-load closing of converter transformers often leads to accidents of misactivation of the gas relay. However, the current study lacks the internal characterization of the gas relay under the action of inrush current, and it is impossible to judge whether the gas relay under the known working condition will be inoperative due to the impact of oil flow.