The white mark is a typical fault in surface discharge of oil-paper insulation in transformers. There is a certain correlation between the white mark and the deterioration of insulation. Obtaining morphological characteristics of the white mark is significant for the evaluation of insulation, but it is difficult to detect because it easily vanishes. In this paper, the growth of the white mark under extremely uneven electric field is studied under step-stress voltage, and the morphology is observed and partial discharge signals are measured. The results show that the morphology and partial discharge characteristics of the white mark at different stages show a good relationship, and PRPD patterns show a trend of gradually denser discharge. Moreover, the area of the white mark is consistent with the cumulative energy of discharge over time. In the diagnosis of fault, the cumulative energy of discharge，PRPD patterns and N-Q distribution histogram are of reference value.
 

Cumulative discharge energy,Oil-paper insulation,PRPD patterns,Step-stress,the White mark