What are the seven key gases in DGA, and what failure modes do they correspond to?
DGA monitors seven key gases dissolved in transformer oil. Each gas is associated with a specific energy level and fault type.
|
Gas |
Formula |
Energy Threshold |
Associated Fault Type |
|
Hydrogen |
H₂ |
Low / Medium |
Partial discharge (corona), electrolysis, rust |
|
Methane |
CH₄ |
Low |
Sparking / low-energy arcing |
|
Ethane |
C₂H₆ |
Low |
Thermal oil degradation (< 300°C) |
|
Ethylene |
C₂H₄ |
High |
Thermal hot spot (300°–700°C) - winding overheating |
|
Acetylene |
C₂H₂ |
Very High |
Arcing (> 700°C) - the most dangerous fault gas |
|
CO |
CO |
Medium |
Cellulose (paper) thermal degradation (> 200°C) |
|
CO₂ |
CO₂ |
Low |
Normal cellulose aging (use CO₂/CO ratio) |
Key diagnostic relationships:
• H₂ alone or H₂ + CH₄ → Partial discharge (corona).
• C₂H₂ → Arcing. Even trace amounts (≥ 1–5 ppm) require immediate action.
• C₂H₄ dominant → High-temperature thermal fault (overheated windings).
• CO₂/CO > 10 → Normal aging; CO₂/CO < 3 → Active thermal fault in paper.
Background gas levels:
• All transformers have some background gas from normal operation.
• Trending is more important than any single absolute value.
• A 20% monthly rise demands attention even if absolute values are below alarm thresholds.
Gas production vs. fault energy:
Low energy → H₂ → CH₄ → C₂H₆ → C₂H₄ → C₂H₂ ← High energy
(PD) (thermal) (arcing)