The Impact of Air Gaps between Needle and Polymer on Electrical Treeing in Needle-Plane Samples

Hualong Zheng, Fang Liu, Simon Rowland, Siyuan Chen, Zepeng Lv

Research output: Contribution to journalArticlepeer-review


Electrical trees are widely grown in laboratories from metallic needle electrodes to enable the degradation process to proceed rapidly at voltages of ~10 kV AC. Here it is shown that cone-shaped air gaps may form at a needle tip, even when they are not visible optically. Using X-ray imaging techniques, samples with and without air gaps up to 55 μm long have been characterized. The presence of air gaps led to partial discharge (PD) patterns resembling void-discharges. Prior to tree initiation, discharge characteristics evolved with increased voltage increments and increased phase lag behind the applied voltage. These changes to PD characteristics reflect a change in the physical state of the air gap. Although samples with air gaps generally initiated trees quicker than those with no gap, PD activities did not appear to play a decisive role in tree initiation times. However, initial voids were found to have major impact on the subsequent tree growth. In samples with persistent pre-tree air-gap PDs, trees grew larger branchy structures with higher PD magnitudes. In addition, the PDs became more asymmetric between positive and negative half-cycles than occurred in samples with no air gaps. It is shown that understanding the interface between the needle and polymer at a micron scale is critical to interpretation of laboratory tree growth experiments.
Original languageEnglish
JournalIEEE Transactions on Dielectrics and Electrical Insulation
Publication statusAccepted/In press - 20 Nov 2019


  • electrical tree
  • air gap
  • tree initiation
  • partial discharge
  • void


Dive into the research topics of 'The Impact of Air Gaps between Needle and Polymer on Electrical Treeing in Needle-Plane Samples'. Together they form a unique fingerprint.

Cite this