AGING STUDY OF ZNO-BASED VARISTORS UNDER ELECTRICAL COMBINED STRESSES FOR DC SYSTEMS ARRESTERS APPLICATION

This paper presents an experimental study on the advantage of the ZnO-based varistors for the protection of de railways network against the transient surges (lightning strikes, switching and temporary overvoltage). New tools of degradation analysis for ZnO-based varistors were used in this work. They consist to measure both classical current-voltage characteristics, and ac impedance under electrical combined stresses for de systems arresters application. It shows that the varistor electrical properties, relaxation time constant tau, distribution function F(tau), and heterogeneity factor beta change with applied stresses. The principal results presented in this paper are: a description of the experimental apparatus employed for the combined tests the characteristics before, during and after the ageing an attempt for interpretation of the characteristics. The evaluation of this degradation is obtained by two methods: measurement of the current-voltage I(U) characteristics measurement of complex impedances which estimate the degree of homogeneity of the potentiel barriers accounting for the non-linearity. A correlation between the two methods is made.