Effect of Crystalline Morphology on DC-Prestressed Breakdown Characteristics of PP-based Cable Insulation

Polypropylene (PP) as a thermoplastic cable insulation material has the advantage of higher working temperature, better insulation performance and recyclability than traditional cross-linked polyethylene (XLPE). This work focuses on the effects of isothermal crystallization (IC) of PP on its electrical properties. PP with different IC time is prepared and their crystallization morphology is characterized by polarized optical microscopy (POM). The space charge distribution of the samples is measured by pulsed electro-acoustic (PEA) method, and the DC breakdown strength after the hetero-polar DC prestress of samples is carried out by the ball-plate electrode. With the IC time increasing from 0 to 10 min, the injection and accumulation of space charge is significantly inhibited. The DC breakdown strength with hetero-polar DC prestress of PP with an IC time of 10 min are both markedly higher than that of non-isothermal crystallized samples. Nevertheless, further increasing IC time to 30 min will accelerate the space charge injection and transport greatly, and the DC breakdown strength after hetero-polar DC prestress is reduced, due to large-sized spherulites. It is concluded that IC can improve the integrity of spherulites and reduce the defects in amorphous regions, thus improving the electrical performance of PP-based insulation for HVDC cables.