Life Estimation of HVDC Cables Subjected to Qualification Test Conditions

The goal of the Master Thesis is estimating the life of HVDC XLPE-insulated cables subjected to the Qualification Tests conditions according to CIGRÉ Technical Brochure 496 and for different values of the coefficients (a) and (b). During the Electrical Type Test (TT), a series of load cycles (LC) with DC voltage UT=1.85 U0 (rated voltage) are applied in three stages, i.e.: • 12 cycles lasting 24 hours each with a negative polarity of the applied voltage (12 days). • 12 cycles lasting 24 hours each with a positive polarity of the applied voltage (12 days). • 3 cycles lasting 48 hours each with a positive polarity of the applied voltage (6 days). according to CIGRÉ technical brochure 496, Load Cycles are of two types: 1. A 24-hour Load Cycle consists of 8 hours heating (with steady conductor temperature equal to the rated one during at least the last 2 hours), followed by 16 hours of natural cooling. 2. A 48-hour Load Cycle consists of 24 hours heating (with steady conductor temperature equal to the rated one during at least the last 18 hours), followed by 24 hours of natural cooling. Results: -The phenomenon called “Field Inversion” takes place only in the case of high values of “a” and “b” coefficients where the outer part of the insulation is stressed more than the inner part. -In case of low “a” and “b” that the lower those values are, the more the inner part of the insulation is stressed. -The life of the cable under the Type Test condition (around 90 days) is three times longer than the Type Test duration (30 days), considering the worst-case which corresponds to low values of “a” and “b”. -The loss of life in one 48-hour Load Cycle (LC) is twice that in two 24-hour LC (equivalent to the same duration of 48 hours). -For the same values of a and b, the inversion of the life curve over the insulation thickness in the Pre-Qualification Test is greater than that in the Type Test because of the High Load period in PQ test