A direct current transmission system and a method for preparation including an electrical conductor layer, an inner semiconductive layer covering the conductor layer, an insulation layer provided on the semi-conductive layer including laminated polymer material and impregnated with a high viscosity fluid, and including an inner part, a middle part, an outer part, and an outer semi-conductive layer covering the insulation layer. The inner part has a first thickness, the middle part has a second thickness and the outer part has a third thickness, whereby the second thickness is greater than the first thickness and greater than the third thickness. The laminated polymer material within each part has a constant thickness and constant ratio of polymer to laminated material and wherein at least one of the parts has a thickness ratio of less than 35%.