HVDC Cable Design and Space Charge Accumulation. Part 3: Effect of Temperature Gradient.

It is well known that polymeric HVDC cables suffer greatly from space charge accumulation due to considerable modifications of the electric field distribution with respect to the geometric “laplacian” field. The design of these cables, especially for EHV cables (exceeding 200 kV), can be a complex task, since service conditions (in terms of temperature distribution across cable thickness, charge accumulation in the insulation bulk, etc.) may differ markedly from the factors considered during the design stage. In solid polymeric insulation, if the applied electric field exceeds the threshold for space charge accumulation, injected charge from the electrodes may build up in traps distributed in the insulation bulk. It has been shown also, that the inner and outer semicon layers can play a major role in the extent of space charge accumulation, which has been demonstrated to depend significantly on the kind of semicon/insulation interface. Moreover, the presence of interfaces between adjacent insulating layers (e.g., in cable accessories) can act as additional trap distributions, which give rise to space charge accumulation due to electrode injection and interface polarization. A deviation from the geometric field can occur also when a cable is supplied by dc voltage and the insulation is subjected to a temperature gradient caused by heating due to Joule losses originating from the current in the conductor. Since the electrical conductivity of any insulating material depends (exponentially) on temperature, a temperature gradient (T=dT(r)/dr, in cylindrical coordinates, where r is the radius) generates a conductivity gradient that results, eventually, in a steady state charge distribution, ρtg(r). Therefore, the electric field profile at steady-state can be significantly different from that derived by considering only the cylindrical geometry of the cable insulation. If ρtg is sufficiently large, the electric field at the outer semicon can become higher than the field at the inner semicon. This phenomenon, known as “field inversion”, has been studied in the past, although only a few results quantifying the space charge profile ρtg associated with temperature gradient are reported in literature. This is probably due to the difficulty of separating the contributions of injected charge and bulk charge due to temperature gradient, which could be a critical task, since literature provides evidence of the existence of a threshold for space charge injection dependent upon temperature. Moreover, a high sensitivity detection system is required to measure the space charge ρtg associated with temperature gradient. This charge amount, in fact, could be in the order of 10-100 mC/m3, for temperature gradients usually applied to cable insulation. The aim of this paper is, thus, to investigate the extent of space charge accumulation due to a temperature gradient in comparison with other charge supply mechanisms, particularly injection from electrodes. For this purpose, space charge measurements were carried out on HVDC cable models under application of different temperature gradients across the cable insulation, above and close to the threshold field for space charge accumulation, and the main results, consisting of space charge patterns and extracted quantities, are discussed here.