Based on years of experience in substation operation and management, lightning strikes account for over 80% of all lightning strike accidents in rural power systems, especially in mountainous areas where the rate can exceed 90%. Therefore, lightning protection for rural power grids must be emphasized during rural power grid upgrades to ensure safe power supply and improve the reliability of rural power grids. 1. Installation of Surge Arresters on the High-Voltage Side During rural power grid upgrades, metal oxide surge arresters should be installed on the 10kV side of distribution transformers. The closer the arrester is to the transformer, the better the protection effect; generally, it should be installed inside the drop-out fuse. The residual voltage of the surge arrester must be less than the withstand voltage of the distribution transformer to effectively protect it. The grounding terminal of the surge arrester should be directly connected to the metal casing of the distribution transformer. It is not permitted to independently ground the surge arrester via a down conductor. This is because the residual voltage of the surge arrester is only 17kV to 50kV, meaning its equivalent resistance under impulse is only 3.4Ω to 10Ω. However, the grounding resistance of an independently grounded circuit might be around 10Ω, and even higher in rural mountainous areas. Therefore, when lightning current flows through, the potential might be quite high. If the surge arrester is independently grounded, the potential from both the surge arrester and the grounding resistance is superimposed before being applied to the transformer, potentially damaging the transformer insulation. If the surge arrester's grounding terminal is directly connected to the transformer's metal casing, the potential does not act on the transformer's insulation, making the insulation safer. However, the potential of the transformer's metal casing will be very high (equal to IR), potentially causing an inverted voltage from the transformer's metal casing to the low-voltage side. Therefore, the neutral point on the low-voltage side must also be connected to the transformer's metal casing. This connection method is called three-point (the grounding terminal of the high-voltage surge arrester, the neutral point of the low-voltage winding, and the transformer's metal casing) joint grounding. When the transformer capacity is 100kVA or higher, the grounding resistance should be reduced to below 4Ω; when the transformer capacity is less than 100kVA, a grounding resistance below 10Ω is sufficient. When the three points are connected together and grounded, when lightning strikes the high-voltage side and the surge arrester discharges, the transformer insulation is subjected to the residual voltage of the surge arrester, and the voltage drop on the grounding device does not act on the transformer insulation. This is beneficial to the transformer protection and can reduce the risk of insulation breakdown between the high-voltage winding and the transformer metal shell. In order to prevent the instantaneous rise of the neutral point potential on the low-voltage side of the transformer from affecting the safety of users, an auxiliary grounding wire (repeated grounding) can be installed near the user. Transformers protected in this way will still have some lightning damage accidents during operation. This is because the general distribution transformer does not have a low-voltage surge arrester installed on the low-voltage side. At this time, not only will damage occur on the low-voltage side, but damage will also occur on the high-voltage side. The damage mechanism is three: (1) Lightning strikes the low-voltage line directly or the low-voltage line is subjected to induced lightning, which damages the insulation on the low-voltage side. (2) Lightning strikes on the low-voltage side damage the insulation on the high-voltage side. This is because, through electromagnetic coupling, an overvoltage proportional to the transformer turns ratio (forward transformation process) will also appear on the high-voltage side winding. Since the insulation margin on the high-voltage side is smaller than that on the low-voltage side, it may cause insulation damage on the high-voltage side. (3) Lightning strikes the high-voltage line directly or the high-voltage line is subjected to induced lightning. At this time, the surge arrester will operate, generating a voltage drop on the grounding resistance. This voltage drop will act on the neutral point on the low-voltage side, and the low-voltage side outgoing line is equivalent to being grounded through the conductor wave resistance. Therefore, most of the high potential generated on the grounding wire is applied to the low-voltage side outgoing line. 2 Simultaneously, install surge arresters on the low-voltage side. To solve the above problems, low-voltage surge arresters can be installed on the low-voltage side. With low-voltage surge arresters, the overvoltage value appearing at both ends of the low-voltage winding can be limited, and the high-voltage winding can generally be protected during forward and reverse transformation processes. Especially in high-altitude areas, lightning often strikes on hillsides, sometimes even lower than power lines. During operation, the low-voltage side of the lightning strike or the impact of reverse surges can easily cause insulation breakdown accidents in distribution transformers. Therefore, in the design and installation of distribution transformers in rural mountainous areas, it is required to install a set of breakdown fuses or low-voltage surge arresters on the low-voltage outgoing side to protect the secondary windings of the transformer and to prevent damage to the high-voltage winding insulation when overvoltage surges reach the low-voltage windings. It is evident that installing low-voltage surge arresters or breakdown fuses on the low-voltage side is necessary, especially in rural areas prone to lightning. Sometimes, after a lightning strike, the insulation breakdown between the transformer turns is only a partial discharge, allowing it to continue operating with difficulty. However, after a period of time, the fault expands, forcing a shutdown. This is an accident caused by partial insulation breakdown during thunderstorms, which expands in clear weather, often creating the illusion that the damage was not caused by lightning.