Compatibility issues in high voltage DC cable insulation development

In the last decade, growing efforts were made to replace crosslinked XLPE insulations with thermoplastic ones in high-voltage DC cables. The main reason for these development projects is the injection and trapping of charge carriers under DC conditions, leading to a field distribution within the insulation layer different from HVAC cables. Thermoplastic cable insulation is favorable in many respects, except for its thermal stability. So far, HDPE/LDPE and binary or ternary PP-based blends have been tried. The former offers a limited advantage in heat resistance, while the latter is too hard at high PP contents and loses thermomechanical properties at high elastomer contents. In this paper, the compatibility of binary polyolefin blends is first briefly reviewed; then, the cable-specific properties are presented together with examples taken from the literature. Deep trap formation, low conductivity, and optimum breakdown properties are observed in HDPE/LDPE blends under specific crystallization conditions where a fine-grained structure is formed. It results in a proper concentration of the traps, but these are not accumulated at the spherulite boundaries. Trap density and energy are also modulated by the relaxation processes. The future belongs to the PP-based blends, where several compatibilizing agents (copolymers, elastomers, in situ reactions) have been tried to find the balance between electrical, mechanical, and thermal properties. In these materials, again, fine, close to co-continuous structures should be achieved to reach the required properties, but the aromatic and polar comonomer content also contributes to the formation of deep traps in relatively uniform distribution. The PP phase must remain continuous to maintain the necessary thermomechanical properties above the Tg of the soft components.