Beyond the Duval Triangle: A Multi-Method DGA Diagnosis Workflow for Asset Managers

The Duval Triangle is the most widely used DGA interpretation method — but it is not the only one, and relying on a single diagnostic method leaves significant information unused. This article presents a practical multi-method workflow that combines the strengths of each approach for more confident fault diagnosis.

Method 1: Key Gas Method (First Pass)

The Key Gas method is the simplest and fastest DGA screening tool. It identifies the dominant gas and maps it to the most likely fault type based on formation temperature. H₂ dominant → partial discharge or corona. C₂H₄ dominant → high-temperature thermal fault. C₂H₂ present in any significant quantity → arcing (C₂H₂ forms only above 700°C). CO and CO₂ elevated → paper/cellulose degradation.

The Key Gas method should be your first pass — if acetylene is present, you have an arcing fault and should initiate immediate investigation regardless of what other methods indicate. If only hydrogen is elevated, you likely have PD or a developing thermal fault — monitor and trend.

Method 2: Duval Triangle 1 (Primary Classification)

Using only three gases (%CH₄, %C₂H₄, %C₂H₂ normalized to 100%), Duval Triangle 1 classifies the fault into one of seven zones: PD, T1, T2, T3, D1, D2, or DT. The method always produces a classification — no ‘unresolved’ results. Plot your point and read the zone.

Use Duval Triangle 1 as your primary diagnostic method. Its determinism and graphical intuitiveness make it ideal for communicating findings to non-specialist stakeholders — ‘the transformer is in the D1 zone’ is clearer than ‘the C₂H₂/C₂H₄ ratio is 0.37 which is borderline according to IEC table A.3’.

Method 3: IEC 60599 Ratios (Secondary Confirmation)

Calculate the three IEC ratios: C₂H₂/C₂H₄, CH₄/H₂, and C₂H₄/C₂H₆. Compare against the standard interpretation table. If the IEC ratios agree with the Duval Triangle classification, confidence is high. If they disagree or return ‘unresolved’, investigate further — the discrepancy may indicate a mixed fault or a fault in transition.

Method 4: Rogers Ratio (IEEE Context)

The Rogers Ratio method uses the same three gas ratios as IEC 60599 but with different coding ranges and fault mappings. It is the traditional method referenced in IEEE C57.104. Run Rogers as a check against IEC — agreement between the two ratio methods adds confidence. Disagreement may reflect the different coding boundaries.

Method 5: CO₂/CO Ratio (Paper Involvement)

The CO₂/CO ratio indicates whether cellulose (paper) insulation is involved in the fault. CO₂/CO < 3: paper is being carbonized — serious, involves solid insulation. CO₂/CO 3-10: normal range. CO₂/CO > 10: paper aging, general oxidation — less urgent but indicates long-term insulation degradation.

This is a critical but often overlooked diagnostic. A thermal fault in oil only (T1-T3) may be manageable. The same thermal fault involving paper insulation is far more serious because paper is not replaceable without a complete rewind.

Practical Workflow

1. Key Gas screening: Is C₂H₂ present? Is CO significantly elevated?
2. Duval Triangle 1: Get the primary fault zone classification
3. IEC 60599 ratios: Confirm or flag discrepancies
4. CO₂/CO ratio: Assess paper involvement
5. Rate-of-change: Is the fault developing rapidly (>10% per day) or slowly?
6. Synthesis: If all methods agree, act with confidence. If they disagree, the transformer needs further investigation — possibly including electrical tests (PF, DF, DGA at multiple oil sampling points) or internal inspection.

Complete Diagnostic Methods Reference → Duval Triangle Guide →