The Effect of Electronic Characteristics of Molecules in Ester Insulating Oils on Streamer Propagation

Ester insulating oils are the preferred dielectric fluids of choice for oil-immersed transformers because of their superior fire safety, environmental friendliness, sustainability, and satisfying operating efficiency compared with traditional mineral insulating oil. However, the ease of streamer propagation and low breakdown voltage in ester fluids at positive lightning impulse (LI) voltage pose a crucial challenge to the safe operation of transformers, significantly limiting their use in power transformers. Currently, most available studies have focused on the morphological analysis of streamers without further investigation at the molecule level. In this work, we investigated the effect of ionization potential (IP) and electron affinity (EA) of molecules in three types of ester fluids-palm fatty acid ester (PFAE), soybean (FR3) and rapeseed (RDB) insulating oils-on streamer propagation at positive LI voltage. The calculated IP of PFAE oil (8.8 eV) is higher than the other two oils (7.5 eV). 66% faster streamer propagation and 25% lower positive LI breakdown voltage were found in PFAE oil compared with FR3 and RDB oils under positive LI voltage. Under the same applied voltage, the streamer in PFAE oil travelled faster and stopped at a farther distance than in FR3 and RDB oils. The lower EA and viscosity of PFAE oil equipped electrons with higher kinetic energy, leading to a fiercer collision between electrons and molecules and thus a large number of excited molecules. In contrast, relatively higher EAs and viscosities of FR3 and RDB oils limited electron-molecule collision, resulting in a lack of excited molecules for streamer propagation.