In order to investigate the mechanism of cellulose bridges’ generation in oil-paper insulation and their effects on characteristics of partial discharge under DC voltage

a high voltage direct current（HVDC） experimental platform was set up. Based on the horizontal and vertical needle-plane model

characteristics including bridge shape

partial discharge inception voltage（PDIV）

partial discharge repetition rate

and partial discharge apparent charges were all recorded and analyzed. The obtained results show that

under the influence of electric field

cellulose chains at the surface of pressboards will be broken and detach into oil to form bridges. According to stress conditions

bridges present in 3 forms

namely

pile-like

nonthrough-like

and through-like. Due to the effect of cellulose bridges

PDIV of oil-paper insulation decreases compared with no-bridge condition. The degree of this inclination reaches the maximum at an oil distance between 12.5 mm and 15 mm

and among three kinds of bridges

pile-like bridge introduces the most severe influence. PD repetition rate of oil-paper insulation increases

and partial discharge process exhibits "expansion phenomena" on time

partial discharge（PD） charges of oil-paper insulation decrease. Strong PD happens at the end of intensive PD period

after which PD amplitude drops among the sparse PD period. Further analysis indicate that cellulose bridges distort the distribution and recovering process of electric field in oil

which consequently cause the above-mentioned phenomenon.