THERMAL MODELING OF ZINC-OXIDE VARISTORS - SIMULATIONS AND MEASUREMENTS

The surge arresters currently used for protection of electrical equipments against transient overvoltages are based on ZnO-based varistor. Indeed, these polycrystalline ceramics offer remarkable characteristics such as a capability of absorbing high energy, a very short response time and a strongly non linear current voltage characteristic. These properties are exploited for minimizing the overvoltage level. The resistivity of a varistor under an electrical shock drops sharply. This gives rise to an absorption of excessive energy which causes heating of the element. Moreover in the usual applications, the varistors are subjected to a permanent voltage, the resulting leakage current which strongly depends on the temperature. The heat generated leads to a thermal runaway as soon as the equilibrium between the dissipated energy (especially by convection) and the Joule heating, is disrupted. Consequently a study of thermal behaviour and its influence on the electrical phenomena is fundamental to the dimension of the surge arrester. We have undertaken a simulation of the electro-thermal behaviour of varistors in the dynamic regime in order to analytically determine the operating point and the limiting conditions of stability. Besides, a comparison of the simulated values with the measured one has allowed us to determine a certain number of physical parameters and thus confirm the validity of our approach.