| This work is concerned with the evaluation of present and new designs of tap-lead insulation of a distribution transformer with barrel-type coils. To accomplish this, simulations, based on lumped parameter equivalent circuits, and test measurements were carried out to investigate transient impulse voltage distribution in a distribution transformer coils, and coils with new tap-lead insulation designs. In order to establish the equivalent circuit models, an accurate and flexible technique was proposed to calculate lumped inductive parameters. The models were validated by a good agreement between the simulation results and test measurements through investigating the capacitive effects of core and tank, the effect of various equivalent circuits and the effect of tap connections under impulse voltage application. By using the simulation results of transient over-voltage distribution as boundary conditions, electric field stresses on insulation near the input end and tap-leads were evaluated through quasi-static field analysis based on Finite Element Method. With conservative factors already built in, the field analysis results show that the present tap-lead insulation design of three layers of paper is conservative and it can be reduced to two layers of paper or even one layer of paper without any problem in electrical point of view. |
