The electrical model and harmonic characteristics of ZnO varistor considering unipoilar impulse aging polarity effect

•An improved electrical model of ZnO varistor considering the polarity effect caused by unipolar impulse aging was proposed.•Simulation results based on the proposed model are in a good agreement with those by the tests.•The improved electrical model provides high accuracy in the researches of the aging criteria of arresters.•The even-order harmonic characteristics can be considered as the impulse aging criteria of varistors. The frequent and high amplitude unipolar impulse currents during the process of switching-on AC filter banks can accelerate the aging of arresters in some ultrahigh-voltage direct-current (UHVDC) and HVDC transmission systems, and result in polarity effect of zinc-oxide (ZnO) varistors. Hence, an improved ZnO varistor electrical model considering the polarity effect caused by unipolar impulse aging was proposed under power frequency voltage. Accelerated aging test under a negative polarity 15/35 µs impulse current was conducted on ZnO varistor samples to validate the proposed model. Furthermore, the current waveform and harmonic characteristics of the leakage current and its resistive component were simulated. Comparative analysis revealed that the simulation results were generally consistent with the test results. Based on the results, it's inferred that the variation of even-order harmonics are attributed to the polarity effect, and can be considered as one of the aging criterion of arresters.