Breakdown Characteristics of Synthetic Ester Oil Considering Electrode Configurations and Large Gap Distances Under Positive Lightning Impulse

Ester oils have many advantages such as high fire point, high water saturation, and biodegradability compared to mineral oil. The application of ester oils is expanding to high-voltage transformers with the increasing demand for environmentally friendly power equipment. Therefore, it is essential to investigate the dielectric characteristics of ester oils considering large gap distances for the development of environmentally friendly high-voltage transformer. However, research into the breakdown characteristics of ester oil considering large gap distances is insufficient. In this paper, the breakdown characteristics of the synthetic ester (MIDEL 7131) were investigated according to electrode configurations and gap distances. The breakdown tests were performed using rod-rod (R1-R1) and rod-plate (R1-P) electrode configurations with a gap distance of 50–150 mm under a positive lightning impulse voltage. In addition, the influence of tip radius (1 mm, 2 mm, 3 mm) on the breakdown characteristics in rod-plate electrode configuration was investigated. From the experiment results, it was confirmed that the breakdown voltage of MIDEL 7131 increased by 29-68% as the gap distance increased. The dielectric strength of MIDEL 7131 was 13-47% lower than mineral oil at the same gap distance. In addition, it was verified that the dielectric strength of MIDEL 7131 improved by over 130% according to increment of tip radius. The influence of tip radius on the breakdown characteristics was significant in MIDEL 7131, while it was relatively insignificant for mineral oil.