Study on the Unipolar Impulse Aging of ZnO Varistors and Their Condition Monitoring Methods Based on the Basic and Even-Order Harmonics of the Leakage Current Resistive Component

The occurrence of frequent high impulse currents during the process of switching-on AC filter banks can accelerate the aging of arresters in ultrahigh-voltage direct-current (UHVDC) and HVDC transmission systems, threatening the safe operation of converter stations. Therefore, impulse aging tests with different current amplitudes were performed on zinc oxide (ZnO) varistors under a negative polarity 15/35 μs impulse current to provide a basis for the aging evaluation criteria of AC filter arresters in UHVDC and HVDC transmission systems. The ratios of even-order to basic resistive harmonic components linearly increase with the number of impulse groups and suddenly decrease after complete damage occurs. The variation trends of the ratios of even-order to basic resistive harmonic components were analyzed and discussed based on the current waveform and amplitude-frequency characteristics of the resistive component and the DC V-I characteristics. Based on the results, a new criterion for the unipolar impulse aging of the ZnO varistor is proposed, i.e., the ratio of second to basic resistive harmonic components, and the application of ZnO varistor even-order harmonic characteristics in online monitoring is briefly discussed.