Statistical investigation on the electrical responses of ZnO-based varistors to different impulse currents

The electrical responses of commercial ZnO-based varistors from the same batch under 4/10 μs, 8/20 μs and 2 ms square-wave impulse currents were investigated with a focus on the statistical characteristics of their residual voltage. Two types of experiments were carried out, including single and repeated impulse currents. For the single-impulse experiment, it was found that under all three types of impulses, the standard deviation (3σ) of the residual voltage exhibited two-stage dependence on the impulse energy, i.e., it was flat under a critical energy and then increased sharply. The increase of 3σ could not be attributed to the electrical degradation, which is confirmed by the repeated impulse experiment. Instead, it was suggested that it depended on the impulse waveform, microstructural disorder and the thermal effect, implying an intrinsic property of ZnO-based varistor ceramics. In fact, it was shown that although the pre-breakdown property deteriorated gradually with repeated impulses, 3σ under high current region remained nearly unchanged, which is due to the different conduction mechanisms. Under high current region, the conduction is ZnO-grain controlled. Therefore, the grain resistance, its distribution and temperature dependence are of importance to determine the current path as well as 3σ. In addition, the failure modes of samples under impulse currents were also investigated. It was found that under 8/20 μs and 4/10 μs impulse currents, flashover was the main destruction mode; while puncture and cracking dominated under 2 ms square-wave impulse current.