Conductivity measurements of electrical insulating oils

Mineral oils play - in combination with cellulose (paper and pressboard), a vital part in the insulation system of large transformers and bushings. The dielectric stress within such insulation systems during high voltage direct current (HVDC) network operation is defined by a mixed stress that consists of AC, DC and transient components. Exemplary, the valve winding of converter transformers is to be named here. Profound knowledge of materials parameters like, for example, electrical conductivity, is crucial for a safe and reliable design of HVDC equipment. First of all, the electrical conductivity of insulating oil is a material parameter and differs with oil type and manufacturer. These differences can reach easily one or even several orders of magnitude. Furthermore, the electrical conductivity is strongly dependent on temperature and on electrical field strength. The electrical field strength differs generally between material measurements and actual operating conditions. Finally the electrical conductivity can also vary with ageing, generally it will increase with advancing ageing. These influences are all together very inconvenient for the design engineer as the electrical field stress within an insulation system can change significantly during the operational life cycle of electrical equipment. For this work, measurements of electrical conductivity have been made for insulating (mineral) oils with test cells according to the IEC 60247 standard. Two different oils have been tested. The temperature has been varied between room temperature (22°C) and 90°C. It could be demonstrated, that there is a strong influence of the oil type and oil temperature on the electrical conductivity.