The Influence of an Air Gap at the Needle/Polymer Interface on Electrical Tree Growth

The impact of air gaps between a metallic needle tip and the polymer on electrical tree growth has been studied in low density polyethylene (LDPE). Gaps of 115 µm, 212 µm and 1037 µm have been experimentally investigated. Corona discharges of different types were measured within those air gaps. Accelerated treeing to breakdown was found in the sample with a 115 µm air gap. In this case the streamer coronas were slightly higher in magnitude than the maximum partial discharge (PD) during the tree growth in the no-gap sample. In contrast, lower corona discharge magnitudes were associated with larger air gaps, and electrical trees gradually stopped propagating to the counter electrodes and developed into pine-branch tree structures. This work highlights the differences between electrical trees that effectively grew from a metallic electrode in direct contact with the polymer and a point of corona source.