Special considerations for insulation design of high voltage delta connected windings

The insulation structure in a high voltage transformer requires a very focused analysis in the design stage. The transformer is subjected to different overvoltage stresses during factory testing and in operation. Therefore, the transformer must be modeled in the transient voltage program, all resulting electric stresses must be modeled in the electric field program and compared to the industry accepted withstand curves. In the case of the wye connected windings with graded insulation, the winding end dielectric stresses can be reduced by placing the line lead at the axial center. However, for the delta connected winding, which is fully insulated, this option is not possible. Also, in a delta HV winding, it is critical to check the dielectric stresses for different test conditions. During lightning impulse, switching impulse and applied voltage test conditions, the terminal connections are different and so the resulting stresses are also different. This paper will discuss the analysis for selected transformers with high voltage winding connected in wye and in delta to highlight the special considerations needed for insulation design of high voltage delta winding. The analysis would include the stress conditions between windings of the same phase and between adjacent phases. The electric field analysis under both: (i) strike in oil and (ii) creeping discharge along solid insulation will be included. Also\, the selected material properties are discussed.