The influence of the moisture during the electrical conductivity measurement on the high density impregnated pressboard
The knowledge of DC behavior of the insulating system is going to be more and more important because of new applications in field of converter transformers. The DC field stresses are determined by means of insulating material conductivity which is normally a combination of oil, paper and pressboard. One of the parameters which has an influence on the electrical conductivity is moisture content of the insulation material. Water appears in transformers as an unwanted substance which affects transformer life dramatically and leads to decreasing of dielectical strength of insulation. Besides this effect of the moisture, it is also interesting to know if the moisture content of solid insulation has an enormous influence on its electrical conductivity because the variation of electrical conductivity moderates also electric field distribution in insulation system. So, this contribution deals with the conduction mechanisms in oil-impregnated high density pressboard and their dependency on water content of the pressboard samples at DC stresses. The investigation shows that a variation of the moisture up to 0.8% has no observable influences on the conductivity of impregnated pressboards. It is also worthwhile to mention that an exponential increase of the electrical conductivity with moisture is observed for moisture level of higher than 0.8%.