Transformer DGA Explained: Fault Gases, Ratios, and Alarm Limits

A transformer usually announces a developing fault years before it fails, and it does so in gas: every thermal and electrical fault inside the tank cracks the oil into a specific mix of seven dissolved gases. Dissolved gas analysis (DGA) is the discipline of reading that mix, and two documents govern it - IEC 60599:2022 for interpretation [4] and IEEE C57.104-2019 for concentration norms [5].

Camlin builds the TOTUS online monitors that run this analysis continuously at the transformer [1]. This guide covers what each gas means, the six fault categories, the ratio methods that turn concentrations into a diagnosis, and where alarm limits actually come from.

The 7 Fault Gases and What Each One Means (IEC 60599)

Each gas forms at a different energy level, so the mix identifies the fault [4]:

Gas	Formed by	Points to
Hydrogen (H₂)	low-energy electrical stress	partial discharge; rises in almost every fault
Methane (CH₄)	low-temperature oil breakdown	early overheating
Ethane (C₂H₆)	low-temperature oil breakdown	T1 thermal faults
Ethylene (C₂H₄)	high-temperature oil breakdown	T2/T3 overheating
Acetylene (C₂H₂)	arc-level energy	D2 arcing; small amounts in D1
Carbon monoxide (CO)	cellulose degradation	paper insulation involvement
Carbon dioxide (CO₂)	cellulose degradation	paper ageing, read with CO

The headline rule: hydrogen is the universal early flag, acetylene is the gas that forms in quantity only at arc temperatures, and the two carbon oxides are the only window into the paper - the insulation that cannot be replaced.

The 6 Fault Categories: PD, D1, D2, T1 to T3 (IEC 60599)

IEC 60599 sorts every diagnosis into six categories [4]: PD (partial discharge), D1 (low-energy discharge - sparking and tracking), D2 (high-energy discharge - arcing), and three thermal tiers - T1 below 300 °C, T2 from 300 °C to 700 °C, and T3 above 700 °C. The categories matter because they carry different urgency: a T1 hot spot can be watched and trended; a D2 arcing signature is an outage conversation, not a monitoring one.

Ratios and the Duval Triangle: Reading the Mix

Absolute concentrations mislead - a large old transformer holds more oil, so the same fault reads lower in ppm. Interpretation therefore works on proportions [4]. The Duval Triangle plots the relative percentages of CH₄, C₂H₄, and C₂H₂ into seven zones covering PD, D1, D2, T1, T2, T3, and a mixed thermal-electrical zone. The IEC ratio method uses three quotients - C₂H₂/C₂H₄, CH₄/H₂, and C₂H₄/C₂H₆ - to reach the same categories numerically, and the CO₂/CO ratio flags whether the paper is involved [4]. CIGRE Technical Brochure 296 documents the limits of reading any single sample in isolation - the diagnosis lives in the trend, not the snapshot [6].

Alarm Limits: The Norms Are Not Your Limits (IEEE C57.104)

The concentration tables in IEEE C57.104-2019 are population statistics - 90th-percentile values derived from a large transformer fleet - not limits for your unit [5]. A 30-year-old transformer with a stable elevated hydrogen baseline is not failing; a young unit whose ethylene doubles in a month is, even if every reading sits under the table value. The defensible alarm scheme combines the published norms with the unit's own commissioning baseline and rate of change [5], and never quotes generic ppm thresholds from memory - the Camlin transformer monitoring specification guide covers how that translates into commissioning settings.

Online DGA vs Lab Sampling: Both, Not Either

An online monitor and the laboratory answer different questions. Camlin's TOTUS G5 measures the five headline gases plus moisture continuously using photo-acoustic spectroscopy, which needs no carrier gas and avoids the frequent recalibration of gas chromatography [2]; the TOTUS G9 extends to nine gases and integrates partial discharge, bushing, and through-fault-current monitoring in one unit [3]. What the monitor buys is time resolution: fault dynamics develop over days to weeks, between annual lab samples. What the lab keeps is the certified record - periodic laboratory DGA remains the reference for disputes and asset records, with the monitor trending between samples.

Common Interpretation Mistakes

The 5 recurring errors against IEC 60599 and IEEE C57.104:

• Treating one sample as a verdict. IEC 60599 ties resampling interval to the suspected fault type [4], and CIGRE TB 296 documents how single-point interpretation misleads [6]; diagnose the trend.
• Reading C57.104 tables as unit limits. They are fleet 90th-percentile norms [5]; the unit's own baseline and rate of change carry the alarm.
• Declaring arcing on acetylene alone. Where a tap-changer compartment communicates with the main tank, its normal switching gases can migrate [4]; rule that path out before condemning the windings.
• Ignoring CO and CO₂ because the hydrocarbon gases look clean. Paper degradation is the ageing that cannot be reversed, and the carbon oxides are its only gas signature [4].
• Expecting DGA to predict failure. DGA detects developing thermal and electrical faults; it does not forecast an imminent dielectric breakdown, and no monitor turns a D2 signature into a safe transformer.

What to Specify When Enquiring: 5 Items

A DGA monitoring enquiry that quotes cleanly states:

1. The asset - voltage class, rating, age, and criticality (grid transformer, generator step-up, or critical distribution unit)
2. Gas set - 5-gas G5 for DGA-and-moisture duty, or 9-gas G9 where partial discharge, bushing, and through-fault monitoring belong in the same box [2][3]
3. Installation - available oil valves and mounting position; confirm valve compatibility on the unit datasheet
4. Communications - the substation automation integration (IEC 61850 or otherwise) the alarms must reach
5. Baseline plan - commissioning baseline sampling and the lab-correlation schedule the alarms will be judged against [5]

Sourcing and Support in Pakistan

Pacific Engineering & Automation is the authorized Camlin reseller in Pakistan, supplying the TOTUS monitor range for transmission, K-Electric metro grid, DISCO, and IPP transformer fleets - with specification support that starts from the fault categories the fleet actually needs to see, not from a brochure gas count.

To put continuous DGA on a critical transformer, request a catalogue or quotation or contact our engineering team.

Field-derived case studies will be added to this post as Pacific Engineering & Automation accumulates engagement records. The current version is grounded in published specifications, regulatory documentation, and standards body references.

Sources

1. Camlin Energy - Transformer Monitoring - TOTUS monitoring suite overview
2. Camlin TOTUS G5 - 5-gas online DGA monitor
3. Camlin TOTUS G9 - 9-gas transformer monitor with PD, bushing, and through-fault monitoring
4. IEC 60599:2022 - Mineral oil-filled electrical equipment in service, guidance on the interpretation of dissolved and free gases analysis (4th ed.)
5. IEEE C57.104-2019 - IEEE Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers
6. CIGRE Technical Brochure 296 - Recent developments on the interpretation of dissolved gas analysis in transformers