Effect of Crystalline Morphology on Electrical Tree Morphology and Growth Characteristics of PP Insulation: From Mesoscopic to Macroscopic

This article aims to investigate the effects of crystalline morphology on electrical tree morphology and growth characteristics of isotactic polypropylene (iPP). The crystalline morphology of iPP is adjusted by altering the crystallized temperature, and the mesoscopic electrical treeing in these samples with a thickness of
$280 \mu \text{m}$
is studied. The polarizing microscope (POM) images show that an electrical tree is easier to grow along the spherulites boundary and form a bush tree within spherulites. Compared with samples crystallized at a cooling rate of 2.5 °C/min and the isothermal temperature of 132 °C, the spherulite size of the sample crystallized at a cooling rate of 10 °C/min significantly decreases, resulting in an obvious decrease of the accumulated damage area and length of the mesoscopic electrical tree. The macroscopic electrical treeing is also conducted in the iPP samples with different crystalline morphologies with a thickness of 2 mm. There exists a relation between the mesoscopic and macroscopic electrical growth characteristics by using the Pearson correlation coefficient. It is concluded that the reduction of the spherulite size and the spherulite boundary gap limits the partial discharge (PD) and the development of an electrical tree in the amorphous region at the spherulite interface, making it easier to form the bush tree and leading to improved resistance to electrical treeing.