Study of Dissolved Gas Analysis under Electrical and Thermal Stresses for Natural Esters used in Power Transformers

  • Sitao Li

Student thesis: Master of Philosophy


Mineral oil has been traditionally used as an insulating liquid in power transformers for over a century, and Dissolved Gas Analysis (DGA) technique has been used for decades as one of the most useful diagnosis tools to assess the conditions of mineral oil filled transformers. However, due to increasing awareness of environmental protection and fire safety, there is a trend of replacing mineral oil with environmentally friendly natural esters; DGA data interpretation method should then be studied, if necessary revised, in order to be applicable for natural ester filled transformers.This thesis covers experimental studies on performances of a mineral oil (Gemini X) and a natural ester (FR3) in terms of fault gas generation. Laboratory simulated faults include electrical sparks, electrical partial discharges (PD) and high temperature thermal hotspot types. The electrical sparking fault was generated by using a sharp needle electrode with a tip radius of curvature of 5 micrometers, a 2.57 L sealed test vessel was designed and built with the TM8 online DGA monitoring system, and two CTs were used to measure the high frequency and power frequency components of the sparking current, respectively. The electrical PD fault was simulated using the same test system but under lower voltages, and a traditional PD detector was used to record the characteristics of PD signals, including the repetition rate and amplitude. The hotspot thermal fault was generated by heating up a copper element locally in a 2.73 L sealed test vessel, and three thermocouples were used to measure the temperatures of the heating element.Furthermore, the dissolved fault gases in oil were measured by both the online DGA monitoring system and the oil analysis laboratory, and the DGA results were also compared.The main findings of this thesis are outlined below: FR3 generates similar amounts of fault gases to Gemini X under sparking faults. Under the same sparking energy (per J), FR3 generates fault gases 25% higher than Gemini X.  FR3 generates higher amounts of fault gases than Gemini X under PD faults. Under the same PD amplitude, the gas generation in FR3 is much higher than that in Gemini X due to a higher PD repetition rate in FR3. FR3 generates less amount of fault gases than Gemini X under high temperature thermal faults (>300 ºC). This indicates that FR3 is more thermally stable than Gemini X. DGA results obtained by the TM8 online monitor are comparable to those from laboratory analysis, within a deviation of 30% under all the faults.
Date of Award31 Dec 2012
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorZhongdong Wang (Supervisor)


  • Online DGA
  • Natural ester
  • Transformer faults

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