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| AC Conductivity with High Frequency Relaxation and Breakdown Characteristics of Epoxy Resin under Bipolar Square Wave Voltage |
| Jiang Qihang1, Wang Weiwang1, Zhong Yu1, Li Shengtao1, Xu Yongsheng2 |
1. State Key Laboratory of Electrical Insulation and Power Equipment Xi’an Jiaotong University Xi’an 710049 China;
2. State Key Laboratory of HVDC Electric Power Research Institute China Southern Power Grid Guangzhou 510080 China |
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Abstract 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.
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Received: 11 November 2022
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2024, Vol. 39 
