Electrical Treeing in Low-density Polyethylene in DC fields and under DC Stresses Superimposed with AC ripples

UoM administered thesis: Phd

  • Authors:
  • Fang Liu

Abstract

HVDC transmission is playing a critical role in modern societies with the development of electricity generation from renewable energy worldwide. Electrical treeing is one type of long-term ageing in polymeric insulation and will eventually lead to insulation failure. While it has been intensively investigated in AC fields, very limited research has been conducted on treeing in DC fields and in superimposition fields which can well represent the working conditions in HVDC networks, i.e., a higher DC component with AC ripples. This work researched treeing in low-density polyethylene with a typical needle-plane geometry. It began with the effect of the interface between the metallic needle and the polymer on treeing in AC fields, and then investigated initiation and growth under DC stresses in both polarities (+DC and -DC hereafter) and under DC stresses superimposed with AC ripples (+DC plus AC ripples and -DC plus AC ripples hereafter). Optically invisible air gaps of 10-55 µm in height had a possibility to form naturally at the metal-polymer interfaces and affected initiation and the subsequent growth in AC fields. Under +DC, tree initiation lengths, were at least one order of magnitude longer than in AC fields and the initiation voltage was close to the short-time local breakdown voltage. While it was difficult to employ PDs as treeing indicators under +DC, under +DC plus AC ripples differences in PD phase distribution and PD energy were found between samples with and without trees. Under -DC, tree initiation occurred over a wider range of voltage magnitudes within a shorter time and had a shorter initiation length than under +DC. Considerable step-like length propagation was seen shortly after initiation. PDs could be measured prior to tree initiation and a comb-like appearance in the PD magnitude variation trend was seen during treeing, however, it did not form PD clusters. Under -DC plus AC ripples, a slim bouquet tree structure which was short in length and narrow in width developed. In 2D projections, channels covered all of pixels of the treeing area; in 3D XCT reconstructions well-interconnected islands are apparent and its 3D fractal dimension, was much larger than its 2D equivalent. PDs had combined characteristics of negative DC and AC fields, i.e., comb-like appearance with a wing-like PRPD pattern, as well as its own features. Further fine channels developed from the existing tree structure while the DC source was ramping down, with and without an AC voltage superimposed onto -DC and regardless of the length and treeing area of the existing structure. These phenomena are explained in terms of space charge injection and accumulation with polarity effects due to the differences in nature between holes and electrons.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date31 Dec 2021