The analysis of DC and AC conductivity in the detection of water tree degradation in XLPE cables

Water tree degradation in underground XLPE insulated cables is a growing problem with many cables experiencing failure well before their predicted design life. Time domain dielectric testing techniques such as return voltage (RV) measurement and polarisation-depolarisation current (PDC) measurement and frequency domain techniques, broadly referred to as frequency domain spectroscopy (FDS) provide ways of detecting water tree degradation in cables. This paper describes the results from PDC and FDS measurements performed on three groups of cables; refurbished field aged cables, non-refurbished field aged cables and new cables. An analysis of the DC and AC conductivity of the cables and how they affect the relaxation processes of a dielectric is undertaken. Finally, recommendations are made about the condition assessment of water tree degraded XLPE cables using the techniques described and the estimated condition of the test cables is given.