Electrical Properties of Insulating Liquids Based on Molecular Properties Calculated by Density Functional Theory

Abstract

For environmental and fire safety reasons, natural and synthetic ester insulating liquids are increasingly being investigated as alternatives to mineral insulating oils for electrical equipment applications. The three main types of insulating liquids exhibit different electrical properties due to obvious differences in molecular structure. To help clarify the reasons for the differences in insulating properties between different insulating oils, the ionization potentials (IPs) and electron affinities (EAs) of the main component molecules in the gas and liquid phases are based on molecular properties calculated by the density functional theory (DFT). In order to directly demonstrate the charge polarization effect induced by an electric field, we calculate the electron density differences ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \rho$ </tex-math></inline-formula> ) of typical molecular liquid-phase structures before and after applying 5.14, 51.4, 257, and 514 MV/cm electric fields, and plot the isosurface map of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \rho $ </tex-math></inline-formula> and the corresponding charge displacement curve. The effects of different electric field intensities can be analyzed intuitively and quantitatively. By calculating the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the primary molecules, the photoexcitation and electrical conductivity of three types of insulating liquids are analyzed. This study can help understand the mechanism of pre-breakdown discharge in the three main types of insulating liquids and the utility of various additives. Furthermore, it also provides a valuable reference of molecular level calculation and analysis results for the streamer generation mechanism of liquid dielectric and modification of ester insulating oil.