Analysis of Variable Frequency Dissipation Factor Violation at Low & High Frequency in Bushings

Conventionally, the Dissipation factor (Tan delta) test at 50/60Hz has been used by most utilities for condition assessment of bushings. However, with the advent of variable frequency tan delta assessment, it is possible to capture the dissipation factor response of the bushing across a much wider frequency range. This can help capture many incipient faults. Many bushings have been removed from service based on the detection of abnormality in tan delta values at low & high frequencies during periodic testing. This paper covers two such instances where violations were observed at low & high-frequency ranges and the correlation of the results with the detailed inspection of the removed bushings.