I have a DGA report with concentrations of seven gases. How do I turn these numbers into a diagnosis?
A: Three internationally recognized ratio methods convert gas concentrations into fault type diagnosis. Use all three for cross-verification.
Method 1: IEC 60599 / IEEE C57.104 Duval Triangle (most widely used)
|
Zone |
Fault Type |
Typical Gas Signature |
|
PD |
Partial discharge |
H₂ dominant, small CH₄ |
|
D1 |
Low-energy discharge (sparking) |
C₂H₂ > 4% of total HC |
|
D2 |
High-energy discharge (arcing) |
C₂H₂ + C₂H₄ dominant |
|
T1 |
Thermal fault < 300°C |
CH₄ + C₂H₆ dominant |
|
T2 |
Thermal fault 300°–700°C |
C₂H₄ > CH₄ |
|
T3 |
Thermal fault > 700°C |
C₂H₄ + H₂ high |
|
DT |
Thermal + electrical (mixed) |
Mixed pattern |
Method 2: IEEE C57.104 Gas Concentration Limits
|
Gas |
Condition 1 (Normal) |
Condition 2 (Caution) |
Condition 3 (Alert) |
Condition 4 (High Risk) |
|
H₂ |
≤ 100 ppm |
101–200 ppm |
201–300 ppm |
> 300 ppm |
|
CH₄ |
≤ 75 ppm |
76–125 ppm |
126–200 ppm |
> 200 ppm |
|
C₂H₄ |
≤ 100 ppm |
101–200 ppm |
201–300 ppm |
> 300 ppm |
|
C₂H₂ |
≤ 3 ppm |
4–10 ppm |
11–35 ppm |
> 35 ppm |
|
CO |
≤ 350 ppm |
351–570 ppm |
571–900 ppm |
> 900 ppm |
|
TDCG |
≤ 720 ppm |
721–1,920 ppm |
1,921–4,630 ppm |
> 4,630 ppm |
Diagnostic workflow:
1. Compare each gas to IEEE C57.104 Condition levels → assess urgency.
2. Calculate Duval Triangle ratios → identify fault type.
3. Cross-check with key gas ratios (Rogers/Doernenburg).
4. Review historical trend - is rate of change accelerating?
5. If ambiguous or generation rate > 30%/month, reduce sampling interval.