Abstract: Due to the limitations of specific insulation structures and the influence of technical process，the growth of electrical trees in high-voltage power equipments with polymer as main insulation materials is more complicated than those in uniform materials，especially in equipment employing semi-crystalline polymer，so it is difficult to characterize by mathematical means. The experiment reported in this paper is the first attempt to focus on the electrical treeing rules in both inner and outer insulation layers of high-voltage XLPE power cable. Through statistical experiments and the SEM observation，large difference in treeing character between the inner and outer layers of cable insulation due to the different crystalline status are found. Electrical trees initiating in the inner layer of insulation has shorter initiation time，higher growth rate and greater variety of shapes compared than in the outer layers. The initiation and propagation mechanisms are also discussed.

Keywords: 
• electrical tree /
• crystal states /
• statistical regularity /
• inner and outer insulation layers