Relationship between the photoionization and fast streamers in Ester-based insulating oil

Abstract

Under impulse voltage, ester-based insulating oil (EO) is more likely to produce fast streamers than mineral insulating oil (MO). This paper explores the generation mechanism of fast streamers from a molecular perspective. First, the energy of EO to reach the first excited state is lower than that of MO, which is achieved through ultraviolet-visible light absorption experiments and molecular computation and is almost unaffected by the electric field. During the discharge process, photons may mainly come from molecular de-excitation, which leads to the neutral oil molecules transition and occurs photoionization. This study established a plasma photoionization model related to the change of first excitation energy and ionization energy at an electric field. Compared with MO, the electric field strength at the streamer head of EO is enhanced by the large number of charged particles produced by photoionization. The higher electric field further promotes the occurrence of photoionization, forming a positive feedback mechanism and accelerating the generation of fast streamers. The simulation results are consistent with the experimental results. It is found that field ionization is one of the main mechanisms that drives the streamers forward, and photoionization may be the cause of the generation of fast streamers in EO.