Literature Study on Condition of Tertiary of Interbus Transformer and Alternative Protection

Abstract

Interbus Transformer (IBT) plays important role in the reliability of power system interconnecting two different voltage levels thus enable the power interchange. Failure in the IBTs' bushings, will instantly lead to the power interruption and outage. Normally, IBTs are constructed of three windings with primary and secondary in wye and the tertiary in delta. The primary and secondary are loaded, with tertiary mostly unloaded and provide the isolating system due to the unbalance from the other two windings. Operating bushings above the nominal voltage tends to reduce the operating cycle. Displacement of absolute reference from phase to phase voltage thus will raise the applied voltage exceeds the nominal voltage. Theoretically, applying operating voltage of 1.571 x U 0 to the tertiary bushing continuously is likely to reduce the life time of bushing to approximately 2.5 years. Online monitoring technique is introduced to monitor the changes in the bushings due to the degradation of insulation quality. Literature study focusing on the ideal condition of a bushing against the real loading cycle in operation also considered to estimate the insulation degradation mechanism. This study put more efforts in analyzing the bushing condition as the function of loading pattern and the applied voltage. Bushing online monitoring data are attached as well to describe the occurrences inside the bushing. At the end of this study, options on the protection scheme are described with fault analysis. 