A New Criterion for Optimal Dielectric Design of High Voltage Bushing Internal Shields in Large Power Transformer

The High Voltage (HV) bushings are the most sensitive components of power transformers, they provide electrical insulation between the HV line and grounded tank, and also provide necessary mechanical support. The majority of transformer breakdowns are attributed to the HV bushings failures which are graded with conductive layers to improve their dielectric strength. The design of bushings is quite challenging with tradeoff between the axial and radial electric stresses. The bushing design is also governed by its mounting angle and transformer tank configuration to meet the internal and external flashover clearances and creepage requirements. At higher voltage levels, the configuration of HV bushing shield and grounded structural components have significant impact on dielectric withstand strength other than the clearances. Hence, it is very important to study the electrode configuration inside transformer tank for a particular bushing mounting arrangement before finalizing the bushing internal shield design. The 3D electrostatic field analysis of high voltage generator transformer was carried out for different bushing shield configurations to evaluate the electric stress distribution and the dielectric breakdown strength of the equipment was verified experimentally. The authors recommend to study the dielectric design of transformer in tandem with transformer configuration and a new evaluation criterion for optimal design of bushing internal shield is proposed.