Research On Failure Mode Of Zinc Oxide Varistor Based On Multiple Aging Model

As an important part of modern lightning protection technology, Surge protective devices always suffer from aging deterioration problem that leads to serious consequences. There are imperfection in the recent studies and the current national standard. Dynamic judgment of aging deterioration of SPD cannot accurately be given. So we cannot replace the broken SPD appropriately, left us great hidden troubleTo solve the problem that the research on heat dissipation potential of the Metal Oxide Varistor(MOV) in the thermal fusing and thermal degradation process strongly depends on actual experiment in the laboratory which contains a large number of human factors, Transient Thermal Impedance(TTI) parameters as well as the Transient Thermal Impedance Model(TTIM) were put forward based on contrast experiment and the research on heat dissipation potential of the Metal Oxide Varistor chips. The influence of chip diameter and thermal fusing current to heat dissipation potential of the Metal Oxide Varistor chips was analyzed in the paper. Experimental results show that the heat loss of Metal Oxide Varistor chips in the thermal fusing and thermal degradation process, which changes with time as well as temperature is not only associated with chip diameter but also associated with the current applied through the Metal Oxide Varistor chips. Experimental results also show that with the reduction of the potential barrier height and the development of the thermal melting and fusing process, the heat dissipation potential of the Metal Oxide Varistor chips increases in a significant way. The Transient Thermal Impedance Model put forward in this paper, which can quantitatively reflect the heat dissipation potential of the Metal Oxide Varistor chips, was verified by the experimental results. Theoretical basis of the research on heat dissipation potential was established.Impact aging experiment based on the tolerance of Zinc Oxide varistor under continuous Power Frequency overvoltage was designed to solve the problem that the change of breakdown voltage is too small to tell the aging degree of Zinc Oxide varistor chip. Different situations of Impact aging and the tolerance of Zinc Oxide varistor under different amplitude of overvoltage were studied, the relationship between the internal current during the tolerance (Iin) and the time is given. Experimental result was studied with Dual Schottky barrier theory and Voronoi grid model. The result shows that the larger the aging degree impacted by 8/20μs current pulse is,the larger the initial value of Iin is; The increased rate of over time is proportional to the initial aging degree; the tolerance time is inversely proportional to the aging degree of Zinc Oxide varistor chip; the larger the amplitude of overvoltage is, the shorter the tolerance time is when the aging situations are equal.To solve the problem that Metal Oxide Varistor(MOV)suffer from overvoltage in the actual work environment, a impact capability tolerance experiment with Transient over voltage pretreatment was designed. The impact capability under different values of AC transient overvoltage for 5 seconds was analyzed. The result shows that the breakdown voltage of MOV will rise after the pretreatment as well as the number of times that tolerance of 8/20μs current impact wave. After the pretreatment, the breakdown voltage of MOV has a "decrease-increase-decrease" tendency during the subsequent impact process. That is very different from the same tendency without the pretreatment. The Zinc Oxide grain boundary barrier height change process under AC transient overvoltage was obtained, using the Dual Schottky barrier conductive theory and has an experimental verification.