Study on moisture influence on electrical conductivity of natural ester fluid and mineral oil

HVDC equipment is normally stressed with AC, DC and super-imposed stresses during its operation time. Contrary to AC field distribution, DC field stresses are determined by the conductivity of the insulating materials, which consist of insulating liquid and cellulose-based transformer board. As a result, a deep knowledge of the permittivity and conductivity of barrier-oil insulation materials are necessary for construction of the reliable insulation systems. To be considered as viable insulating fluids for HVDC equipment, the electrical oil conductivity of alternative dielectric liquids should thoroughly be investigated dependent on different parameters, which affect the behavior of oil conductivity. One of the parameters influencing electrical conductivity is moisture content of oil. Water appears in transformers as an unwanted substance affecting transformer life dramatically and leading to a decrease of dielectric strength of insulation. Besides this effect of the moisture, it is also essential to know if the moisture content of insulating liquid has an influence on its electrical conductivity. In this contribution, a fundamental study of electrical conductivity of Envirotemp™ FR3™ fluid is presented in term of water content variation of oil samples. The measurements are established in comparison with mineral oil used as conventional insulating liquid in HVDC converters. The effect of moisture during conductivity measurement is studied at different field strengths and two different measurement temperatures. The measurement results show that the rate of field strength dependency for oil conductivity is strongly dependent on level of moisture content for both investigated liquids.