CIGRE Technical Brochure 771 is the most comprehensive working-group report on dissolved gas analysis (DGA) interpretation methodology published to date. Produced by the joint working group that includes the originator of the Duval methods, it advances the earlier CIGRE brochures by tying severity assessment to the type and the physical location of a fault, and by setting out the full numerical machinery behind the Duval graphical methods. Its findings on bushings and wind-turbine transformers were carried into the current edition of IEC 60599.
What it covers
The brochure targets mineral oil-filled power transformers and their accessories — wind-farm transformers, bushings, and on-load tap-changers of both compartment and in-tank types. Its two defining contributions are conceptual. First, it expands the conventional fault taxonomy with additional stress categories — stray gassing, overheating, possible paper carbonization, and a thermal fault confined to the oil — that earlier schemes could not separate. Second, it replaces the single typical-value threshold with a graded severity concept that escalates through several levels, each tied to a recommended change in sampling intensity or intervention.
Crucially, the brochure shows that where a fault sits — in the oil or in the paper — changes the severity picture dramatically: the same characteristic gas can be benign in oil yet pre-failure in paper. It provides the complete zone-boundary coordinates for the Duval Triangles and Pentagons, dedicated treatment of carbon-oxide gases for cellulose assessment, the influence of the oxygen-to-nitrogen balance on typical values, tap-changer gas-pattern evolution, and the markedly different gassing behaviour of wind-farm transformers and bushings. It is built on an exceptionally large DGA case database.
Why it matters in practice
The single typical-value approach answers only whether a result is unusual. TB 771 answers the questions that follow: how urgent is it, what is the most likely mechanism, and — decisively — is the paper involved? Because faults in paper can become dangerous at gas levels that would be unremarkable in oil, the location question reframes risk entirely. The brochure also benchmarks the older ratio methods honestly, documenting their tendency to misclassify or return no diagnosis, and steers practitioners toward the graphical methods backed by its database. Its frank treatment of why high carbon monoxide alone is often oxidation rather than paper damage corrects a common over-diagnosis.
How we use it
TB 771 is our reference for multi-level severity interpretation and for the Duval zone geometry. For mineral oil transformer reports we use its graded levels as the default framework, and where a Duval method has confirmed the fault type we apply the corresponding fault-specific threshold. When acetylene is elevated, our first question is always whether the fault is in oil or in paper, settled with carbon-oxide ratios, furans, and acoustic methods. We do not apply its mineral oil thresholds to wind-farm transformers, where stray gassing and corona discharge dominate and are of lower concern, nor to ester fluids, which call for their own interpretation frameworks.