On-site dielectric diagnostics of power cables using the isothermal relaxation current measurements

The destruction free determination of the ageing status of a polymeric cable insulation is a significant issue in the assessment of the cable residual life. Ageing of a polymer changes the electrical, physical, mechanical and morphological properties of the insulation. Using the computer aided isothermal-relaxation-current-analysis (IRC-analysis) the depolarization after DC poling of the insulation is measurable and the separation of different time dependant relaxation mechanisms is feasible. The machine intelligent diagnostic tools for the ageing classification and the residual strength prognosis of installed medium voltage cable is described. With the calibration performed in the field at one of the major German cable manufacturers using destructive methods (voltage and water tree tests) and the development of the machine intelligent diagnostic software, a nondestructive test procedure for already installed XLPE cable systems is created. The diagnosis reliability of the IRC-analysis based on several on-site tests is also discussed