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<p>Thermal ageing of power cables may cause premature breakdown if the cables are operated at elevated temperatures, a relation usually described by Arrhenius’ law. The purpose of the present study is to investigate if any significant ageing takes place under normal operational temperatures. The investigation shows that thermal degradation at these comparably low temperatures is hardly reported and seems not to be of practical concern to transmission system operators. This may be the result of a conservative mind-set during the dimensioning of power systems, here in the case of power cables, which means that the lines are generally lightly loaded. Based on the literature found or, to be more precise, on the literature not found it seems to be very difficult to determine that thermal ageing should be problematic for operational power cables at all, in particular as compared to laboratory investigations at very high temperatures, where thermal ageing clearly takes place. To this conclusion contributes the fact that failure statistics do not correlate failures with severity of loading, and that the laboratory studies (which mosly conclude thermal ageing to be problematic) exposes the materials to unrealistic conditions.</p>

<p>Submarine cables used in harsh environmental conditions are often strengthened by means of protective structures, in addition to the armour, for  enhanced mechanical durability. The added exterior protection shields the cable against damage during installation and operation. These, polymeric or<br />metallic protective structures, contribute to the temperature increase in the cables by reducing the natural cooling processes. <br />In this paper are described investigations on the thermal conditions in and outside a three-phased armoured submarine cable equipped with a fully enclosed mechanical protection. The investigations are conducted through a full-scale experiment, including mitigation techniques. <br />In the full-scale experiment, the effects of the protective pipe result in a temperature increase when compared to an experiment using a cable with no protective pipe. The measurements show that the addition of the mechanical protection may result in the phase conductor temperatures to reach and<br />exceed the critical 90 °C limit, even under otherwise normal operating conditions.<br />The effect of measures to mitigate the increase in phase conductor temperature was also investigated. At set intervals a symmetrical distribution of holes<br />were drilled into the protective pipe. This mitigation technique resulted in a temperature decrease of approximately 9 °C in phase conductor temperature.</p>