Montmorillonite (MMT)/low-density polyethylene (LDPE) nanocomposites have excellent partial discharge resistance and electrical tree resistance compared with pure LDPE. However, the MMT/LDPE nanocomposites have low breakdown strength due to the poor compatibility between nano-MMT and LDPE. In order to improve the breakdown strength of the MMT/LDPE composites without changing the LDPE matrix, an MMT/SiO 2 /LDPE multielement composite was prepared by melt blending, and its breakdown strength and electrical tree resistance properties were investigated. The results show that the MMT/SiO 2 /LDPE multielement composite has excellent breakdown strength and electrical tree resistance properties compared with the MMT/LDPE composite. The reasons for the excellent insulation properties of the MMT/SiO 2 /LDPE multielement composites were analyzed by exploring the effects of MMT and SiO 2 on the microstructure and trap characteristics of LDPE.

To investigate the characteristic of electrical tree propagation in semi-crystalline polymers, the low-density polyethylene (LDPE) samples containing electrical trees are cut into slices by using ultramicrotome. Then the slice samples are etched by potassium permanganate etchant. Finally, the crystalline structure and the electrical tree propagation path in samples are observed by polarized light microscopy (PLM). According to the observation, the LDPE spherocrystal structure model is established on the basis of crystallization kinetics and morphology of polymers. And the electrical tree growth process in LDPE is discussed based on the free volume breakdown theory, the molecular chain relaxation theory, the electromechanical force theory, the thermal expansion effect and the space charge shielding effect.

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