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

This study investigates the influence of accelerated thermal ageing 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 over extended ageing periods of 90, 200, and 500 hours. 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 DC breakdown voltage (DCBDV) values derived from the experimental data. The experimental data are validated and modelled by employing 3-parameter Weibull distribution. The study reveals that FR3 exhibits minimal variation in breakdown voltage (BDV) under thermal ageing and maintains higher BDV than MO under both polarities. However, increasing the ageing 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 ageing on conductivity and overall dielectric strength. The analysis uncovers distinct differences in the ageing characteristics of MO and NEO-FR3. It is seen that FR3 exhibits a significant rise in DC conductivity over time, while its breakdown voltage does not decrease proportionally. On the other hand, MO is less affected by changes in conductivity but is more prone to breakdown voltage, especially when subjected to negative polarities.