Influence Analysis on Defects in Bushing Core at Grid Side of Converter Transformer

Abstract

The high voltage bushing on the grid side of UHV converter transformer is located at the connection position between the grid side winding of converter transformer and the AC side system, which is the key equipment of UHVDC transmission system. In actual operation, the bushing will bear high temperature, large load current and additional load for a long time. In addition, due to the limitation of process level. During the rolling process of bushing capacitor core, there are inevitably some defects such as plate wrinkle, plate offset and unreliable grounding of the end plate. Under the long-term action of electricity, heat and stress, small defects are likely to develop into larger defects or even cause faults. Therefore, it is of great significance to study the influence of different defects on the electric field distribution in the bushing core and the partial discharge initial voltage (PDIV) of the bushing. On the basis of 550kV capacitive bushing, this paper designed and manufactured four scaled models. The rated working voltage of scaled models are 31.8kV. And the corresponding finite element simulation models are also established. The simulation results show that the wrinkle defect and offset defect will cause serious electric field distortion, resulting in a sharp increase of electric field at the defect. Ungrounded defects will completely change the distribution of electric field in the bushing, making the electric field concentrated in the insulating oil between the flange and the end plate. The measurement of PDIV shows that the wrinkle defect and offset defect can significantly reduce the PDIV which makes the bushing prone to PD at a lower voltage level and lead to fault. The initial voltage drop of ungrounded partial discharge is less, but the discharge amount is large. Therefore, the research of this paper has guiding significance for timely finding the original defects of bushing, preventing the development of defects, and solving the problem of frequent failure of converter bushing.