Gaseous characteristics of impulse breakdown initiation process in transformer oil

The statistical characteristics of the impulse breakdown time lag in transformer oil are studied over a wide range of pulse width, and the statistical discharge time lag and the discharge formation time lag are calculated by the Laue's pattern. Then a physical model for the discharge initiation process in quasi-uniform field is built. It is supposed that the field emission current generated by the small protrusion tips on the cathode surface could heat the liquid near the surface. When the liquid is superheated, nucleation sites would be generated. The nucleation sites would continue to expansion and elongate with the continuous heat supply until the gas in the bubble discharges. In this paper, the nucleation time is calculated based on the vaporization nucleation theory. A theoretical equation of the statistical discharge lag and the strength of the electric field is proposed based on the F-N theory. It is shown that the equation is consistent with the experimental data, indicating the accuracy of the proposed physical model.