Evaluation of Consistency for Cable Degradation Diagnosis using VLF tan δ

Very low frequency(VLF) tan measurements are widely used to evaluate the insulation degradation of power cables. In Korea,δ diagnostic evaluations are generally performed using an Index R derived from multiple tan parameters. However, the conventionalδ Index R approach has limited applicability when certain factors, such as skirt, cannot be measured. From the analysis of more than 6,000 sets of domestic underground cable data, approximately 15% of the data were found to be unsuitable for Index R calculation due to missing or unmeasurable parameters. To overcome this limitation, three diagnostic parameters(reproducibility, coefficient of determination, and sum of norm) were applied, and various parameter combinations were analyzed through a statistical approach to identify the most effective combinations for condition assessment. Based on this analysis, an alternative method for cable degradation evaluation was proposed for cases where Index R computation is not feasible.By applying the optimal combination, degradation grading became possible even for cable sections where Index R could not be calculated, demonstrating its value as a supplementary diagnostic tool. This approach can support practical decisions on cable maintenance and replacement without requiring additional testing.