Research on the Electrical Properties and Microstructural Characteristics of ZnO Varistors Under Impulse Current

Zinc oxide (ZnO) varistors are a core component of surge arresters; their failure can directly affect the secure and reliable operation of power equipment. Therefore, this paper conducts an impulse degradation test on ZnO varistors, combining electrical and microstructural tests to systematically explore the intrinsic correlation mechanism between the electrical properties and microstructural characteristics. Test results show that this type of ZnO varistor is susceptible to side-glaze surface flashover under an impulse current with a waveform of 8/20 μs and an amplitude of 27 kA, and the discharge branches exhibit an extension from the negative electrode towards the positive electrode. Moreover, surface flashover causes the formation of local conductive channels in the side glaze layer, resulting in a significant drop in the direct-current (DC) reference voltage U1mA. However, the residual voltage U10kA increases slightly with an increase in the number of impulse groups, with a change in amplitude of less than 1.5%. Additionally, the microstructural testing reveals that the impulse currents cause the bismuth (Bi) element in ZnO grains to precipitate and form more Bi-rich phases at the grain boundaries. This results in an increase in the thickness of the grain boundary layer, which is negatively correlated with the U1mA. Meanwhile, the grain morphology and size distribution of brand-new samples, samples with different degrees of degradation, and samples with side-glaze surface flashover damage are not significantly different. This is consistent with the fact that the change range of the residual voltage U10kA during impulse degradation is very small. This test phenomenon indicates that the failure of this type of ZnO varistor to withstand an impulse current with a waveform of 8/20 μs and an amplitude of 27 kA is mainly due to changes in the volt-ampere properties of the small-current regions caused by ion migration within the grain boundary layer. This research provides an experimental basis and theoretical support for improving the impulse withstand capacity of ZnO varistors in their design.