Condition assessment of XLPE cable insulation using short-time polarisation and depolarisation current measurements

The authors describe how a commercially available test set has been used to make preliminary determinations of condition of medium voltage underground power cables with extended cross-linked polyethylene cable insulation at remote sites in north-east Queensland. Polarisation and depolarisation currents are measured with an injection voltage of 1 and 5 kV with observed polarisation and depolarisation periods of 600 and 60 s, respectively. The condition of cables is estimated on the basis of a newly defined apparent conductivity, which is based on difference between polarisation and depolarisation currents in the first 20 and 60 s, and on a degree of nonlinearity factor which is the ratio of apparent conductivity at different voltages. Procedures needed to obtain consistent field test results are recommended as are diagnostic criteria to provide an indication of cable insulation condition. It is concluded that while the technique can provide evidence of poor insulation quality, it is also possible to distinguish between water trees in insulation and poor joint insulation.