1 Introduction China has 2,400 county-level rural power grids and 280 urban power grids, with millions of distribution transformers. Given China's vast territory and frequent thunderstorms (100-130 thunderstorm days annually in some southern regions), distribution transformers are severely affected by lightning strikes. This not only causes significant economic losses to power supply companies but also seriously impacts power supply reliability. Therefore, to prevent lightning strikes from damaging distribution transformers and ensure their safe operation, it is necessary to analyze lightning protection measures for each transformer and selectively adopt appropriate lightning protection measures. 2 Lightning Protection Measures for Distribution Transformers (1) Install surge arresters on the high-voltage side of the distribution transformer. According to SDJ7-79 "Technical Specification for Overvoltage Protection Design of Power Equipment," the high-voltage side of the distribution transformer should generally be protected by surge arresters. The grounding wire of the surge arrester, the neutral point on the low-voltage side of the transformer, and the metal casing of the transformer should be connected together for grounding. This is also the lightning protection measure recommended in the Ministry-issued DL/T620-1997 "Overvoltage Protection and Insulation Coordination of AC Electrical Installations". However, extensive research and operational experience have shown that even when surge arresters are used only on the high-voltage side, damage still occurs under the influence of lightning surges. The annual damage rate is generally 1% in most areas, reaching about 5% in areas with frequent lightning strikes, and as high as 50% in some areas with exceptionally strong lightning activity during 100 thunderstorm days. The main reason for this is the forward and reverse transformation overvoltage caused by lightning surges entering the high-voltage windings of the distribution transformer. The mechanism of forward and reverse transformation overvoltage is as follows: ① Reverse transformation overvoltage. When a lightning surge enters the 3-10kV side, causing the surge arrester to operate, a large amount of impulse current flows through the grounding resistance, generating a voltage drop. This voltage drop acts on the neutral point of the low-voltage winding, raising the neutral point potential. When the low-voltage line is relatively long, the low-voltage line is equivalent to wave impedance grounding. Therefore, under the influence of the neutral point potential, a large inrush current flows through the low-voltage winding. The inrush currents flowing through the three-phase windings are in the same direction and equal in magnitude. The magnetic flux they generate induces an extremely high pulse potential in the high-voltage winding according to the transformer turns ratio. The three-phase pulse potentials are in the same direction and equal in magnitude. Since the high-voltage winding is connected in a star configuration and the neutral point is not grounded, although there is a pulse potential in the high-voltage winding, there is no inrush current. The inrush current only flows in the low-voltage winding, and there is no corresponding inrush current in the high-voltage winding to balance it. Therefore, the inrush current in the low-voltage winding becomes entirely a magnetizing current, generating a large zero-sequence magnetic flux, which induces a very high potential on the high-voltage side. Since the potential at the high-voltage winding output terminal is fixed by the residual voltage of the surge arrester, this induced potential is distributed along the winding, with the largest amplitude at the neutral point. Therefore, the neutral point insulation is prone to breakdown. At the same time, the potential gradient between layers and between turns also increases accordingly, which may cause interlayer and interturn insulation breakdown in other parts. This overvoltage is first caused by the high voltage incoming wave, and then induced by the low voltage electromagnetic induction to the high voltage winding. This is usually called reverse transformation. ② Positive transformation overvoltage. The so-called positive transformation overvoltage means that when the lightning wave enters from the low voltage line, there is an impact current in the low voltage winding of the distribution transformer. This impact current also generates an induced electromotive force on the high voltage winding according to the turns ratio, which greatly increases the neutral point potential on the high voltage side, and the gradient voltage between layers and between turns also increases accordingly. This process of generating induced overvoltage on the high voltage side due to the low voltage incoming wave is called positive transformation. Experiments show that when the low voltage incoming wave is 10kV and the grounding resistance is 5Ω, the gradient voltage between layers on the high voltage winding sometimes exceeds the full wave impact strength of the interlayer insulation of the distribution transformer by more than twice. In this case, the interlayer insulation of the transformer will definitely break down. (2) Install ordinary valve-type surge arresters or metal oxide surge arresters on the low voltage side of the distribution transformer. The wiring of this protection method is as follows: the grounding wires of the high and low voltage surge arresters of the transformer, the neutral point of the low voltage side and the metal shell of the transformer are connected together for grounding (or three points are connected together). Operational experience and experimental research show that for distribution transformers with good insulation, lightning damage accidents caused by forward and reverse transformation overvoltages can still occur even when surge arresters are installed only on the high voltage side. This is because the surge arresters installed on the high voltage side are powerless against forward or reverse transformation overvoltages. The interlayer gradient under the action of forward and reverse transformation overvoltages is proportional to the number of turns of the transformer and is related to the distribution of the windings. The beginning, middle and end of the windings may be damaged, but the end is more dangerous. Installing surge arresters on the low voltage side can limit the forward and reverse transformation overvoltages to a certain range. (3) Protection method with separate grounding of high and low voltage sides. The wiring of this protection method is that the surge arrester on the high voltage side is grounded separately, the surge arrester on the low voltage side is not installed, the neutral point of the low voltage side and the metal shell of the transformer are connected together and grounded separately from the high voltage side grounding. Studies have shown that this protection method can basically eliminate reverse transformation overvoltage by utilizing the attenuation effect of the ground on lightning waves. For forward transformation overvoltage, calculations show that when the grounding resistance on the low-voltage side is reduced from 10Ω to 2.5Ω, the forward transformation overvoltage on the high-voltage side can be reduced by about 40%. If the grounding body on the low-voltage side is properly treated, the forward transformation overvoltage can be eliminated. This protection method is simple and economical, but it has high requirements for the grounding resistance on the low-voltage side and has certain promotion value. There are various lightning protection measures for distribution transformers. In addition to those listed above, there are also measures such as adding a balancing winding to the core of the distribution transformer to suppress forward and reverse transformation overvoltages and installing metal oxide surge arresters inside the distribution transformer. 3 Application of lightning protection measures for distribution transformers Through the above analysis, it can be seen that various lightning protection measures have their own characteristics. Each region should rationally select appropriate lightning protection measures according to the intensity of lightning activity on thunderstorm days. (1) In plains and other areas with few lightning, the annual damage rate of distribution transformers is low, and only the method of installing surge arresters on the high-voltage side of the distribution transformer can be adopted. (2) In areas with frequent thunderstorms, it is recommended to install surge arresters on both the high-voltage and low-voltage sides of the distribution transformer. (3) In areas with frequent thunderstorms, a single lightning protection measure is often ineffective. A comprehensive lightning protection measure is recommended, i.e., a surge arrester with separate grounding on the high-voltage side, and separate grounding on the low-voltage side, including the surge arrester, the neutral point on the low-voltage side, and the metal casing of the transformer. (4) In areas with severe thunderstorms, especially those with a high annual damage rate of distribution transformers, if comprehensive lightning protection measures still do not achieve good lightning protection results, a balancing winding should be added to the core of the distribution transformer (i.e., a new type of lightning arrester) or a metal oxide surge arrester should be installed inside the distribution transformer, based on a technical and economic comparison. 4 Conclusion There are various lightning protection measures for distribution transformers, and the actual conditions of distribution transformer installation locations vary from place to place. Adapting to local conditions, rationally selecting lightning protection measures, and emphasizing and strengthening the operation and management of distribution transformers will certainly improve the effectiveness of lightning protection for distribution transformers.