Flashover time delay characteristics of grooved insulators under nanosecond pulse voltage in vacuum

For dielectric-vacuum compound insulation systems, the discharges usually occur across the insulator surface with an onset voltage much lower than the breakdown voltage for vacuum gap of the same length, which is the main cause leading to insulation failure. It has always been a hot topic to improve the surface insulation strength in vacuum. In this paper, the relationship between the flashover time delay and electric field is studied under nanosecond impulse voltage. Experiment results suggest that the flashover time delay was proportional to N-th power of electric field for grooved insulators with different parameters. For grooved insulator, the flashover channel prefer to develop along the grooves rather than pass over the groove wall. The blocking effect of grooves to the electron multiplication in cathode-anode direction leads to the increased time delay. A narrow band of electric field with respect to time delay is formed over the flat samples. The breakdown electric field of a grooved insulator was increased by more than 100% than that of a flat one. The flashover electric field tends to be saturated when the groove depth is more than 2 mm for both PTFE and PMMA, which is correlated with the trajectory of electrons travelling across insulator surface. Grooves near electrode is more effective to increase surface insulation strength than grooves in middle area, which can be of great help to guide the structure optimization in engineering application.