CIGRE WG A2.37 reported, windings, tap changers and bushing related failures are the major contributors to power transformer failure. About, 17% of transformer breakdowns are caused by bushing failure often followed by explosions, tank ruptures, fires and even human harm. Even though, construction, design and materials have evolved, aging and operational stress still affect the reliability of HV and EHV bushings, therefore, it is imperative to use a non-intrusive and non-destructive testing method to provide assertive information on the condition of HV and EHV bushings. To date, the insulation system in HV and EHV bushings is mainly oil-impregnated paper (OIP) where Dielectric Frequency Response (DFR) has gained more application in the field, and deference amongst other methods, due to its practicality and comprehensive analysis.
This article describes the application of DFR and HV DFR as a preventive method to evaluate the condition and prioritize maintenance activities on HV and EHV bushings and relates some case studies.