Dielectric response methods for diagnostics of power transformers

The dryness and aging state of oil-paper insulation is a key factor in both the short- and long-term reliability of a power transformer, since moisture has deleterious effects on dielectric integrity and insulation aging rates. Today, the water content of the cellulose in an in-service transformer is determined indirectly via moisture measured from oil samples, according to IEC 60422. This report summarizes the work performed by Cigre Task Force 15.01.09, and presents conclusions regarding the state of the knowledge on the applicability of the current diagnostic techniques.