Simulation Study on Long Gap Breakdown Characteristics of Natural Ester under Lightning Shock
Zheng Hanbo1, Yang Hang1, Feng Yongji1, Lü Weijie1, Peng Qingjun2
1. Guangxi Key Laboratory of Intelligent Control and Maintenance of Power Equipment Guangxi University Nanning 530004 China; 2. Electric Power Research Institute of Yunnan Power Grid Corporation Kunming 650200 China
Abstract：Natural ester, which has a greater ignition point and a better rate of biodegradation than conventional dielectric liquid mineral oil, has emerged as a prospective substitute for the former in light of the present green development trend of power grid equipment. It differ from mineral oil in terms of chemical and molecular structure, which impacts how well they insulate. Dielectric breakdown is preceded by streamer in liquid dielectric under high electric field magnitude. Natural ester streamer has a higher propensity to become 3rd mode fast streamer when compared to mineral oil. Natural esters also suffer from the substantial issue of having insufficient lightning shock resistance when working in areas with strong local fields and wide discharge gaps. This paper develops an enhanced model of natural ester streamer to analyze the process underlying the unique breakdown characteristics of natural ester by investigating the streamer development characteristics. First, the continuous medium model of carriers in streamer is used to explain the mechanism of charge generation and capture, and the physical process of streamer development is studied from a microscopic perspective, with special attention to the changes of charge density, electric field magnitude and other parameters during the dynamic process of discharge. Secondly, the ionization potentials of different triglyceride molecules representing natural esters were calculated with the help of density flooding theory (DFT) and wave function analysis. Third, considering the temperature for the carrier mobility in the streamer development, and finally adding the standard lightning impulse voltage simulated by subtracting the two exponential functions, the simulation data of the streamer development of natural esters are calculated. Simulation results for natural esters show that unlike mineral oil, the breakdown voltage and acceleration voltage of natural esters are very close to each other, leading to the easy transition from slow to fast streamer. Discussion of the ionization of different ionized molecules in the development of streamer reveals that in the early stage of development, the ionization of low ionization potential molecules contributes most of the free electrons and positive ions, and the ionization of high ionization potential molecules is limited. And with the increase of voltage, as the ionization potential difference between high and low ionization potential molecules of natural esters is not large, the high ionization potentialy molecules quickly participate in ionization, injecting enough carriers for the 2nd mode streamer, and the streamer completes the transition from the 2nd mode slow streamer to the 3rd mode fast streamer. In long oil gaps, the breakdown voltage of natural esters is much smaller than that of mineral oil. The breakdown of long gaps requires higher voltage, which makes the insulating oil subject to high field magnitude. The difference in ionization potential and other aspects between natural ester and mineral oil intensifies the performance of both in breakdown under different gaps. Since the unique breakdown properties of natural esters are closely related to the ionization potential of molecules, the distribution characteristics of ionization potential and electron affinity energy of natural esters induced by molecular configuration are less studied. Although some studies have been conducted to show the correlation between molecular configuration and electrical properties, only a few features related to molecular structure and the conformational relationship between electronic structure properties and electrical properties have been attempted to explain, and the intrinsic mechanism of molecular structure affecting electrical properties has not been revealed. The future direction should clarify the conformational relationship between the molecular structure and electrical properties of natural esters, use molecular design to improve the electrical properties of natural esters, and expand the application scenarios of natural esters.
郑含博, 杨杭, 凤永吉, 吕伟杰, 彭庆军. 雷电冲击下天然酯的长间隙击穿特性仿真研究[J]. 电工技术学报, 2023, 38(23): 6451-6460.
Zheng Hanbo, Yang Hang, Feng Yongji, Lü Weijie, Peng Qingjun. Simulation Study on Long Gap Breakdown Characteristics of Natural Ester under Lightning Shock. Transactions of China Electrotechnical Society, 2023, 38(23): 6451-6460.
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2023, Vol. 38 
