Dissolved Gas Analysis (DGA)
DGA is one of the most valuable diagnostic tools available. It is a procedure used to assess the condition of an oil-filled transformer from an analysis of the gases dissolved in the cooling/insulating medium. It is a well established technique that is cost effective, providing essential information from a relatively simple, non-destructive test based upon oil sampling. Whilst the analysis is normally done in a laboratory, on-line devices are also available. The results reveal much about the health of the plant including its present condition, any changes that are taking place, the degradation effects of overload, ageing, the inception of minor faults and the most likely cause of major failures.
Oil sampling from transformers is equivalent to blood sampling from humans.
It should be noted that a severe fault may also produce free gases that collect in the Buchholz relay. This closely associated topic is dealt with in Tutorial T5
Taking an Oil Sample
It is important that oil samples are taken carefully to avoid contamination or the loss of gas. Techniques vary from the use of syringes to kits made up from bungs, tubes and sealed bottles. Opening a drain valve, filling a bucket and pouring the contents into a jar will not produce meaningful results.
In the Laboratory
In the laboratory the mixture of gases must be extracted from the oil, for example by the application of a vacuum. The mixture is then passed through a chromatograph where the individual components are separated, identified and quantified. The results are normally presented in tabular form with each gas listed together with the quantity found in parts per million (ppm) by volume.
Interpreting the Results
Interpreting the results is a specialist science. With knowledge and experience the results of a DGA test can be used to produce a detailed and accurate profile of an individual item of plant. This is made possible by the fact that different conditions within a transformer give rise to different quantities and types of gas. For example, acetylene is only produced by arcing.
The Gases Measured
The main gases that are measured and their sources are as follows:
|
From the oil |
|
|
Hydrogen |
H2 |
| Methane | CH4 |
| Ethane | C2H6 |
| Ethylene | C2H4 |
| Acetylene | C2H2 |
| From the paper | |
| Carbon Dioxide | CO2 |
| Carbon Monoxide | CO |
The Application of DGA
DGA can be used in a variety of ways
such as:
 |
On-line sampling for continuous monitoring - see Catalogue |
|
One-off sampling with the results checked against statistical norms |
|
Periodic sampling of a single item to establish trends |
|
Selective sampling of large numbers with statistical predictions for the remainder |
|
Mass sampling on a routine basis to collect detailed historical data |
|
Fault analysis after a Buchholz alarm or trip |
Responding to Abnormal
Results
Abnormal results are
likely to require follow-up action in the form of more frequent sampling and closer
monitoring. Internal conditions that produce gases include over-heating,
partial discharges and arcing. Where discharges or arcing are taking place
techniques that enable insulation defects to be located with accuracy
have reached an advanced stage of development - see Catalogue
(Field Services)
Other Information Available from Oil Sampling
This tutorial deals briefly with the subject of DGA. However, transformer oil contains a great deal more information than is available from an understanding of the gases dissolved in it. Other parameters that need to be taken into account include moisture content, acidity, dielectric strength, the presence of furans etc. These are all important but outside the scope of this tutorial
End