The Degradation of ZnO Varistors Packed with Epoxy Insulation Layers

Li, J, Li, S, Alim, M. A. and Chen, G (2003) The Degradation of ZnO Varistors Packed with Epoxy Insulation Layers. The 2nd International Conference on Insulation Condition Monitoring of Electrical Plant, , Chongqing, China. 27 - 30 Oct 2003. pp. 137-140 .

Abstract

Zinc oxide (ZnO) varistor is a highly non-linear semiconducting device that is widely used as a transient voltage surge protector in electrical and electronic systems. One of the major challenges in the continuous development of varistors has been to reduce the long-term degradation. The degradation can take a number of forms when the varistor is subjected to a series of pulses or to a constant applied dc or ac voltage. There will be a distinct asymmetry in the I-V characteristics once the degradation has taken place, with the leakage current being greater in both directions of the applied field. Because varistors are usually operated under the voltage, any increase in leakage current is of major importance. This is especially so for the station-class varistors that are used in power distribution systems, because the power dissipated within the varistor represents a loss in distributable power. Under extreme conditions, particularly at elevated temperatures, the increase in leakage current can be sufficient for the associated Joule heating to cause thermal runaway of the varistor.

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