Dielectric relaxation in medium-voltage cable insulation

This paper describes dielectric relaxation in 11 kV and 33 kV cables. Relaxation is measured by discharging the insulation from a steady D.C. potential and recording an absorption-current transient, which persists after discharge of the geometric capacitance. The technique is attractive as a simple means of in situ assessment of degradation in underground cables. With appropriate transformation the transient may be interpreted in terms of the physical processes that govern the relaxation and the structure in which they occur. The aim of the present work is to characterize relaxation in the impregnated-paper and polymer systems used in medium-voltage cables, and to identify features that indicate the degree of ageing.