Restoring reliability of aged bushings

Three major cause of failure had been established for bushings with over 20 years of service namely: seals damaged, leaks at expansion gas chamber and corrosion at bushing tap. In the paper, each root cause of failure is illustrated with true faults occurred while in service. Then detailed analysis is presented, and more important, some corrective measures to prevent bushing explosion had been established. Those include inspection procedures that had been applied to identify and remove from service damaged bushings. Furthermore, those bushings had been disassembled and investigated at bushing repair facilities. Internal inspection confirmed suspected damaged and promote corrective maintenance actions. Results demonstrate that paper insulation is largely unaffected by these root causes if treated at early stages. Most important, the dielectric strength of the main insulation is restored after vacuum treatment and impregnation with new oil. Bushing reliability had been evaluated in laboratory condition with impulse, AC overvoltage and partial discharge measurements, demonstrating that it is possible to restore reliability on aged bushings, if the three main cause of failure are corrected on time.