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The choice of the materials used as the dielectric media in large transformers continues to be a combination of pressboard and mineral oil. However, a number of failures in the field have been attributed to the surface discharge or creeping discharge over the oil/pressboard interface. This paper reports on an experiment to study surface discharge over oil/pressboard. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD) from inception to flashover. The max PD charge, pattern, frequency and current pulse were measured during the flashover process. Four oil/pressboard specimens with different aging degrees were used and their influence on the flashover process over oil/paper interface was studied in this experiment. The results indicate that the max PD charge and PD frequency increase with the increase of applied voltage. The oil aging has great influence while the pressboard aging has little influence on PD characteristic over oil/pressboard. The PD pattern and current pulse shape show great difference for various applied voltages. The results therefore suggest that the PD detection on the oil/pressboard can be used effectively to implement the early monitor of surface discharge thereby to avoid the formation of surface flashover. [ABSTRACT FROM AUTHOR]