What it does
The Duval Triangle classifies active faults inside oil-filled transformers by looking at the ratio of three fault gases — methane (CH₄), ethylene (C₂H₄), and acetylene (C₂H₂) — and plotting that ratio on a ternary diagram. The point’s zone on the triangle maps to one of seven fault types, ranging from partial discharge at low energy through to high-energy arcing.
Developed by Michel Duval in the 1970s and refined since, the method is reproduced in IEEE C57.104-2019 and IEC 60599 as a first-line screening tool. It works because the three gases respond to fault energy differently — acetylene appears only at temperatures high enough to imply arcing, methane dominates in thermal faults below a few hundred degrees, and ethylene climbs as thermal activity intensifies.
The gases it uses
CH₄ · methane
Dominant product of low-temperature thermal activity and oil overheating.
C₂H₄ · ethylene
Rises with thermal fault temperature — a high-ethylene profile implies hot-metal contact.
C₂H₂ · acetylene
Effectively a marker for arcing. Any appreciable acetylene pushes the call toward the D1 or D2 zone.
The seven fault zones
A classified sample lands in exactly one of these zones. The severity column is the app’s 1–5 ranking — used to color the diagnosis card and the trend chips — not a Duval designation.
| Code | Fault | Severity |
|---|---|---|
| PD | Partial discharge | Watch |
| T1 | Thermal fault, low temperature (< 300 °C) | Watch |
| T2 | Thermal fault, medium temperature (300–700 °C) | Developing |
| T3 | Thermal fault, high temperature (> 700 °C) | Active thermal fault |
| DT | Mixed thermal and electrical fault | Developing |
| D1 | Low-energy discharge | Sparking |
| D2 | High-energy discharge | Arcing |
When to reach for it
The Duval Triangle is the right first diagnostic tool once the three key gases add up to something measurable. This app withholds a classification when CH₄ + C₂H₄ + C₂H₂ is below 10 ppm total — below that floor the percentages swing wildly on lab noise and the call isn’t trustworthy.
It’s designed for mineral-oil transformers. Natural-ester fluids (FR3 and similar) have different thermal gassing signatures, and applying Triangle 1 to them without care will bias the call toward T2/T3 territory. Duval later authored dedicated triangles for ester fluids; we’ll add those as separate models rather than branching this one.
Strengths and limits
Strengths: the Triangle almost always produces a fault call once gases are detectable, and it’s robust to sample-to-sample variation. Two labs reporting the same oil usually end up in the same zone even when their absolute ppm values differ.
Limits: Triangle 1 can’t distinguish arcing in paper insulation from arcing in oil alone, can’t flag stray gassing from certain oil additives, and doesn’t speak to cellulose ageing (that’s CO/CO₂ territory). Used as the first diagnostic tool and combining it with the Duval Pentagon, Rogers Ratio, and a trend across multiple samples is how the final diagnosis actually gets made in practice.
References
- IEEE Std C57.104-2019, Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers (Annex G covers the Triangle).
- IEC 60599:2022, Mineral oil-filled electrical equipment in service — Guide to the interpretation of dissolved and free gases analysis.
- M. Duval, “A review of faults detectable by gas-in-oil analysis in transformers,” IEEE Electrical Insulation Magazine, 2002.