Condition Assessment of Bushings Using DFR

One important milestone for a wider acceptance of DFR (Dielectric Frequency Response) in condition assessment of capacitance graded bushings was the publication of IEEE C57.12.200 in 2022. Since then, additional experience has been collected. Baseline testing using DFR for new bushing installations to allow for more accurate condition assessment during service is a well-established method which is encouraged by the authors. It is however important to understand the influence of fringing effects caused by the different electrical context when testing is carried on installed bushings, compared to the separate routine testing in the manufacturer's facility required by the bushing standards. Some examples of bushing degradation and methods for result analysis are outlined in IEEE C57.12.200, however additional are available for both oil impregnated as well as dry concepts. The additional findings are based on extensive practical use of DFR. The most important conclusion is that testing at 1400 V provides more reliable results when accessing the condition of bushings, compared to testing at lower voltages.