Discharge characteristics of streamer in liquid dielectric under lightning impulse voltage

Liquid dielectrics are widely used as insulating medium in the electrical insulation system, and often withstand various impulse voltage occurrences of discharge. To understand the physical behaviors of discharge phenomena in dielectric liquid, adopting needle-sphere electrode geometry in this paper, the dynamic state of streamer and the dynamic characteristics of charged particles in liquid dielectric are investigated under lightning impulse voltage. The change process of positive ions, negative ions, and electrons with the formation and development of the streamer are described by two-dimensional continuity equations, and coupled with the Poisson´s equation described the electric field strength. The research results show that, during the process of impulse rise, the movement of the free charge generated by liquid molecule ionization only near the sharp needle electrode, the streamer generated by migration of the electric field propagates towards liquid-gap slowly as well. However, the streamer velocity increases an order of magnitude when a critical voltage is reached, then the streamer will accelerate development towards sphere electrode. We also find that in liquid dielectric at the same peak voltage, rise times influence the shape and velocity of the streamers. According to these conclusions enable ones easily understand breakdown mechanism of the liquid dielectrics that are reasonably applied to power system fields.