Performance Evolution and Ageing Characteristics of Crosslinked Polyethylene Insulation for HVDC Cables After Field Operation

Abstract

The ± 200 kV direct current (DC) cables of the Zhoushan Transmission Project were targeted, and crosslinked polyethylene (XLPE) insulation samples were prepared from faulty and spare cables. Samples were tested for oxidation induction period, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermally stimulated depolarisation current, space charge and DC dielectric strength. The evolution of performance parameters was analysed, and the insulation ageing characteristics of onsite operating DC cables were deduced. The results show that compared with spare cables, the carbonyl index of operating cables decreases, the crystallinity increases and DC dielectric strength increases slightly. It is inferred being related to the reduced content of antioxidants and crosslinking by-products and improved molecular chain and crystal microstructure of XLPE insulation. It is suggested that the insulation ageing of high voltage DC XLPE cable is divided into two stages. In the early stage, the secondary crosslinking and recrystallisation processes are dominant, and the thermal and electric effects make the material properties basically stable or even slightly improved. In the middle and later stage, the thermal oxygen reaction is dominant, the microstructure of the material is destroyed, with a large number of macromolecular chains broken and some crystalline regions transformed into amorphous ones, and the insulation properties degraded significantly.