## 1 Introduction  
The DC grounding electrode is an indispensable and vital part of the ultra-high-voltage direct current transmission project. The unipolar ground operation carries all the system current, and the bipolar operation plays a role in holding the unbalanced current, pinning the neutral point potential of the converter valve, and protecting the safety of the converter valve [1, 2, 3]. Thousands of amperes of current pass through the diversion cable and the distribution cable, and then enters the feeder rod to diffuse into the nearby coke layer and soil, which will lead to the heating and aging of the coke layer. In severe cases, it will cause the burning of the grounding electrode and the electrochemical corrosion problem, resulting in the fracture of the connection between the distribution cable and the feeder rod, which seriously affects the electrical performance of the grounding electrode [4, 5, 6, 7]. Two grounding electrode burning accidents occurred in the \(\pm\)500 kV DC grounding electrode from Gezhouba to Shanghai South Bridge within 14 months of operation. After excavation and inspection, it was found that the drainage cable was burned and the feeder rod was seriously corroded to breaking, which seriously threatened the normal operation of the Geshang DC transmission system [8; 9]. Therefore, the detection of the operating state of the DC grounding electrode has become an urgent problem that needs to be solved. At present, the detection of the DC grounding electrode operation status in practical engineering mainly includes operation environment parameter monitoring, electrical parameter detection, and physical inspection. (1) In the aspect of operating environment parameters monitoring, References [10; 11] provide insights into the maintenance and repair of DC grounding electrodes by analyzing the impact of coke bed ageing and grounding electrode corrosion on the electrical performance of the grounding electrodes. References [12; 13; 14] utilize data monitoring modules to monitor soil temperature, soil moisture, water temperature, water level, and feeder current. The health status of the grounding electrode is then assessed through wireless transmission integration and data processing analysis. (2) In terms of electrical parameter detection, References [15; 16] designed an online monitoring system for the ultra-high-voltage direct current (UHVDC) transmission grounding electrode. Through remote monitoring of the grounding electrode current and observation of important operating data such as the well water level, the operation state diagnosis of the grounding electrode is realized. Reference [17] selects characteristics such as grounding resistance, step voltage, contact potential, current distribution, and operational environment temperature of the grounding electrode to achieve status monitoring of the DC grounding electrode. (3) In terms of physical inspection, it is mainly based on daily inspection and monitoring, regular tests, and on-site excavation sampling methods [18]. However, there are some shortcomings in the above methods. For example, the electrical parameter test needs to be carried out regularly according to the test cycle, and the interval time is long. The on-site excavation work faces challenges such as great difficulty and randomness [19], and it is difficult to obtain the location information of the specific fracture of the DC grounding electrode. Therefore, this paper makes an in-depth study on the breakage position of the connection between the distribution cable and the feed rod in the local area of the DC grounding electrode. The breakage of the grounding electrode mentioned in the subsequent paper refers to this problem. This paper first analyzes the current distribution law of the DC grounding electrode feeder rod, then studies the surface potential distribution characteristics after the breaking of the local area of the grounding electrode.