In the present paper, we study the effect of trap depth for shallow traps on the surface flashover performance in vacuum; that is on deepening the trap depth, the flashover performance is firstly enhanced and then weakened, only when the trap depth changes in the certain range, the best surface flashover performance in vacuum is exhibited. Further to a previous study, we extensively study wide range of trap depths by changing the aggregation structure of semi-crystalline cross-linked polyethylene (XLPE). In the present study, we firstly annealed the XLPE at 135 o C for 10 minutes then cooled it at different conditions to get samples with different trap depths. We found that the trap depth deepened on increasing the grain size. Furthermore, we found that the flashover performances of the samples (Cooled at a rate of 0.5 o C /min) were enhanced by 76% and 19%, respectively under dc and impulse voltage. These results demonstrate heat-treatment as an effective way to enhance the flashover performance in vacuum for the semi-crystalline polymers.Index Terms -Cross-linked polyethylene, Surface flashover in vacuum, Aggregation structure, trap, heat treatment.

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