Insulating materials evaluation for Cold Dielectric superconducting cables

The realisation of the so-called "cold dielectric" cable, where the insulation is kept at a cryogenic temperature, entails the choice of an optimised insulation system, with properties that are the best compromise between physical-electrical characteristics and design requirements. To this purpose, a specific study was planned to qualify different materials. On the basis of literature results, mostly carried out at liquid helium temperature, polypropylene laminate (PPL), paper, high-density polyethylene (HDPE) fiber and ethylene-propylene rubber (EPR) were chosen as potential candidates for the final use in liquid nitrogen. Electrical properties of materials were evaluated and compared by means of different models. Flat and cylindrical models were used to characterise the intrinsic properties of materials and determine the impact of the technological processes, respectively. Short term and long term tests were carried out at liquid nitrogen temperature and at different absolute pressures for the evaluation of electrical endurance. As far as short term tests are concerned, AC and lightning impulse (LI) breakdown step tests were performed. Long term tests were carried out under AC field; furthermore, power factor and partial discharge measurements were made to evaluate the impregnation process. The results of the whole testing procedure indicate that PPL shows the best behaviour for the application at cryogenic temperature in a superconducting cable.