Breakdown failure in insulation material is one of the key problems that threaten the safe operation of high-voltage direct current cable. In this work, the effect of boron nitride nanosheets (BNNSs) concentration, space charge and temperature on DC breakdown strength have been explored. Cross-linked polyethylene (XLPE)/BNNS nanocomposites were prepared by the melt blending method, and the basic characteristics of nanoparticles and composite were characterised. The experimental results indicate that DC breakdown strength of nanocomposite can be effectively improved when a small amount of BN nanosheet is doped into the matrix. The breakdown strength of the sample reaches the maximum value of 407.52 kV/mm when BNNS content is 0.5 wt%, which is about 33% higher than that of pure XLPE. Further, the effect of space charge on the breakdown of nanocomposites has been studied by pre-injecting charges. For the samples with different BNNS contents, all the breakdown strength present ascending trend when the polarity of the applied voltage is the same as that of the pre-injected charges. Besides, it can be found that the breakdown strength of the XLPE/BNNSs composite decreases significantly at 50°C, which is due to more charge accumulation at 50°C. It reaches 2.06 × 10 −8 C which increases by about 2.2 times than the room temperature.

A semi-conductive shielding layer plays an important role in the uniform electric field for a high-voltage cable. The electric-thermal properties of the semi-conductive layer and insulation layer directly affect the overall insulation performance of the cable. The physicochemical performances of semi-conductive composites are firstly analysed herein. Furthermore, electric-thermal properties of the semi-conductive layer and insulation layer are discussed. The experimental results show that the thermal conductivity of the commercial semi-conductive layer is about twice that of the insulation layer, owing to the effect of carbon black. The thermal expansion coefficient of the insulation layer rises from 1.86 � 10 −4 /K at 25°C to 3.20 � 10 −4 /K at 90°C. By contrast, the semiconductive layer begins to slowly decline at a certain temperature, and decreases significantly to 2.25 � 10 −4 /K at 80°C, owing to the effect of ethylene-vinyl acetate copolymer (EVA). The electrical experiments show that the resistivity of semi-conductive composite gradually rises with an increase in temperature, and gradually declines with an increase in the carbon black content. The dc breakdown strength of the composite structure of the semi-conductive layer/insulation layer decreases significantly with an increase in temperature, decreasing from 307 kV/mm at 25°C to 203 kV/mm at 90°C. At four typical temperatures, the breakdown strength reaches the maximum value when the carbon black content is 25 phr. It is about 15% and 19% higher than carbon black contents of 20 and 30 phr. These findings have reference significance for high-voltage cable breakdown fault analysis and material selection in cable design. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

This paper describes the effects of α-Al2O3 nanosheets on the direct current voltage breakdown strength and space charge accumulation in crosslinked polyethylene/α-Al2O3 nanocomposites. The α-Al2O3 nanosheets with a uniform size and high aspect ratio were synthesized, surface-modified, and characterized. The α-Al2O3 nanosheets were uniformly distributed into a crosslinked polyethylene matrix by mechanical blending and hot-press crosslinking. Direct current breakdown testing, electrical conductivity tests, and measurements of space charge indicated that the addition of α-Al2O3 nanosheets introduced a large number of deep traps, blocked the charge injection, and decreased the charge carrier mobility, thereby significantly reducing the conductivity (from 3.25 × 10−13 S/m to 1.04 × 10−13 S/m), improving the direct current breakdown strength (from 220 to 320 kV/mm) and suppressing the space charge accumulation in the crosslinked polyethylene matrix. Besides, the results of direct current breakdown testing and electrical conductivity tests also showed that the surface modification of α-Al2O3 nanosheets effectively improved the direct current breakdown strength and reduced the conductivity of crosslinked polyethylene/α-Al2O3 nanocomposites.

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