Properties Of Electrical Tree Growing In Ldpe/montmorillonite Nano-composite

Low density polyethylene is widely applied in high and extrahigh voltage power cable insulation because of the excellent dielectric and heat resistant performance. As to higher voltage rating polymer cables, electrical tree aging problem is more outstanding. Low density polyethylene is a major insulation materials of the power cable, analysis of propagation characteristics of electrical tree in LDPE is one of the most important ways of evaluating insulation performances of such kind of material. This paper presented a type of low-density polyethylene (LDPE)/montmorillonite (MMT) nano-composites prepared by a melting intercalation process. The design of the electrical tree propagation experiment system is based on the needle-plate model, and the electrical tree propagation processes are investigated with the living microscope camera and partial discharge measurement system. Comparative analysis of electrical tree growing properties in LDPE and LDPE/MMT nano-composites. The author of this article does experimental research in following aspects:X-ray diffractometry(XRD)and transmission electron microscopy(TEM)were used to investigate the intercalation behavior and microstructure of composites.The intercalation effect of MMT in polymer and the factors affecting the effect were discussed. Resistance thermal properties, mechanical properties and Electrical Breakdown properties were also studied,and all these properties improved.The LDPE samples used for electrical tree experiments are obtained through being cooled in ice water,air and silicone oil. Experimental results show that growing velocities and expansion coefficients of electrical trees in LDPE sample cooled in silicone oil are smaller than those of electrical trees in LDPE sample cooled in ice water and air. According to further analysis of the test results of differential scanning calorimetry,the LDPE cooled in silicon oil has the highest degree of crystallinity,the most uniform distribution of crystal grain size and lower growing of electrical tree, while the LDPE cooled in ice water has the lowest degree of crystallinity and electrical tree in the quickest propagation and lighter color.Growing procedures and morphology of electrical trees in the nano-composites were observed in the experiments under a constant voltage stress. Growing rates and expansion coefficients of the electrical trees in the nano-composites were measured and statistical characteristics of partial discharges (PD) during electrical tree growing procedures were analyzed. The mechanism of electrical trees in the LDPE/MMT nano-composites restrained by MMT nano-particles was illustrated through analysis of crystallization behavior of the nano-composites. Experiments and analysis results showed that MMT nano-particles improved the crystallinity degrees and decreased the grain sizes of LDPE. Quantities and repetition rates of PDs in electrical trees of the nano-composites were meanwhile reduced by MMT nano-particles during growing procedures of electrical trees and initiation and growth of electrical tress were postponed by MMT nano-particles.Based on the combination of the experimental results and the theories about electrical tree initiation, including Maxwell electrical-mechanical theory, defect theory and trap theory, the inhibition mechanism of added MMT nano-particles to initiation and growth of electrical trees in low density polythylene is explained.