Permittivity matching effects on coil section of oil-immersed transformer

The lightning impulse breakdown voltage of the coil section of oil-immersed transformers is dominated by the partial discharge voltage of the wedge-shaped oil gaps made of the paper insulated conductors, mineral oil, and the coil section spacers. We have studied the permittivity matching effects on the lightning impulse breakdown voltage of the coil section models by means of the experiments and the electric field analyses. The relationship between the critical electric field E in the wedge-shaped oil gap when the breakdown occurred, and the gap length d was acquired. Moreover, the breakdown strength of the coil section models were estimated by using the above relationship within 6% error compared to the measured average values.