Effects of mechanical stretching on space charge behaviors of PP/POE blend for HVDC cables

Polypropylene (PP) blended with polyolefin elastomer (POE) has been considered as a potential choice to replace XLPE as HVDC cable insulating material, which shows excellent performance in electrical, mechanical and thermal properties. The space charge accumulation, formed under dc stress in PP/POE blend, would distort the local electric field, and furtherly lead to partial discharge or premature breakdown. During the processes of fabrication, installation and operation, the cable insulating material may be exposed to mechanical stress. In order to study the influence of mechanical stretching on space charge behaviors, PP/POE blends were prepared and stretched to different ratios of 1, 1.1, 1.2, 1.3 and 1.4, and the space charge behaviors in polarization and depolarization processes were measured and analyzed. The results indicate that a larger number of space charges accumulate in stretched specimens after polarization for the same time, and they decay faster than those in original specimens during depolarization process. Besides, the trap depth decreases with the increasing elongation ratio and then increases at a higher elongation ratio. The microstructure changes of PP/POE blend after mechanical stretching is considered to be responsible for the observed phenomena.