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Duval Triangle 1 reduces three dissolved-gas concentrations — methane (CH4), ethylene (C2H4) and acetylene (C2H2) — to a single point inside an equilateral triangle. Each gas is taken as a percentage of the three-gas total, those three percentages add up to 100 %, and they fix one position and only one. That is the arithmetic this page does for you, and it plots the point.
Enter the three concentrations in ppm. Gases not used by Triangle 1 (H2, C2H6, CO, CO2, N2) are not part of this calculation.
What this tool does and does not do. It normalises the three gases to 100 % and places the point at the exact position required by a Duval Triangle 1 plot, which is the step that is awkward to do by hand. It deliberately does not name a fault zone: the zone boundaries are straight lines across the triangle defined in IEC 60599 Annex B (Figure B.1) and IEEE C57.104, and reading the zone from the printed diagram is the step that carries the diagnostic decision. A commercial DGA report should be interpreted against the standard edition your laboratory works to.
How to use the plotter
- Copy the CH4, C2H4 and C2H2 figures from your laboratory report — they are normally given in ppm (µL/L). Leave every other gas out; hydrogen, ethane, carbon monoxide, carbon dioxide and nitrogen do not belong in this sum.
- Enter the three numbers and press Plot point. The panel below the form shows each gas as a percentage of the three-gas total and places the point on the triangle.
- Open the Duval Triangle 1 diagram — IEC 60599 Annex B, Figure B.1, or the equivalent figure in IEEE C57.104 — and read off the zone your point has landed in.
Worked example
A report lists CH4 = 30 ppm, C2H4 = 50 ppm and C2H2 = 70 ppm. Their sum is 150 ppm, so the normalised values are 20.00 % CH4, 33.33 % C2H4 and 46.67 % C2H2. Enter 30, 50 and 70 and the point lands at exactly the position you would reach with a ruler on the printed diagram — no rounding, no interpolation.
What the three gases tell you
Duval picked these three because they are generated at three different energy levels, so their proportions act as a rough thermometer for what is happening inside the tank.
| Gas | Energy needed to form it | Usually points to |
|---|---|---|
| Methane, CH4 | Low | Partial discharge and low-temperature thermal faults |
| Ethylene, C2H4 | Intermediate | Overheating of paper and oil — thermal faults |
| Acetylene, C2H2 | High | Arcing and high-temperature faults |
The seven zones you are reading
The triangle is divided into seven regions. The names come from the fault codes in IEC 60599, and each one describes a failure mode rather than a severity level.
| Zone | Fault class | What it describes |
|---|---|---|
| PD | Partial discharge | Spark-type discharges that pit the paper and burn pinholes through it |
| D1 | Discharge of low energy | Low-energy arcing — carbonised perforation or surface tracking of paper, carbon particles in the oil |
| D2 | Discharge of high energy | Arcing with power follow-through: extensive paper damage, heavy carbon formation, metal fusion, gas alarms and tripping |
| T1 | Thermal fault, below 300 °C | Overheating of paper or oil at low temperature |
| T2 | Thermal fault, 300 °C to 700 °C | Overheating in the middle range, typically a hot spot in the winding or core |
| T3 | Thermal fault, above 700 °C | Severe local heating — carbonised oil, discoloured metal, metal fusion above 1000 °C |
| DT | Mixed | Thermal and electrical faults occurring together |
Those descriptions follow the fault classes defined in IEC 60599 and summarised in Duval’s own review of the method. The boundaries between the zones are the printed lines in Annex B, Figure B.1. This page does not redraw them: the boundary is where the diagnostic decision is made, and a plotter that guessed at the lines from rounded numbers would be wrong exactly where the interesting samples sit — on the boundary itself.
Why a ratio method, not ppm thresholds
An absolute concentration tells you how much gas is present. The proportion between the three gases tells you what kind of fault produced it. One hundred ppm of acetylene means something different in a 500 MVA generator transformer than in a small distribution unit, but the acetylene-to-ethylene relationship points to the same mechanism in both.
The second advantage is that the triangle is a closed system. Every combination of three non-zero gases lands somewhere inside it, so a result can never come back as “unresolved”. In Duval’s comparison of interpretation methods, the triangle left 0 % of samples unresolved with 4 % wrong diagnoses, against 15 % unresolved for the IEC ratio codes and 33 % for the Rogers method. That is why it is still the default method in both IEC 60599 and IEEE C57.104.
Five ways to put the point in the wrong place
- Feeding it the total instead of the three gases. Only CH4, C2H4 and C2H2 go in. Including hydrogen or ethane in the sum silently shifts every percentage.
- Normalising twice. If your report already hands you percentages of the three-gas total, convert them back to ppm before entering them, or the tool will normalise an already-normalised figure.
- Treating “not detected” as zero. Most laboratories report a detection limit rather than a true zero. Typing 0 pins the point to the edge of the triangle, where a small change can move it across a boundary.
- Mixing methods. Several Duval triangles and pentagons exist for different equipment. Check that the laboratory report you are working from used Triangle 1 for transformer oil.
- Reading one sample as a diagnosis. The triangle classifies a gas pattern. A rising trend across two consecutive samples says considerably more than a single point does.
Frequently asked questions
Which gases does Duval Triangle 1 use?
Only three: methane (CH4), ethylene (C2H4) and acetylene (C2H2). Hydrogen, ethane, carbon monoxide, carbon dioxide and nitrogen are used by other DGA interpretation methods, not by this triangle.
Should I enter ppm or percentages?
ppm (µL/L). The plotter converts them into percentages of the three-gas total and shows you the working. If your report only gives percentages, convert them back to ppm first so the arithmetic stays traceable.
Where do I read the fault zone from?
From the printed Duval Triangle 1 diagram — IEC 60599 Annex B, Figure B.1, or the equivalent in IEEE C57.104. The plotter fixes the position of the point; naming the zone is the interpretation step, and it stays with whoever signs the DGA report.
What if one of the gases is zero?
The point sits on the edge of the triangle, which is a legitimate position. Only a sample where all three gases read zero cannot be plotted, because there is no proportion left to work out.
Is my data sent anywhere?
No. The calculation runs entirely in your browser. Nothing is submitted, logged or stored, and there is no account to create.
Which edition of the standard should I cite?
IEC 60599:2022 is the current edition and IEEE C57.104-2019 is the current IEEE guide. Check which edition your laboratory reports against before comparing results across years.
Reference standards
- IEC 60599:2022 — mineral oil-filled electrical equipment in service: guidance on the interpretation of dissolved and free gases analysis
- IEEE C57.104-2019 — guide for the interpretation of gases generated in mineral oil-immersed transformers
- M. Duval, Dissolved gas analysis and the Duval Triangle (2006) — the method, its seven zones and its comparison with the ratio codes
Related reading
- Duval triangle method: how to read the three fault zones
- IEC 60599 explained: what the standard actually tells you
- DGA fault detection: from gas pattern to fault type
- All DGA and oil analysis guides
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