Analysis of Key Influencing Factors on the Current-Carrying Capacity of ZnO Varistors Under Impulse Current Based on Finite Element Simulation

The neutral point of a 110 kV transformer is often protected by a discharge gap in parallel with a ZnO arrester for insulation coordination. The current-carrying capacity of the ZnO varistor is crucial for its ability to withstand overvoltage. Insufficient capacity can lead to arrester damage or transformer tripping under high-amplitude overvoltages like lightning. This paper first conducts the current-carrying capacity test of the varistor in a 110kV arrester using a dedicated test platform. Then, a finite element model of the ZnO varistor is built using COMSOL Multiphysics to study its Electro-Thermo-Mechanical behavior under two consecutive impulses. Finally, the impact of varistor size and multiple lightning strikes on its performance is investigated. Simulations show that under high-current impulses, the highest temperature occurs near the side surface due to the skin effect, with a maximum temperature difference of $42.776^{\circ} \mathrm{C}$ between the axis and the side. After impulse aging, both temperature rise and difference intensify. Increasing the varistor radius enhances its current-carrying capacity. Under similar energy absorption, failure in multiple lightning strikes is predominantly thermal runaway, whereas in a single strike, it is more often fracture. These findings provide valuable insights for varistor performance evaluation and failure analysis.