In this work the degradation of transformer mineral oil in oil-immersed ZnO-based surge arresters has been studied. Three different kinds of ZnO varistors: 1-non-coated, 2-resin-coated and 3-glass-coated, have been used in this study. It can be seen that after long-period high thermal constraints in oil there is a deposited layer on the varistors' surface which call disturb the function of the non-coated varistors. It is also observed that the ZnO varistor and its coating have an influence on degradation of the oil.
In this paper the AC ageing of ZnO-based varistors in transformer oil is studied by an accelerated ageing during 6000 hours. Three kinds of varistors have been studied: 1-non-coated, 2-coated by resin and 3-coated by glass . As well the thermal behaviour of varistors has been studied at different temperatures and voltages. It is found that the coating and environmental conditions have a great influence on the thermal behaviour and the lifetime of varistors. The oil improves their thermal behaviour. As well it improves the I(V) characteristics of the coated varistors while for the naked ones it causes a premature degradation. It is seen that glass could be, from the thermal point of view, a good material for coating the varistors.
Protection equipment based on metal oxide varistors is the most common solution today in the field of low voltage surge arrester applications. All ZnO based varistors are power electronic devices made of n-type semiconductor polycrystalline ceramics, having a high degree of non-linearity of current-voltage dependence. This paper presents an original algorithm applied for dimensioning low voltage varistors, used to protect the inverters located in photovoltaic plants. The algorithm consists in 3 steps, one for computing the height corresponding to the desired voltage, another for the determination of the diameter corresponding to a certain energy/volume absorption capacity, and, a third one which is a sort of safety test for conformity.