Influence of Thermal Aging on DC Conductivity and Breakdown Strength of Natural Ester Oils for HVDC Applications

This study investigates the influence of accelerated thermal aging and degradation on the breakdown strength of natural ester oil (NEO-FR3) compared to mineral oil (MO) under dc voltage stresses. The base oils undergo accelerated thermal stresses overextended aging periods of 90, 200, and 500 h. The primary focus is to understand the breakdown phenomena influenced by various oil properties. To ensure the reliability of the results obtained, statistical hypothesis testing is performed to evaluate the repeatability by analyzing the statistical patterns in direct current (dc) breakdown voltage (DCBDV) values derived from the experimental data. The experimental data are validated and modeled by employing a three-parameter Weibull distribution. The study reveals that natural ester oil (FR3) exhibits minimal variation in breakdown voltage (BDV) under thermal aging and maintains higher BDV than MO under both polarities. However, increasing the aging duration decreases the breakdown strength for both oil samples. The majority of the data conforms to Weibull distributions, confirming the reliability of the findings. Moreover, the study explores dc conductivity through frequency domain spectroscopy (FDS), investigating the effects of thermal aging on conductivity and overall dielectric strength. The analysis uncovers distinct differences in the aging characteristics of MO and NEO-FR3. It is seen that FR3 exhibits a significant rise in dc conductivity over time, while its BDV does not decrease proportionally. On the other hand, MO is less affected by changes in conductivity but is more prone to BDV, especially when subjected to negative polarities.