Experimental investigation of the effect of the design parameters of pressboard in mineral oil on creepage discharge propagation

One of the vital causes of the failure of oil transformers is considered as the creepage discharge propagation on pressboard. The authors performed an experimental investigation under an AC step test voltage to analyse the effects of the structure, distance and gluing direction of pressboard on creepage propagation. Flat, ribbed and grooved test specimens were prepared. Further, breakdown experiments were conducted for two gluing directions, parallel and vertical with respect to the electric field. For parallel gluing, the average breakdown voltage (V_BD) of the ribbed and grooved types was 1.2-1.33 times that of the flat type. For vertical gluing, the V_BD of the flat type was 1.28-1.66 times that of the same type with parallel gluing, whereas the values for the ribbed and grooved types were 1.02-1.11 times the corresponding values for parallel gluing. The rib height and groove depth do not seem to contribute greatly to the breakdown strength because most of the flashover propagation appeared not on the pressboard´s surface but at the glued interface. The results of this preliminary investigation may be useful for improving the dielectric creepage strength of solid insulation systems using pressboard as radial spacers, and high-voltage winding lead support parts.