Comparison of Aging Effect of Ester Liquids and Mineral oil in Semi-Uniform Field Geometry under Lightning Impulse Voltage and Standard Compliant AC-Voltage Testing

Breakdown and aging properties of five insulating liquids have been discussed and compared. Additionally, the characteristics in their properties under different voltage polar-ities were analyzed to study the effect of aging on the polarity effect under LI voltage in semi-uniform field. These finding were compared to standard compliant AC breakdown test. After 2330 h and 4350 h, respectively, of aging a change in the aging markers water content (absolute and relative), total acid number and color was seen in all liquids. Viscosity increased only in natural esters up to 10 %. The aging related increase in water content was only alarming for mineral oil, being above 20 %. The effect of aging on the breakdown voltage differed depending on the voltage type and polarity applied. Ageing had the least effect with negative lightning impulse voltage. For the synthetic ester MIDEL 7131 strongest reduction in breakdown voltage was seen under positive lightning impulse voltage. It decreased more than 24 %, from a breakdown voltage above 560kV to 428kV. The change in breakdown voltage in mineral oil amounted to only 3 %. For all other oils decrease of breakdown voltage due to aging accelerated degradation was most pronounced under AC voltage. With the natural esters the decline amounted to 17 %, with the synthetic esters to 26 % and a 38 % decrease of AC breakdown voltage was observed in mineral oil.