Improved Evaluation of the Life Lost by HVDC Extruded Cables due to Long TOVs

Abstract

Previously the author developed a procedure for estimating the life lost by HVDC extruded cables under the long Temporary Over-voltages (TOVs) that may arise in VSC HVDC cable systems. As the topic is gaining interest, here the procedure is improved to target an upper conservative limit of the aging effects of such TOVs. This goal is achieved in two steps. First the line length effect on insulation volume is considered via the so-called enlargement law for HVDC cables, as the longer the cable, the greater the insulation volume, and the number and size of defects; this lowers insulation endurance to applied stresses and raises the fraction of life lost at every single TOV. Second, a life model developed for harmonics superimposed onto rated voltage is included in the procedure to better address the challenge set by the fast peak of long TOVs. The improved procedure is applied to 320-kV and 525-kV cable designs. The results highlight that, with long lengths and conservative values of insulation parameters, the improved procedure provides orders-of-magnitude higher loss-of-life fractions, which in the most pessimistic case might be non-negligible for severely-aged cables in service since long time.