Influence of electrode material on conductivity measurements under DC stresses

The optimization of design and operation of converter transformers during test and operation needs understanding of insulation system behavior under AC, DC and mixed stress. In order to calculate the electrical field under all kinds of stresses, the material properties have to be known. The common metals exposed to noteworthy electric stress inside power transformers are copper (conductors) and aluminum (shields). Stainless steel is a material which is often used in dielectric material test arrangements. In typical arrangements bare metals as well as metals covered with insulators are found. In this contribution the influence of different bare electrode materials onto the conductivity values of mineral oil and onto their function of time is investigated. The behavior of each electrode material is also investigated at different measuring temperatures which shall be representative for the conditions of insulation during transformer operation.