Faults inside a transformer break down oil and paper. The fragments form gases that dissolve in the oil. A lab measures them in a small oil sample, and the gas mix tells you what kind of fault is active.
How DGA works
Why gases form
Mineral oil is a mix of hydrocarbon molecules. Paper is cellulose. Heat and electrical discharges break their chemical bonds. The fragments recombine into small gas molecules.
Which gases form depends on the energy. Low energy breaks only weak C–H bonds and gives hydrogen and methane. More heat breaks C–C bonds and gives ethane, then ethylene. Only very high energy, as in an arc, forms the triple bond of acetylene. Paper gives carbon monoxide and carbon dioxide.
| Gas | Mainly from |
|---|
| H₂ hydrogen | Partial discharge; also part of almost every fault; stray gassing |
| CH₄ methane | Low-temperature heating of oil; partial discharge |
| C₂H₆ ethane | Heating below about 300 °C; stray gassing |
| C₂H₄ ethylene | Oil heated above about 300 °C; dominant above 700 °C |
| C₂H₂ acetylene | Sparks and arcs; traces from very hot thermal faults |
| CO, CO₂ | Ageing and overheating of paper |
How a sample is taken
Oil is drawn into a gas-tight glass syringe at the sampling valve (114.3 on this unit). The lab extracts the dissolved gas and measures it by gas chromatography (IEC 60567, ASTM D3612). Results are in ppm, the same as µl/l.
Three questions, in order
- Is there a problem? Compare each gas with the IEEE C57.104-2019 norms (Tables 1 and 2), and check the change since the last sample (Table 3) and the rate of increase (Table 4). The result is DGA Status 1, 2 or 3.
- What kind of fault? Use a diagnosis method only when at least one gas is above Table 1. At low levels the ratios are mostly measurement noise.
- How serious, and what next? Combine fault energy, rate of increase, location clues and other tests.
The six methods
Key gas looks at the dominant gas. Doernenburg, Rogers and IEC 60599 compare gas ratios against tables. Duval Triangle 1 and Duval Pentagon 1 plot relative gas shares on a chart divided into fault zones; Pentagon 2 then splits thermal faults into O, C and T3-H. Pick a method below to see how it works, step by step, on the selected sample.