Q: I hear about the "Duval Triangle" a lot in DGA interpretation. How does it work?
A: The Duval Triangle (by Michel Duval, IREQ, now IEEE C57.104) is the most widely used graphical method for DGA fault classification using a ternary diagram.
The three gas ratios (Triangle 1 - mineral oil):
• %CH₄ = CH₄ / (CH₄ + C₂H₄ + C₂H₂) × 100
• %C₂H₄ = C₂H₄ / (CH₄ + C₂H₄ + C₂H₂) × 100
• %C₂H₂ = C₂H₂ / (CH₄ + C₂H₄ + C₂H₂) × 100
|
Zone |
Fault Type |
Description |
|
PD |
Partial Discharge |
Low-energy corona, sparking in voids |
|
D1 |
Low-energy Discharge |
Sparking, tracking (glow discharge) |
|
D2 |
High-energy Discharge |
Arcing with power follow-through |
|
T1 |
Thermal < 300°C |
Overheated insulation (winding or core) |
|
T2 |
Thermal 300°–700°C |
Hot spot, bad electrical connection |
|
T3 |
Thermal > 700°C |
Severe hot spot, possible arcing + thermal |
|
DT |
Mixed Thermal + Discharge |
Combination fault |
Example diagnosis:
A 150 MVA transformer: CH₄ = 180 ppm, C₂H₄ = 280 ppm, C₂H₂ = 5 ppm
%CH₄ = 38.7%, %C₂H₄ = 60.2%, %C₂H₂ = 1.1%
→ Falls in T2 zone → thermal fault 300–700°C.
High C₂H₄ + low C₂H₂ → likely a bad bolted connection or tap-changer contact.
Action: Inspect tap-changer contacts. Schedule outage if C₂H₄ continues rising.
Other Duval Triangles:
|
Triangle |
Application |
Key Difference |
|
Triangle 1 |
Mineral oil transformers (standard) |
CH₄, C₂H₄, C₂H₂ |
|
Triangle 2 |
Thermal fault zone refinement |
CH₄, C₂H₄, C₂H₆ |
|
Triangle 3 |
Low-temperature thermal faults |
C₂H₄, C₂H₆, CH₄ |
|
Triangle 4 |
Load Tap Changers (LTCs) |
Same gases, different zone boundaries |
|
Triangle 5 |
Natural ester (vegetable oil) |
Adjusted boundaries for ester oils |
Limitations:
• Requires at least three hydrocarbon gases above detection limits.
• Less reliable when one gas > 80% of total HC.
• Cannot distinguish multiple simultaneous fault types.
• Always use with gas generation rates (ppm/day or ppm/month).