Life-based Geometric Design of HVDC Cables. Part 2: Effect of Electrical and Thermal Transients

This study, in two parts, investigates the design of HVDC cables depending on electrothermal life modelling of the insulating material. The first part investigated the way in which different parameters affect the design of an HVDC cable in steady state conditions, i.e. with known nominal voltage and ampacity. This paper investigates the effect of electrical (i.e. voltage) transients, such as voltage polarity reversals, in addition to thermal transients (such as load cycles of cable current) on the electrothermal life map of HVDC cables, and their influence on cable feasibility. An ad hoc MATLAB code has been developed for this study. Results show a considerable negative effect of electrical transients on the expected life. A more severe effect of electrical transiens is noticed for materials with a lower value of the temperature coefficient of conductivity, due to a longer duration of transients. On the contrary, thermal transients associated with load cycles – which mostly imply current values equal to or lower than cable ampacity – imply insulation temperature values always lower than the maximum permissible temperature, thus they have a positive effect on the electrothermal life, since the thermal stress applied on the cable insulation will be reduced, allowing for smaller cable geometries for a fixed voltage and ampacity rating. It must be noticed that only the electrothermal life is investigated in this paper, and other types of stresses (e.g. mechanical stress) may also affect insulation life, but this is out of the scope of this study.