학술논문
环氧树脂高频松弛的交流电导与双极性方波击穿特性 / AC Conductivity with High Frequency Relaxation and Breakdown Characteristics of Epoxy Resin under Bipolar Square Wave Voltage
Document Type
Academic Journal
Author
Source
电工技术学报 / Transactions of China Electrotechnical Society. 39(4):1159-1171
Subject
Language
Chinese
ISSN
1000-6753
Abstract
高频非正弦电压下环氧树脂击穿表现出较强的频率依赖性,并且存在复杂的宽频电-热耦合特性.高频电压下环氧树脂交流电导,特别是载流子非线性输运过程是理解高频绝缘击穿的关键.该文以两种电工环氧树脂为研究对象,采用宽频介电谱研究环氧试样交流电导的频率与温度依赖关系,通过Almond-West模型分析得到低频下交流类直流电导率与温度的关系,低频电导随温度变化更符合 Vogel-Fulcher-Tammann 模型.基于 Havriliak-Negami 复合介质多分散松弛极化理论,分析高频、高温下的松弛行为,认为高频交流电导率主要由松弛过程α 产生的极化损耗引起.通过高频双极性方波击穿测试平台,测试了重复频率双极性方波电压(500 Hz,1 kHz,2 kHz,3 kHz)下环氧试样的击穿场强,基于三参数Weibull分布统计分析特征击穿场强随频率的关系,发现击穿场强与频率呈负相关.研究提出考虑Poole-Frenkel效应的高频高场交流电导模型,并建立高频自由体积随频率变化的击穿模型.研究指出高频下分子链段振动效应增强,导致交流电导增强;高频下分子链运动滞后于频率响应,导致总体自由体积增大,造成高频击穿场强下降.研究结果对高频绝缘破坏与可靠性研究具有重要意义.
As one of the solid insulation materials of high-frequency transformers(HFT),epoxy resin presents good performance under high-frequency non-sinusoidal electrical voltage.Its insulation performance depends strongly on voltage frequency.Due to the local temperature rise in high-frequency transformers,the electrothermal effect can increase the conductivity of epoxy resin.Nevertheless,the AC conductivity from the dielectric spectrum measurement cannot be equivalent to the AC conductivity characteristics under high-frequency non-sinusoidal voltage.Therefore,this paper studies the AC conductivity characteristics under high-frequency square voltage by the Fourier decomposition.The mechanism underlying the AC conductivity of epoxy resin on the breakdown at high frequency is discussed by improving the free volume breakdown theory.The relationship between the AC conductivity and the high-frequency breakdown of epoxy resin is analyzed by the high-frequency relaxation characteristics and high-frequency high-field conduction theory. Two kinds of epoxy resin samples E1 and E2 were prepared using the same process and method.According to the results of glass transition temperature Tg and AC conductivity,the Almond-West(A-W)model and Havriliak-Negami(H-N)multi-dispersion relaxation polarization model were used to fit and analyze the AC conductivity spectrum at different temperatures,and the DC conductivity of epoxy resin at different temperatures was also obtained.Then,the temperature and frequency dependence of relaxation processes α and δ are analyzed according to the fitting results of the H-N model.According to the insulation breakdown measurement results,the breakdown characteristics of two epoxy samples within 500～3 000 Hz were analyzed.Based on the statistical results of the three-parameter Weibull distribution,the relationship between the featured breakdown field strength and the frequency was obtained.According to Jonscher's universal dielectric response model,the Poole-Frenkel effect,DC conductivity,and the frequency and electric field effects on AC conductivity under bipolar square wave voltage are analyzed.The free volume breakdown model is proposed to analyze the influence of frequency on the AC conductivity and high-frequency breakdown in the epoxy resin samples. According to the fitting results of the A-W model and the H-N model,the AC conductivity is mainly determined by the polarization loss caused by the relaxation process α above Tg.The relaxation process α depends on the dipole turning polarization,and the dipole concentration in the epoxy resin sample with a high epoxy value is high.The relaxation process δ is determined by the DC conductivity process,and δ process is related to the carrier migration across the barrier.The breakdown strengths of two epoxy resin samples decrease with the increase in frequency.The breakdown strength of E1 sample is greater than that of E2. The conclusions are as follows according to the measured results,fitting results,and model analysis.(1)The AC conductivity at high frequency and high field includes dipole conductivity at low field,Poole-Frenkel emission of charge carriers under high-frequency electric field,and DC conductivity.The AC conductivity increases with the increase in frequency or electric field.(2)The hysteresis effect of movements of molecular chain segments under the high-frequency electric fields becomes obvious as the frequency increases.The maximum length of the unoccupied equivalent free volume between the molecular chains increases,reducing breakdown strength at high-frequency voltage.In addition,the molecular group spacing decreases with the increase in frequency,leading to the decrease of carrier hopping potential barrier and the increase in AC conductivity of epoxy resin samples.
As one of the solid insulation materials of high-frequency transformers(HFT),epoxy resin presents good performance under high-frequency non-sinusoidal electrical voltage.Its insulation performance depends strongly on voltage frequency.Due to the local temperature rise in high-frequency transformers,the electrothermal effect can increase the conductivity of epoxy resin.Nevertheless,the AC conductivity from the dielectric spectrum measurement cannot be equivalent to the AC conductivity characteristics under high-frequency non-sinusoidal voltage.Therefore,this paper studies the AC conductivity characteristics under high-frequency square voltage by the Fourier decomposition.The mechanism underlying the AC conductivity of epoxy resin on the breakdown at high frequency is discussed by improving the free volume breakdown theory.The relationship between the AC conductivity and the high-frequency breakdown of epoxy resin is analyzed by the high-frequency relaxation characteristics and high-frequency high-field conduction theory. Two kinds of epoxy resin samples E1 and E2 were prepared using the same process and method.According to the results of glass transition temperature Tg and AC conductivity,the Almond-West(A-W)model and Havriliak-Negami(H-N)multi-dispersion relaxation polarization model were used to fit and analyze the AC conductivity spectrum at different temperatures,and the DC conductivity of epoxy resin at different temperatures was also obtained.Then,the temperature and frequency dependence of relaxation processes α and δ are analyzed according to the fitting results of the H-N model.According to the insulation breakdown measurement results,the breakdown characteristics of two epoxy samples within 500～3 000 Hz were analyzed.Based on the statistical results of the three-parameter Weibull distribution,the relationship between the featured breakdown field strength and the frequency was obtained.According to Jonscher's universal dielectric response model,the Poole-Frenkel effect,DC conductivity,and the frequency and electric field effects on AC conductivity under bipolar square wave voltage are analyzed.The free volume breakdown model is proposed to analyze the influence of frequency on the AC conductivity and high-frequency breakdown in the epoxy resin samples. According to the fitting results of the A-W model and the H-N model,the AC conductivity is mainly determined by the polarization loss caused by the relaxation process α above Tg.The relaxation process α depends on the dipole turning polarization,and the dipole concentration in the epoxy resin sample with a high epoxy value is high.The relaxation process δ is determined by the DC conductivity process,and δ process is related to the carrier migration across the barrier.The breakdown strengths of two epoxy resin samples decrease with the increase in frequency.The breakdown strength of E1 sample is greater than that of E2. The conclusions are as follows according to the measured results,fitting results,and model analysis.(1)The AC conductivity at high frequency and high field includes dipole conductivity at low field,Poole-Frenkel emission of charge carriers under high-frequency electric field,and DC conductivity.The AC conductivity increases with the increase in frequency or electric field.(2)The hysteresis effect of movements of molecular chain segments under the high-frequency electric fields becomes obvious as the frequency increases.The maximum length of the unoccupied equivalent free volume between the molecular chains increases,reducing breakdown strength at high-frequency voltage.In addition,the molecular group spacing decreases with the increase in frequency,leading to the decrease of carrier hopping potential barrier and the increase in AC conductivity of epoxy resin samples.