White marks on a power transformer pressboard will greatly reduce the oil-paper insulation performance

but it is difficult to observe the white marks on the pressboard during the transformer maintenance period. In order to study the carbonized marks on pressboard surfaces in power transformers

the needle-plane electrodes were selected. The typical partial discharge characteristics were measured by the pulse current method.Moreover

the tendency of average value

maximum amplitude

number of discharges and energy per second were analyzed under supplied voltage

and the high-speed camera was used to study the surface tracking on pressboard. According to the common tendency of the partial discharge characteristic parameters

the overall partial discharge tendency can be divided into five stages

there is a one-to-one relationship between the variation of the partial discharge characteristic parameters and the development of the white mark. The second to fifth stages correspond to the generation

rapid extension

slow extension

and final flashover of white mark; The pressboard surface was severely broken by observing white mark under the scanning electron microscopy. The mechanism of white mark formation and development was analyzed based on experimental phenomena.Therefore

the different processes of white mark can be reflected by the variation of the partial discharge characteristic parameters

which be used to provide a reference for studying the process of fibers broken on the pressboard in power transformers.