The Effects of Transient Overvoltages on the Reliability of HVDC Extruded Cables. Part 2: Superimposed Switching Impulses

In a previous paper (Part 1) the authors have performed the simulative evaluation of the life lost by HVDC extruded cables subjected to long TOVs occurring in VSC HVDC cable systems. The conclusions were that it cannot be completely ruled out that the field levels and the duration of such TOVs are a potential threat to certain cable designs, and tests other than Load Cycling Type Tests after CIGRÉ Technical Brochure 496 could assess quantitatively the potential threat associated with long TOVs. For these reasons, in this Part 2 superimposed switching impulses applied during Type Tests after TB 496 are analyzed in order to check if such impulses might be equivalent to long TOVs as for their electro-thermal stress effects on extruded insulation. The analysis is similar to that carried out for long TOVs in Part 1, i.e., it consists of: calculating the electric field during superimposed switching impulses; comparing it with the field during long TOVs; calculating with Miner's law of cumulated damage the life lost during superimposed switching impulses and comparing it with the loss of life during TOVs. A brief literature survey of potential high field effects during long TOVs is also reported.