Life-cycle-cost assessment and correlation between degradation-diagnosis parameter and degradation degree

The effectiveness of diagnosis method is closely related to the correlation between the parameter used for diagnosis criterion (degradation parameter) and the degree of degradation. In this paper, as an example, two parameters, the maximum water-tree length and dielectric loss tan δ, for water-tree degradation in power cables were compared. Assuming that both the parameters could be measured non-destructively and the costs for diagnosis were the same, the minimum total life-cycle costs which corresponded to the optimized diagnostic parameters, e.g. diagnosis interval, criterion, etc., were calculated. As a result, the total cost for the case of diagnosis based on the maximum water-tree length was much lower than that for the case of tan δ. This suggested that the maximum water-tree length reflects the degradation degree much better than the dielectric loss (tan δ), and practically the non-destructive diagnosis method based on the measurement of parameters related to water-tree length, e.g. residual-charge measurement, is expected to be more efficient.