This text shows an example of possible evaluation of results of DGA measuring for six transformers of the same type. The evaluation possibilities include evaluation based on absolute values of measured concentrations, on ratio criteria, relative assessment of machines and auxiliary ratios.
The article introduces also an auxiliary ratio ELDIAG based on the CO2 / sum CxHy (CO2 / S) relation which is very useful when oil de-gassing occurs and when concentration of decomposition gases range in trace amounts. In these cases most of criteria fails and it is not possible to indicate possible defects. The ratio reacts also to higher concentration of gases. It was empirically found out that its value is > 100 in case of defect free machines, for suspicious systems it is in the range < 100 to 10, and the range under 10 concerns machines with a provable defect. All damaged machines in the database showed decimal values or lower.
It arises from the example that the simple evaluation may hide the case of two independent defects that may be evaluated as one single defect.
To be able to apply the known ratio criteria, it is necessary to recalculate the measured gas concentrations as if they were obtained from degassed oil. It is also recommended to correct the results with regard to temperature. The studies have shown that due to the specific construction of bushings in comparison with insulating system of transformers it is not possible to use criteria for transformers. It was also found that in cases of possibility of withdrawal of an oil sample for traditional degassing, the measured values are considerably influenced after ventilation of the bushing.
