STUDY OF THE ZNO-BASED VARISTORS BEHAVIOUR IN MINERAL OIL FOR REALIZATION OF AN OIL-DAYERSED ZNO SURGE-ARRESTER

The existing surge-arresters exhibit some problems because of the environmental atmosphere pollution. Since a few years there is an interest for installing the Zno-based varistors inside the transformer. By this methode we shall be able to eliminate these problems and also to have a shorter distance between the arrester and the transformer winding, which can provide a better protection. Electrical and thermal behaviours of ZnO-based varistors in air have been already well studied. In this work their behaviour in mineral oil (naphtenic hydro-carboris) is studied. An ageing process on three types of varistors : I-non-coated, 2-coated by resin and 3-coated by glass is accomplished. Tlfis ageing has been realized during 6000 hours in two circumstances : Firstly, with application of a power frequency alternative voltage with an r.m.s. value equal to 60% of the threshold voltage (V1mA: DC voltage when DC current of 1mA flows, at 20*C to the varistors at 100*C and secondly without any polarization but at the same temperature. As well, high amplitude current impulses of 65KA,5/15 pS have been applied to the varistors in oil. By means of these experiments we have studied the behaviour of three types of ZnO varistors in oil. The thermal exchange coefficient between varistor and ambient atmosphere is also measured both in oil and air. It can be seen that the oil improves the the thermal and electrical performances of the coated varistofs, while it degrades the non-coated ones.