RESEARCH ON THE BREAKDOWN UNDER TYPE TEST OF NONPRESSURIZED PAPER-INSULATED HVDC CABLES

One of the most interesting and important problems, met during the dev elopment of non-pressurized impregnated paper cables for high voltage direct current, arose from the introduction of a long-duration type te st at thermal cycles under a DC steady voltage of twice the working vo ltage. DC cables manufactured according to a traditional paper insulat ing technique showed during the cooling phase of a thermal cycle a cat astrophic fall of the dielectric strength of the order of 50 %. The re ason for this fall is analysed both theoretically and experimentally a nd the way to overcome the obstacle is explained and discussed.