Incipient failures analysis of high voltage bushings

High voltage bushing failures are one of the main causes that affect reliability and availability of high voltage transformers and reactors. Not only reliability and availability are affected, but also failures in these components could produce fires, damages in nearby equipment and injuries to people working in the proximity. It was shown that performing periodic diagnosis or implementing continuous monitoring help to avoid terminal failures. In order to expand knowledge regarding how high voltage bushings age and how the signs of ageing could be detected, the results of the analysis of six high voltage bushings removed preventively from service are presented in this work. These bushings had been operating for between twenty and thirty-three years before being removed. Three of them were pulled out preventively because diagnoses performed over them showed risk of failure. The analysis includes test of dissipation factor (tan δ) and capacitance, tip-up, frequency domain spectroscopy performed during their life in service and lab tests conducted when they were out of services: dissipation factor and capacitance, frequency domain spectroscopy, tip-up, dissipation factor at nominal voltage, partial discharges, DGA and oil analysis performed in laboratory. At the end of the lab test each bushing was disassembled, and a detailed visual inspection was carried out. Oil samples were taken to carry out physical and chemical analysis. Tests results, both field historical and in laboratory, life time in service, visual inspection and oil sample analysis results are presented correlated with the aim of evaluating failure mechanisms that were present, and analyzing which diagnosis tests have shown to be the most suitable to detect their condition.