Study of stray gassing measurements by different methods

Dissolved gas analysis (DGA) is a powerful tool used to monitor the condition of oil filled high voltage electrical equipment, in particular transformers. The aim is to have an early indication of thermal and dielectric faults. The methodology for analysis of dissolved gases and the interpretation has been developed over the last 40 years and that process is still ongoing. One way to improve the method is to gain more understanding of the difference of gassing behavior of the insulating oil itself, so called stray gases, and fault generated gases. The stray gases are formed by chemical reactions at normal transformer operating temperatures. Stray gassing measurements can be performed with different methods and the results may differ depending on method and how the method is set up. Only one standardized method is available, ASTM D 7150, other methods have been suggested in Cigré report 296 [1]. It is very important that the plausibility of the stray gassing results is considered when drawing conclusions since small differences in the test setup might lead to large variations in results. In this paper the authors compare different method to test stray gassing and look at the influence of oil preparation and test conditions such as temperature, time of heating and oxygen concentration. A new developed method with a glove box for sample preparation is introduced, this in order to have a lower and more stable oxygen level before ageing. The difference in stray gassing behavior of inhibited and uninhibited mineral insulating oils has also been studied.