Gas Losses and Gas Management in Transformers

Dissolved gas analysis (DGA) is frequently used for the condition assessment and monitoring of transformers. In the case of free-breathing transformers, gas losses may occur at the phase interface due to the chaotic molecular movement of the dissolved gas molecules. These gas losses and the resulting distortion of the interpretation basis for the DGA can lead to a false assessment of the condition. Furthermore, the relative ratios may shift, which in turn can lead to the misclassification of the present defect. This paper deals with the influences on gas losses and describes a possible consideration for use in the condition assessment. It begins with an explanation of the gas losses problem in more detail based on theoretical considerations and summarised in hypotheses. In addition to the experimental methods used, theoretical model considerations for taking gas losses into account are described below. The results deal on the one hand with the influences on the gas losses (geometry and temperature) and the other hand with the application of the developed model for the consideration of the gas losses. In detail, the results of the gassing behaviour of different complex geometries - simple phase interfaces, a transformer model and a distribution transformer - are described in more detail and it is shown that a correction of the degassing with the model is possible within certain limits. Furthermore, the methodologies for determining the model are discussed in this paper.