Stray Gassing of Oil in HV Transformers

Currently produced insulating oils tend to generate gases at relatively low temperatures (90–200 °C). This phenomenon, defined by CIGRE as stray gassing (SG), can hinder the diagnostics of oil-filled electrical equipment based on the DGA method, because high amounts of hydrogen H2 and saturated hydrocarbons can sometimes occur in service without an actual fault. The aim and contribution of this article is to present how to distinguish such cases from electrical partial discharges or hot spots in paper, which may be risky for the safe operation of equipment and demand further investigation. Very high levels of H2 and high levels of C2H6 are observed recently in some families of HV transformers in Poland. This article presents DGA results of oil samples taken from these transformers together with results of laboratory tests of stray gassing of these oils performed according to CIGRE procedure. By using Duval Pentagon 2 it is possible to compare stray gassing patterns in the laboratory and in real transformers and evaluate the temperature range of possible defects. Stray gassing/overheating of oil in transformers, within this temperature range and whatever the possible other causes, is not a concern for their safe operation, according to observations published by CIGRE.