Carbonized surface tracking on pressboard cylinder of oil filled transformer under main insulation faults

In recently years, main insulation faults such as short-circuit of primary winding to secondary winding, winding to bushing and winding to ground is becoming main sources for transformer explosion. During the short-circuit issues, creeping discharge along the solid-liquid interface tend to cause tree-like carbonized white marks and black marks to spread on pressboard and lead to flashover. These surface tracking phenomena are good indicators for understanding the development mechanisms of transformer insulation failures. However, up to now most of the research on surface tracking of carbonized marks is carried out in minimized test cells, e.g., need-plane electrode configuration immersed in oil/pressboard samples, the precise modelling of surface tracking along oil/pressboard interface in real transformer is still a technical challenge. In this paper, a single-phase transformer model is manufactured, with internal insulation structure imitating the practical case of a 500kV three-phase split type transformer. AC power source is injected into the high-voltage bushing of the transformer model with applied voltage up to 60kV, whereas a divergent gap is artificially placed surrounding the pressboard cylinder to create main insulation failures. Oscilloscope was used to monitor the voltage level of simulated faults, and microscope was used to observe the carbonation traces on the surface of the pressboard after the simulation experiment. The experimental results show that the surface discharge fault of the pressboard will damage the structure of the pressboard, and the damage severity increases with the increase of fault voltage level