Influence of polypropylene content in PPLP for high voltage insulation at cryogenic temperature

This study compares the dielectric performance of polypropylene-laminated paper (PPLP) insulation impregnated with liquid nitrogen in the context of high-power superconducting cables within the SCARLET European project. Samples with polypropylene (PP) contents of 43 % and 55 % are tested in a representative cable geometry including almost 20 % gap density in the insulation. Breakdown voltage is found to increase with hydrostatic pressure. Weibull analysis predicts similar breakdown field strength at 63 % probability for both kinds of samples (about 89 kV/mm) but a larger scale parameter for 55 %-PP-content sample (about 10 versus 7). This better performance for 55 % PP content is mitigated by a larger brittleness upon breakdown.