Electrical trees in inner and outer layer of different voltage rating XLPE cable insulation

The statistical initiation and propagation characteristics of electrical trees in cross linked polyethylene cables with different voltage rating from 66 kV to 500 kV were investigated, under a constant test voltage of 50 Hz/7 kV. It was found that the characteristics of electrical trees in inner layer of 66 kV cable insulation differed greatly from that in outer layer under the same test condition; however, few differences appeared as to 110 kV and above voltage rating cables. Through X-ray diffraction, differential scanning calorimetry and thermogravimetry method, it was suggested that besides extrusion process, molecular weight of base polymer material and its distribution are the prime factors deciding crystallization state, while the crystallization state and impurity content are responsible for the resistivity to electrical trees. Finally, dense and small spherulites, high crystallinity, high purity level of base polymer material and super-clean production processes are the aims to develop higher voltage rating cables.