What it does
The Duval Pentagon extends the Triangle from three gases to five — adding hydrogen (H₂) and ethane (C₂H₆) alongside CH₄, C₂H₄, and C₂H₂. Each gas owns one axis of a regular pentagon; a sample’s percentage on each axis defines a five-pointed polygon, and the area-weighted centroid of that polygon is the diagnostic point. Where the centroid lands maps to a fault zone — partial discharge, stray gassing of mineral oil, thermal at three temperature bands, low-energy discharge, or high-energy discharge.
Michel Duval introduced the Pentagon in the early 2010s to address two weak spots of the Triangle: it couldn’t distinguish stray gassing of mineral oil (a benign oil-side reaction at moderate temperature) from a real T1/T2 thermal fault, and it couldn’t use the H₂ and C₂H₆ measurements that most labs already report. The Pentagon takes both gases as inputs and adds the dedicated S zone for stray gassing.
The gases it uses
H₂ · hydrogen
Earliest-onset fault gas; dominates corona and partial discharge signatures.
C₂H₆ · ethane
Marker for stray gassing of certain mineral oils and for low-temperature thermal activity.
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 centroid toward the D1 or D2 zone.
How the centroid is computed
The pentagon has radius 40 — meaning each gas’s axis runs 0 → 40 % of the five-gas total, with the 40 % mark sitting exactly on the pentagon vertex. The five sample points form a polygon, and Duval’s algorithm uses the polygon’s area-weighted centroid (the shoelace centroid, not the simple vertex mean) as the diagnostic point.
Why area-weighted: the simple vertex mean would over-weight gases that are below average and under-weight gases that are near the vertex. The shoelace centroid honors the polygon’s actual geometry, so a sample with one large component (say 80 % CH₄) ends up close to the CH₄ vertex rather than washed back toward the center.
The pentagon is applied once the five gases collectively clear 20 ppm — below that floor the percentages swing on lab noise and the centroid stops being meaningful.
Implementation status
Centroid plotted; zone classification pending.
The diagnosis tab for this model plots a sample’s centroid on a labeled pentagon and surfaces the (x, y) coordinate, but the seven Pentagon 1 zone polygons (PD, S, T1, T2, T3, D1, D2) aren’t encoded in the classifier yet. Until they are, use the Duval Triangle call as the primary diagnosis and read the pentagon geometrically against a printed reference.
When to reach for it
Once the zone math lands, the Pentagon is the right complement to the Triangle: it’ll pick up stray gassing the Triangle can’t see, it’ll often agree with the Triangle on D1/D2/T3 calls (which raises confidence), and a disagreement between Pentagon and Triangle on T1/T2 is itself a useful signal that something unusual is happening in the oil chemistry.
Like the Triangle, the Pentagon is calibrated for mineral-oil-filled equipment. Natural-ester fluids (FR3 and similar) have different gassing signatures and benefit from dedicated ester-oil pentagons rather than this one.
References
- M. Duval and L. Lamarre, “The duval pentagon — a new complementary tool for the interpretation of dissolved gas analysis in transformers,” IEEE Electrical Insulation Magazine, vol. 30, no. 6, pp. 9–12, 2014.
- CIGRÉ Technical Brochure 771, Advances in DGA Interpretation (2019) — chapter on the Duval pentagons, including the Pentagon 2 variant for distinguishing carbonization of paper.
- IEEE Std C57.104-2019, Annex G — references the pentagon alongside the Triangle as complementary screening methods.