Effect of Molecular Structure on the Difference of Lightning Impulse Discharge Between Mineral Oil and Natural Ester Using DFT Calculation

The lighting impulse breakdown performance of natural ester (NE) is lower than that of the traditional mineral oil, which is a key point that needs to be considered in high-voltage level and large capacity transformers. The micromolecular structures of NE and mineral oil are quite different, which leads to significant differences in macroperformance. Therefore, molecular models of two types of oil were constructed in this article, and density functional theory (DFT) was used to calculate molecular microscopic parameters, such as ionization potential (IP) and polarization properties under the action of electric fields (EFs) with different directions. The results show that the IP of the NE is smaller than mineral oil, and the IP of the two oils continues to decrease with the increasing EFs, which is mainly because the highest occupied molecular orbital (HOMO) energy level of NE is significantly higher than that of mineral oil under EFs. Therefore, the lightning impulse breakdown voltage of NE will be lower than that of mineral oil, and the electron density difference (EDD) and atomic force of NE are greater than mineral oil under the EF; thus, the electronic structure of NE is more susceptible to EF. The differences of electronic structure between NE and mineral oil cause the discharge threshold of NE lower than that of mineral oil, which makes the discharge streamers in NE tend to develop along the axial direction of the electrode.