Impulse treeing breakdown in polyethylene under non-uniform and quasi-uniform fields

The polarity effect of the impulse tree inception and complete breakdown (puncture) of low-density polyethylene is described as a function of the radius of the needle tip. Furthermore, the morphology of the impulse tree and breakdown channels is examined. It is shown that the puncture field strength E_b for both polarities is about 6 MV/cm at 500 μm and is of the same order as the intrinsic breakdown strength of low-density polyethylene. The shapes of impulse tree and breakdown channels for both polarities are shown for varying tip radius and gap distance. Although the E_b obtained has almost the same value as the intrinsic breakdown strength at larger tip radii, it can be seen that the morphologies of the breakdown channels for the two polarities are different. Especially at larger tip radii, the difference of the morphology is remarkable. One explanation for this difference may be the high-density gaseous plasma and the joule heating in the tree channels