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]. The DGA gas ratios guide works through the IEC ratio limits and Duval zones in full.

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 DGA alarm limits and trending guide covers the norms, baselines, and false positives, and 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