To investigate the inhomogeneous distribution of electric fields in insulating equipment and components, five nonlinear-conductance composite materials based on epoxy resin (EP) (nano-SiC/EP, nano-ZnO/EP, micro-ZnO/EP, nano-SiC/ZnO/EP, and nano-micro-SiC/ZnO/EP), were prepared using nano-SiC, nano-ZnO, and micro-ZnO particles as fillers. The mass fractions of the inorganic fillers were 1, 3, and 5 wt%, respectively. The direct current (DC) voltage characteristics of the composites showed that the electrical conductivities and nonlinear coefficients of the composites utilizing single-filler types increased with increasing inorganic filler content. Under the same conditions, the conductivity and nonlinear coefficient of SiC/EP were both larger than those of the nano-ZnO/EP and micro-ZnO/EP. However, the nonlinear coefficient of the composites was significantly affected by the simultaneous addition of the two inorganic fillers, micro-ZnO and nano-SiC. When the content ratio of micro-ZnO to nano-SiC was 2:3, the nonlinear coefficient of the composite reached a maximum value of 3.506, significantly higher than those of the other samples. Compared with the nano-SiC/EP, micro-ZnO/EP and nano-ZnO/EP composites with 5 wt% inorganic filler, the nonlinear coefficient of the two-filler composite was greater by a factor of 0.82, 2.48, and 5.01, respectively.

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The synergistic effects of zinc oxide (ZnO) Micro/Nano particles simultaneously filled in low-density polyethylene (LDPE) on the space charge characteristics and electrical properties has been investigated by melt blending micro-scale and nanoscale ZnO additive particles into LDPE matrix to prepare Micro-ZnO, Nano-ZnO, and Micro-Nano ZnO/LDPE composites. The morphological structures of composite samples are characterized by Polarizing Light Microscopy (PLM), and the space charge accumulations and insulation performances are correlated in the analyses with Pulse Electronic Acoustic (PEA), DC breakdown field strength, and conductance tests. It is indicated that both the micro and nano ZnO fillers can introduce plenty of heterogeneous nuclei into the LDPE matrix so as to impede the LDPE spherocrystal growth and regularize the crystalline grains in neatly-arranged morphology. By filling microparticles together with nanoparticles of ZnO additives, the space charge accumulations are significantly inhibited under an applied DC voltage and the minimum initial residual charges with the slowest charge decaying rate have been achieved after an electrode short connection. While the micro-nano ZnO/LDPE composites acquire the lowest conductivity, the breakdown strengths of the ZnO/LDPE nanocomposite and micro-nano composite are, respectively, 13.7% and 3.4% higher than that of the neat LDPE material.

Electrical tree occurring in polymer insulation limits the applications and reduces the service lifetime of power equipment. To improve electrical tree resistance in polymer insulation, polyethylene (PE) nanocomposites with nano-montmorillonite (MMT)layered fillers are compounded and investigated in terms of electrical tree characteristics. By an organization process, intercalation and coupling agents are used to modify the inorganic-layered MMT nanofillers in sequence. Polyethylene/nanomontmorillonite (PE/MMT) composites with different MMT contents are prepared by melting intercalation. Fourier transform infrared spectrophotometry is used to determine the chemical characteristics of the MMT in different surface-modification stages. The crystalline morphologies and MMT distributions in the composites are characterized by polarizing microscopy and scanning electron microscopy. The electrical tree propagating characteristics and corresponding conduction properties of the PE and PE/MMT composites are investigated. Results show that the electrical tree resistance of PE/MMT composites significantly improves. The electrical tree length decreases and the fractal dimension increases in PE/MMT composites compared with pure PE. After performing electrical tree initiation, conductive current slightly increases in composites but decreases in PE. This result suggests that the characteristics and conductive mechanism of electrical tree differ between PE and PE/MMT.Index Terms -Polyethylene/nano-montmorillonite composites, electrical tree, conductive current, conductive mechanism of tree channels.