One of the most interesting and important problems, met during the development of non‐pressurized impregnated paper cables for high voltage direct current, arose from the introduction of a long‐duration type test at thermal cycles under a DC steady voltage of twice the working voltage. DC cables manufactured according to a traditional paper insulating technique showed during the cooling phase of a thermal cycle a catastrophic fall of the dielectric strength of the order of 50 %. The reason for this fall is analysed both theoretically and experimentally and the way to overcome the obstacle is explained and discussed.

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