Enhanced charge injection in rough HVDC extruded cable interfaces

The insulation system of an HVDC extruded cable includes a variety of different material interfaces, especially within its cable accessories. Depending on interfacial manufacturing method, different degrees of surface roughness can be found and are here assessed. Based on detailed knowledge of insulation surface roughness, this paper proposes a new approach for calculations of electrical field distributions and injection current densities at the insulation surface. It is shown that local fields at rough material interfaces can be greatly enhanced, resulting also in field reduction in other areas along the surface. These calculations also reveal a distinct field threshold for space charge injection, likely leading to formation of homocharge layer adjacent to the interface. Existence of this homocharge layer at a rough surface (density of approximately 1 C/m(3)) is confirmed by means of space charge measurements.