The Impact of Square Wave and Sine Wave Ripples Superimposed on HVDC on Electrical Tree Growth in Epoxy Resin

  • Faisal Aldawsari

Student thesis: Phd

Abstract

Presence of harmonics generated by power electronic converters can negatively affect insulation materials, potentially leading to electrical treeing, a pre-breakdown phenomenon that can cause failures in high voltage cables and cable accessories. Electrical treeing involves the formation of gaseous channels ranging from approximately 1 to 40 µm in diameter under high electrical stress. While most research on electrical treeing has focused on AC systems, the increasing exploration and development of HVDC systems necessitate further investigation into treeing in HVDC transmission system. In this study, a typical needle-plane geometry with a 3 µm tip radius was embedded 2 ±0.1 mm from the bottom of the epoxy resin. The influence of combined square wave voltage and DC voltages with similar amplitudes were investigated in epoxy resin. Additionally, the effects of ±HVDC voltages superimposed with sine wave and square wave ripples were examined to better simulate the working conditions in DC systems. The primary objective was to understand the effect of steady state ripples, such as those generated by power electronics, on tree growth under HVDC fields. In this study, 15 kV was initially used to ensure tree initiation and to facilitate effective comparison and validation of the findings with existing literature. Then, voltage was gradually reduced from 15 kV to 10 kV to understand the behaviour of the tree and to identify the optimum voltage level for the next set of experiment in chapter 5. The findings reveal that electrical tree growth under DC bias and superimposed HVDC involved three stages: initiation, fast forward tree growth (dark tree growth), and fine tree growth. In contrast, electrical trees without DC bias exhibited five stages of development: tree initiation, fast forward tree growth, fine tree growth, darkening of the fine tree, and reverse tree growth. The average tree growth rate in length under positive 15 kV unipolar square wave increased by 25% compared to negative 15 kV unipolar square wave, whereas the average growth rate in length under positive 13 kV unipolar square wave increased by 18% compared to negative 13 kV unipolar square wave. The average tree initiation time under positive 60 kV DC ±10 kV square wave at 50 Hz was 45 % less than the average initiation time under positive 60 kV DC ±10 kV sine wave. Comparable average tree initiation times were observed between negative 60 kV DC ±10 kV square wave and negative 60 kV DC ±10 kV sine wave. The results show that the ripples were required to initiate trees under the DC voltage. However, the rate of growth in the linear region was not changed significantly between 10 kV bipolar square wave, positive 60 kV DC ±10 kV square wave and negative 60 kV DC ±10 kV square wave. The runaway stage of tree growth happened at shorter lengths in the case with positive at ~1 mm, compared to ~ 1.4 mm in the case with negative and ~1.6 mm in the 10 kV bipolar square wave case. Therefore, electrical trees in positive and negative HVDC fully crossed the insulation faster in than bipolar square wave. Failure happened rapidly after the tree crossed the insulation with HVDC applied, but in the bipolar case a reverse-tree was initiated, presumably because insufficient voltage was applied to cause the system containing trees to break down. These observations are consistent with that the magnitude of the AC component of the waveform essentially drives tree growth, with DC components having a relatively minor impact on growth speeds. However, the breakdown process of an aged sample which already includes the reverse tree is determined by the maximum voltage applied across the sample (i.e. including the DC component).
Date of Award31 Dec 2024
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorSimon Rowland (Supervisor) & Ian Kinloch (Supervisor)

Keywords

  • superimposed HVDC
  • Unipolar square wave
  • Bipolar square wave
  • Tree initiation
  • Epoxy resin
  • Electrical treeing
  • Tree growth

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