Dielectric Characteristics of Polypropylene Laminated Paper in Liquid Nitrogen for Superconducting Devices of the High-Voltage Grid

With fast changing requirements on the electrical grid and the respective necessary efficient energy transmission, voltage quality and grid reliability, high temperature superconducting (HTS) devices gain considerably in importance. For the operation of HTS devices in liquid nitrogen (LN2) at high voltage levels there is a need for reliable and cost-effective insulating materials. Besides the very costly polyimide (PI), the already in LN2 applied polypropylene laminated paper (PPLP) offers a very good alternative. However, to get more information about insulating materials, that have not been used in LN2 yet, the dielectric strength of conventional insulating papers used in insulating oil is investigated at DC and AC voltage and compared with PPLP. In summary the investigations acknowledge an appreciable higher dielectric strength of PPLP compared to common insulating papers, especially at DC voltage. Nevertheless the measured insulating papers show a dielectric strength in LN2, that is in the same range or even superior to the dielectric strength after oil impregnation. Measurement results of undried as also of dried materials are compared and discussed, showing there is no decisive difference in LN2. All things considered the results of the investigations show, that the remarkable breakdown performance and the low dielectric losses of PPLP as also the very good values for common insulating papers enable cost-effective HTS applications even at the high voltage level.