Computer Simulation of ZnO Varistors Failures
- PDF / 411,061 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 20 Downloads / 225 Views
ABSTRACT A simple thermo-mechanical model is applied to evaluate the influence of the nonuniformity of ZnO varistor disks used in surge arresters on their energy handling capability. By solving heat transfer equations for a varistor disk with nonuniform electrical properties, we compute the time dependence of the temperature profile and the distribution of thermal stresses. The model can identify the energy handling limitations of ZnO varistors imposed by three different failure modes: puncture, thermal runaway, and cracking. It conforms to the available failure data, and explains the observation that energy handling improves at high current densities. INTRODUCTION Zinc oxide varistors are multi-component ceramic devices produced by sintering ZnO powder together with small amounts of other oxides. Highly nonlinear current-voltage (IV) characteristics of ZnO varistors are used in electrical surge arresters. They protect electrical equipment from damage by limiting overvoltages and dissipating the associated energy. Therefore, the energy handling capability is crucial. It is defined as the amount of energy that a varistor can absorb before it fails. There are three main failure modes of varistor elements: thermal runaway, puncture, and cracking. The leakage current, and consequently the Joule heating of a varistor, increase with temperature. Thus, if the temperature is raised above the thermal stability temperature T8 , power input may exceed heat dissipation, and thermal runaway occurs. In puncture, a small hole results from melting of the ceramic where high current is concentrated [1]. Nonuniform heating can also cause thermal stresses higher than the failure stress of the material and can lead to cracking [1, 2]. Currents in the nonlinear region of the I-V characteristics tend to concentrate into narrow paths. This current localization has been detected by applying small spot electrodes on the surfaces of varistors, by using infrared cameras [3, 4] and by electroplating techniques [5]. Measurements of the energy handling capability of varistors have been reported [1, 6, 7], but the nature of the failures is not well understood. Puncture has been studied by Eda [1], who showed that at high currents, a hot spot may reach about 800 TC and cause local melting. However, two important factors were omitted by Eda: (1) the influence of the
253 Mat. Res. Soc. Symp. Proc. Vol. 500 ©1998 Materials Research Society
upturn in the I-V characteristics, and (2) that thermal stresses may cause cracking before a puncture can take place. ZnO varistors exhibit a complex dependence of the energy handling capability upon pulse magnitude and duration. Initially the energy handling capability decreases with increasing current, but, as first pointed out by Sakshaug et al. in [6], it increases again if the current becomes very high and the pulse duration becomes very short. It is the main purpose of the present paper to provide a fundamental explanation of the phenomenon. The explanation is of added interest because Ringler et al. [7] h
Data Loading...
