At the end of last year, a 35kV busbar surge arrester explosion occurred at the 110kV Ma'anshan substation of Qujiang County Power Supply Bureau. The substation's main transformer is a three-winding transformer, and the 35kV system uses neutral-to-ground insulation. The 35kV busbar is equipped with one set of zinc oxide surge arresters and three JDJJ-type voltage transformers. The surge arrester model is HY5WZ-42/134, with a rated voltage of 42kV and a continuous operating voltage of 23.4kV. According to the substation operator, the 35kV system issued several grounding signals before the 35kV busbar surge arrester explosion. Our analysis of this accident, ruling out product quality issues and improper operation, suggests that the main causes were system vibration overvoltage due to 35kV line grounding, and the relatively low rated and continuous operating voltage values ​​of the zinc oxide surge arrester. The insulation level of electrical equipment in the neutral-point insulation system is designed based on line voltage. When a single-phase stable ground fault occurs in the system, the phase voltage rises to the line voltage. The rated voltage of the surge arrester in this incident, 42kV, is fully capable of withstanding this. In the event of a lightning strike, based on the residual voltage of the surge arrester at 5kA, its duration is very short (less than 0.1 seconds), so it should not be the primary cause of the surge arrester explosion. For a neutral-point insulated system, when a single-phase ground fault occurs, a capacitive current flows through the fault point, causing the phase voltage of the two ungrounded phases to rise to the line voltage. This forms a zero-sequence loop through the neutral-point grounding of the voltage transformer. Simultaneously, a displacement voltage is generated at the neutral point, becoming the equivalent power source for the oscillating voltage, and the electrical parameters of the zero-sequence loop change. The core inductance parameters of the voltage transformer change with the degree of core saturation. Once an arc ground fault occurs, a transient inrush current is generated in the system. During this instantaneous change, the excitation current of the two ungrounded phases of the high-voltage coil of the voltage transformer suddenly increases, potentially even saturating, and the inductance parameters change accordingly. Series resonance occurs when the equivalent inductive reactance of the core inductor in the zero-sequence circuit is equal to or close to the capacitive reactance of the system's distributed capacitance, i.e., ωL = 1/ωC. Because the extinction time of the grounding arc varies, the voltage transformer does not necessarily generate a large excitation current every time a single-phase ground fault occurs, so series resonance does not always occur. When nonlinear resonance occurs, the overvoltage amplitude can reach 2-3 times the phase voltage, and the duration is long. Moreover, while fundamental frequency resonance occurs, high-frequency and sub-frequency resonances may also be induced; the superposition of these two will cause even greater damage. The rated voltage of metal oxide surge arresters is an important parameter indicating their operating characteristics and is also an indicator of their ability to withstand power frequency voltage. In "AC Gapless Metal Oxide Surge Arresters" (GB11032-89), it is defined as "the maximum permissible effective value of the power frequency voltage applied between the arrester terminals." However, the ability of metal oxide varistors to withstand power frequency voltage is closely related to the duration of the applied voltage, and the duration of the applied voltage is not given in the definition. In this example, the rated voltage of this type of surge arrester is 42kV, which is clearly too low under resonant overvoltage conditions. Although according to the "AC Gapless Metal Oxide Surge Arresters" (GB11032-89), metal oxide surge arresters should have a certain power frequency overvoltage withstand capability, and the 10-second withstand time for neutral point insulated systems should not be less than the rated voltage, under resonant overvoltage conditions, the system voltage that the metal oxide surge arrester withstands far exceeds its rated voltage, and the withstand time is far more than 10 seconds. Continuous operating voltage is an important characteristic parameter of metal oxide surge arresters, greatly affecting operational reliability. In "AC Gapless Metal Oxide Surge Arresters" (GB11032-89), it is defined as the effective value of the power frequency voltage that is allowed to be continuously applied to the arrester terminals during operation. It should cover the highest power frequency voltage that may be continuously applied to the metal oxide surge arrester during power system operation. When resonance occurs, the overvoltage amplitude can reach 2-3 times the phase voltage, far exceeding the system's highest operating phase voltage. Equating the surge arrester's continuous operating voltage to the system's highest operating phase voltage (e.g., in this case, the continuous operating voltage of this type of surge arrester is 24.5kV) is clearly too low. In response to this incident, we believe that surge arresters should be selected with the highest possible rated voltage and continuous operating voltage. For example, the HY5WZ-51/134 surge arrester should be used, with a rated voltage of 51kV and a continuous operating voltage of 40.8kV. Furthermore, various measures should be taken to avoid system resonance and eliminate the adverse effects of excessively high system voltage on equipment.