Accelerated aging and diagnostic testing of 115 kV Type U bushings

Type U bushings have exhibited a poor record of performance in the US and provided utilities with a major asset replacement problem. Causes of failure in Type U bushings are reviewed and diagnostic test results presented following an accelerated aging program performed on six 115 kV Type U bushings retrieved from the field after up to 20 years of service. The six bushings represented three broad classes with two bushings each characterized by low, medium and high dissipation factors. Accelerated aging consisted of a combination of normal line to ground voltage superimposed on increasing load current, i.e., increasing bushing temperature. All diagnostic tests were consistent with one another and showed that the six bushings could be grouped into three categories. The two bushings with the highest dissipation factors failed during accelerated aging. The program also demonstrated the usefulness of the bushing tap connection for performing a wide range of diagnostic measurements. The dissections demonstrated that the failures observed were both principally due to thermal aging.