The solubility of combustible gases in power transformers is important to correctly identify the levels of gases produced in the associated defective conditions before any dissolved gas analysis is attempted. Different combustible gases have different solubility levels in mineral oil [DiGiorgio1].
The following table provides solubility levels of the gases in transformer mineral oil with a static equilibrium at 760mm Hg and 25 °C [DiGiorgio1]:
| Gases | Solubility |
| H2 | 7% by volume |
| N2 | 8.6% |
| CO | 9% |
| O2 | 16% |
| CH4 | 30% |
| CO2 | 120% |
| C2H6 | 280% |
| C2H4 | 280% |
| C2H2 | 400% |
The figure below indicates the relative solubility of the combustible gases as a function of temperature [DiGiorgio1]. Gases such as hydrogen, nitrogen, carbon monoxide, and oxygen have an increasing solubility rate with increasing temperature. The hydrocarbon gases Methane CH4, Acetylene C2H2, Ethylene C2H4, Ethane C2H6, and Carbon Dioxide CO2 have a decreasing solubility rate with increasing temperature. It is noted that both hydrogen and carbon monoxide are not very soluble at low temperatures but the solubility rate does improve drastically for hydrogen at operating temperatures around 70-80°C. The solubility rate of methane is moderate, decreasing with increasing temperatures. These aspects play an important role when assessing the gas production rates from oil samples.
References
| [Digiorgio1] | DiGiorgio, J. B., “Dissolved Gas Analysis of Mineral Oil Insulating Fluids,” DGA Expert System: A Leader in Quality, Value and Experience 1, Northern Technology and Testing, pages 1-17, http://www.nttworldwide.com/tech2102.htm, 2005 |