Gas in oil

The basic principle of the dissolved gas analysis is based on the capacity of the insulating oil to dissolve not only air from the atmosphere, but also cracked gases. These evolve not only from the natural ageing of oil and insulation materials, but above all when thermal or electrical defects occur in transformers. The extent of a defect and its increase over time (trend analysis) can be concluded from the quantity of dissolved gases in the respective oil.

These dissolved gases can be measured with a gas chromatograph (GC). To ensure accurate measurement results, the oil sample should have minimal exposure to ambient air. Therefore, the oil is collected in a gas-tight aluminum bottle during sampling. In the laboratory, 50 ml of oil is extracted from the aluminum bottle using a sealable, gas-tight glass syringe. The oil is then injected directly from the syringe into the TOGA GC 2. Dissolved gases are extracted from the oil under vacuum, and the total gas content is measured using a pressure sensor.

The escaping gases are flushed with the aid of argon (carrier gas) through a separating column. Depending on the structure of the respective gases, these remain on the surface of the column for different lengths of time. In order to ensure clean separation, two different columns are used. Detectors recognise the individual components and record corresponding chromatograms. The larger the area under a peak, the greater the proportion of the corresponding component.