Dissolved Gas Analysis, or DGA Testing, is one of the sharpest sticks in the tool bag of analyses used in a predictive/preventative maintenance plan by transformer oil testing.
The insulating fluids used in transformers and related equipment are used because of their high dielectric strength and chemical stability. However, conditions and faults within the equipment will lead to chemical degradation of the fluid. The intensity of the fault or action determines the nature and degree of the degradation. The fault breakdown products are low molecular weight gaseous compounds that are soluble in the fluid. Analysis of which gases are present, the amount of each, and the ratios to each other are used to determine which fault processes are present in the equipment.
Common Gases Determined
| Gas | Formula | Fault/Atmospheric |
|---|---|---|
| Hydrogen | H2 | Fault Gas |
| Methane | CH4 | Fault Gas |
| Ethane | C2H6 | Fault Gas |
| Ethylene | C2H4 | Fault Gas |
| Acetylene | C2H2 | Fault Gas |
| Carbon Monoxide | CO | Fault Gas |
| Carbon Dioxide | CO2 | Fault Gas |
| Oxygen | O2 | Atmospheric Gas |
| Nitrogen | N2 | Atmospheric Gas |
DGA testing provides the current condition within the equipment by detecting thermal and electrical faults. This may indicate a need for additional electrical diagnostics or evaluation during manufacture or warranty.
After determining the gases present, interpretation of the data can be done in several ways.
Key Gas Analysis
This method is a qualitative interpretation based on the determination of fault types from “key gases” formed at different temperatures.
Ratio Analysis
This is a statistical determination of the ratios of certain gases to each other to further define fault types and thermal or arcing severity of the faults occurring.
Graphical Analysis
This interpretation involves plotting specific gases graphically in nomograms, triangles, or pentagons using intersecting points to indicate areas where the fault conditions are most probable.
Rate of Generation
This evaluation involved tracking the change between successive analyses in the concentration of gases indicated. This can involve a change or delta between 2 analyses or using linear regression analysis over several points of applicable data. This can indicate the severity and rate of progression of the faults occurring.
Unit-Specific and Historical Tracking
These methods of interpretation are enhanced further with specific knowledge of the operating conditions of the equipment involved and historical records of the gases levels over time and those conditions
