Operation Characteristics of 500kV OIP Bushings of Power Transformer

Statistics show that most of the transformer failures are caused by bushings. However, there is still a lack of an effective method for the condition assessment of bushings. In this paper, the fault causes between the draw lead bushing and conductor tube bushing are studied firstly, showing that conductor tube bushings are more likely to cause a fire than the draw lead bushings. The electrical performance of a group of 254 units of 500kV oil impregnated paper (OIP) bushings are further investigated. The distribution of DGA, dielectric loss, DFR, X-wax and high voltage PD results are investigated. It is shown that the presence of X-wax in bushings is highly related to the amount of hydrogen, 5000ppm could be regarded as the minimum caution limit by far according to our database. It is found that bushings with tanδ higher than 0.5% and its variation to name plate Δtanδ more than 38% are most likely in serious condition. Besides, the effect of X-wax on DFR test is found different from moisture. A condition assessment method, established by combining the distributions of tanδ, DGA, DFR and the dismounting findings, is verified with 4 extra cases. This paper could provide useful reference for the condition assessment of bushings in practice.