Positive discharge development in insulating oil: optical observation and simulation

Results of optical investigations of positive discharge channel development in transformer oil under divergent field configuration and impulse voltages to 650 kV are presented. The propagation of two different discharge structures (bush-like, tree-like) and the transition between these structures were observed. The conditions of their development and boundary of the bush-to-tree transition were determined. Anomalous behavior of the breakdown time dependence on applied voltage, consisting of a sharp drop in time at a critical voltage, was found. The dielectric breakdown model modified by introducing the varying channel conduction is presented. In the model discharge channel propagation is described by stochastic development of current and field instabilities. The model describes adequately the main spatial and temporal features of the various discharge structures observed in the experiments. The current and field characteristics of the simulated discharge channels which belong to different structures are compared. The bush-to-tree transition and the anomalous dependence of the breakdown time on voltage were explained by current instability in bush-structure channels.