Stochastic model of breakdown initiation in dielectric liquids under AC voltage

The electric strength of perfluorodibutyl ether and transformer oil in small gaps between hemispherical electrodes was investigated experimentally under an AC voltage of linearly increasing amplitude. The influence of the radius of the hemispherical electrodes, gap lengths and also rate of voltage increase on probability of breakdown initiation was investigated. In the macroscopic stochastic approach, breakdown initiation is described by a macroscopic function μ(E). This function is a probability density of the streamer initiation. The dependences of breakdown initiation probability on various parameters of the experiment were obtained for flat, cylindrical and hemispherical electrodes under DC and AC voltages of linearly increasing amplitude. The method of fixed breakdown probability was developed to reconstruct the function μ(E) in the case of non power-law approximation using experimental data on a series of breakdown voltages. The values of the function μ(E) for transformer oil and perfluorodibutyl ether were determined in the range of electric field 0.3-1.0 MV/cm. Stochastic computer simulations of breakdown inception were carried out.