Al₂O₃/Polyethylene Composite Insulating Materials

With the green and sustainable new energy sources development, a number of DC transmission engineering with ultra-high pressure, high-capacity, and long-distance have been built and applied widely in China. The development of DC transmission has entered a new period. Space charge accumulation is an important problem in the development of DC cables. The suppression of charge accumulation is a key to improve the properties of material.Low density polyethylene ?LDPE?, as a widely used insulation material in the field of AC transmission, has been suffered space charge limitation in the field of HVDC ?High Voltage Direct Cable? transmission. The functional nanoparticles such as MgO and SiO2 were added into LDPE can suppress the space charge accumulation, improve the electric breakdown strength, and control the motion rate of carrier. But the uncertainty of random distribution and the agglomeration of nanoparticles could result in the electrical properties of materials decreasing, which is the bottleneck for HVDC cable development. Al₂O₃ as a commonly thermal conductive particles, also have been used to improve the traces and other characteristics of silicone rubber. It has been widely used in the outdoor insulation, but its application to internal insulation material is still scarely. Therefore, it is of great practical significance to prepare All2O3/LDPE composites with a certain disperse-structured of nano-particles to improve the durability of the composites.Four high performance Al₂O₃/LDPE composites with micro-structured and nano-structured were fabricated by melt method. The morphology of the composites was characterized by scanning electron microscopy ?SEM? and Fourier transform infrared spectroscopy ?FT-IR?. The results showed that the modified Al₂O₃ particles had very good compatibility with LDPE. The following conclusions are obtained for the characterization of the four materials:?1? The breakdown strength of 1 wt% Al₂O₃/LDPE nanocomposite was 468 kV/mm, which was close to the breakdown strength of LDPE. The accumulation of space charge in the material was measured by the electroacoustic pulse method ?PEA? test, it was found that the amount of charge in 1 wt% Al₂O₃/LDPE nanocomposites was less than others, and the charge decay rate of that was the fastest in the all content. The move rate of carrier in the 1 wt% Al₂O₃/LDPE nacocomposites was 9.97×10-12 cm2/V·S, which was least two orders of magnitude lower than that of LDPE.?2? The optimal parameters of two time hot-pressing were selected by factor-level analysis method. Al₂O₃/LDPE nacocomposites with gradient-distributed and sandwich-structured were prepared. The properties of gradient-distributed and sandwich-structured Al₂O₃/LDPE nacocomposites were shown repectively:compared the breakdown strength of the single-layer Al₂O₃/LDPE nanocomposites with the same thickness, the breakdown strength of the gradient-distributed Al₂O₃/LDPE nanocomposites was higher than that of the the single-layer Al₂O₃/LDPE nanocomposites. The space charge accumulation profies were shown that the gradient-distributed Al₂O₃ nanoparticles could introduce trap with a certain arrangement. The gradient-distributed Al₂O₃/LDPE nanocomposites has effectively suppressed the charge injection and reduced the internal space charge accumulation. The sandwich-structured Al₂O₃/LDPE nanocomposites was used to construct the biaxial gradient distribution model of Al₂O₃ nanoparticle. The electrical conductivity of the sandwich-structured Al₂O₃/LDPE nanocomposites ?1%N/0.1%N/1%N? was found to be 2×10-11 S/m at 80 kV/mm field strength by conductivity testing of the materials.?3?Oriented movement of the Al₂O₃ fiber in the molten LDPE has been occured by the orientation of the electric field. The orientation of the fibers was perpendicular to the thickness direction of the materials by SEM test. The space charge profiles were found that space charge accumulation of array-structured Al₂O₃/LDPE composite with the content of 0.2% was least, and the charge decay speed was the fastest in the short circuit. It was shown that this ordered arrangement has benifits to block the carrier transport path, inhibt the formation of electron channels, and reduce the amount of surface charge injection in the material.?4?The breakdown strength of array-structured 0.2% Al₂O₃/LDPE composites was 498 kV/mm, which was significantly higher than that of the other three kinds of Al₂O₃/LDPE compsites. When the electric field was 80 kV/mm, the conductivity of the array-structured 0.2% Al₂O₃/LDPE composites was 4×10-13 S/m.