Effect of Thickness on DC Breakdown Weibull Distribution of XLPE Insulated Cables at High Temperature

The design of XLPE insulation thickness is one of the key tasks in the development and manufacturing of HVDC cables. However, it is difficult to obtain the DC breakdown performance parameters of thick insulation samples in the laboratory, and the design of insulation thickness lacks direct data support. Meanwhile, although there are continuous attention and researches on the thickness effect of breakdown field strength and the breakdown Weibull probability distribution of solid dielectric, there are few studies on the DC breakdown field thickness effect based on XLPE at high temperatures, and there are few reports on the dispersion of the DC breakdown field strength. Therefore, it is of great significance to study the effect of thickness on DC breakdown of XLPE insulation. In this paper, the high temperature DC breakdown experiment of XLPE insulation samples with different thicknesses was carried out, and charge transport and molecular displacement modulated model (CTMD) was used to simulate and analyze the breakdown process. Besides, the characteristic breakdown field strength and Weibull distribution shape parameters of XLPE insulation under high temperature DC voltage are studied with sample thickness. In dielectric insulation, the deep trap captures carriers, forming space charges at the interface of the insulating medium, and the continuous aggregation of space charges make the electric field strength distorted. At the same time, by the Coulomb force, the displacement of the molecular chains make the expansion of the free volume, the charge energy increases dramatically. When the charge energy increases to the point where it can leap over the trap barrier, the insulating material breaks down. DC breakdown experiments were carried out on 160, 300, 400, 550 and 650 μm XLPE specimens at 90℃. The characteristic DC Weibull breakdown field strengths were 264.1, 212.6, 185.0, 147.7, 136.4 kV/mm and the shape parameters of the Weibull distribution were 21.3, 20.5, 19.9, 14.1 and 13.5. The process of carrier transport and energy accumulation in dielectric insulating materials ultimately leads to the breakdown of the dielectric, and this process is closely related to the charge transport parameters. Therefore, three charge transport random variables, which are trap energy level, trap density and carrier mobility, are introduced into the CTMD model. By adjusting the trap charge transport random variables, the standard deviations of trap energy level, trap density and carrier mobility corresponding to XLPE are obtained. The following conclusions are obtained from the experiment and simulations: (1) Thick XLPE insulation has a wider Gaussian distribution trap, making the DC breakdown field strength dispersion more obvious. Therefore, the thickness effects of reduced characteristic breakdown strength and increased breakdown strength dispersion need to be considered simultaneously in the design of high-voltage-rated cable insulation. (2) CTMD simulations yield that the shape parameter of the Weibull distribution decreases with increasing standard deviation of the charge transport random variable, and the dispersion of the breakdown field strength data increases. The dispersion of the characteristic Weibull breakdown field strength varies most significantly with increasing standard deviation of the trap energy level. (3) The high-temperature DC breakdown field strength of XLPE insulation decreases with increasing thickness in an inverse power function relationship, and variations in the standard deviation of the charge transport parameter do not affect the characteristic breakdown field strength. © 2024 China Machine Press. All rights reserved.