The valve-side windings of converter transformers have to endure combined AC-DC voltage in operation. Under DC voltage

solid impurities generated due to PD（partial discharge） may cause great threat to the insulating ability of oil-paper insulation. However

the generation and growth mechanism of solid impurities is still unavailable. In this paper

an oil-paper insulation model was established and the content of micro impurity bridges was analyzed. The generation and growth process of micro impurity bridges under negative DC voltage were investigated. Besides

PD characteristics during the above process were also acquired synchronously. The experimental results show that micro bridge impurities are formed by fiber bundles detached from the surface of the insulating paper under DC electric field. The generation and growth process of micro impurity bridges can be divided into three stages. Among the stages the quantity of micro bridges increases significantly. The diameter of the bridge increases from 100 μm in the first stage to 1 mm in the third stage

along with which is the increase of the PD average charge from 400 p C to 1 000 p C and the PDs rates from dozens to hundreds per minute. At the third stage a ‘hazardous bridge’ emerges

leading to the final breakdown of the insulation. Besides

a ‘cluster discharging’ phenomenon appears in the second stage

which is associated with the generation and the disappearance of micro bridge impurities.