Effects of Oxidative Aging on Dielectric Strength of Natural Ester Oil Under DC Voltage Stresses

The developments in the research and design of HVDC transmission equipment prompted this investigation into determining the electrical strength of insulating oil under direct current (dc) voltage stresses. The manufacturers of oil-filled high-voltage equipment are continually tasked with developing novel insulating liquids with high dielectric strength, efficient heat transfer, and low environmental effects. As new energy sources are developed, these materials must function on dc. To ensure the long-term reliability of power equipment, it is essential to understand the degradation characteristics of insulating oil in long-term operations. Due to the limited literature on the behavior of aged insulating oil in direct voltage stresses, breakdown tests are conducted on a typical electrode arrangement in fresh and oxidatively aged mineral oil (MO) and natural ester oil (NEO) (FR3) in an effort to provide additional understanding on the dc breakdown voltage (DCBDV) of these oils. In this article, the statistical behavior of DCBDV experimental results is analyzed by using normal, two- and three-parameter Weibull distributions by subjecting the oils to thermal stresses at elongated aging hours of 90, 200, and 500 h. The Anderson-Darling test and goodness of fit are applied to verify the experimentally obtained DCBDV data to a specific statistical distribution. Results show that the breakdown strength of FR3 under DCBDV is higher than that of MO in both polarities after oxidative aging. However, with the increase in aging duration, the breakdown strength reduces for both the oil samples. It is observed that the majority of data also confirm to normal and Weibull distributions.