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This paper investigates the pressure dependence of streamer inception and propagation in highly purified insulation oil (Shell Diala D) stressed with impulse and AC voltage. In a point-to-plane electrode configuration the hydrostatic pressure was varied from 0.1 to 1.5 MPa. For both polarities, the minimum breakdown voltage increases with pressure. The time to breakdown and the propagation velocity of all streamer types are pressure-independent as well as the secondary streamer inception. The most obvious pressure-dependent effect is the change of the primary streamer pattem. At atmospheric pressure the highly ramified primary streamer propagates in a sector wider than 180 degrees. With increasing pressure the sector angle diminishes and the ramification reduces.

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