Transformer Fault Types & Gas Signatures — Complete Diagnostic Reference

From Gas Data to Fault Identification

Each type of transformer fault produces a characteristic pattern of dissolved gases. Understanding these patterns — and the physical damage mechanisms behind them — transforms DGA from a monitoring tool into a true diagnostic instrument.

Partial Discharge (PD)

Temperature range: <150°C (low-energy ionization)
Primary gas: H₂ (hydrogen) — typically >80% of total combustible gas
Secondary gases: CH₄ (methane), trace C₂H₆
Key ratio: CH₄/H₂ < 0.1 (indicating PD rather than thermal)
Duval zone: PD

Physical Mechanism

Partial discharge occurs in gas-filled voids within solid insulation — between paper layers, in epoxy spacers, or at electrode-insulation interfaces. The electric field in these voids exceeds the breakdown strength of the gas, causing localized ionization without complete breakdown. Each discharge event:

1. Ionizes gas molecules in the void
2. Bombards the void walls with energetic electrons and ions
3. Breaks C-H bonds in adjacent oil molecules
4. Releases atomic hydrogen that recombines into H₂

Over time, PD erodes the solid insulation surface, progressively enlarging the void and increasing discharge energy — a self-accelerating degradation process.

DGA Signature

Risk Assessment

Low-Energy Discharge (D1)

Temperature range: 150–500°C (low-energy sparking)
Primary gas: H₂ + C₂H₂ (acetylene appears!)
Secondary gases: CH₄, C₂H₄
Key ratio: C₂H₂/C₂H₄ = 0.5–3.0
Duval zone: D1

Physical Mechanism

D1 discharges are small sparks or intermittent flashovers between parts at different potentials — often from:

• Floating shields or screens
• Loose clamping bolts or core grounding straps
• Poorly bonded tank walls or magnetic shunts
• Small carbonized tracking paths on insulation surfaces

The distinguishing feature from PD is the presence of acetylene (C₂H₂) — the temperature in the spark channel exceeds 700°C locally, enough to break triple C-C bonds and form C₂H₂, but the total energy release is low.

DGA Signature

Risk Assessment

Any acetylene in a transformer that previously had zero C₂H₂ demands immediate investigation. Even 2–5 ppm of new C₂H₂ indicates that discharge is occurring somewhere. The transformer may continue operating while investigation proceeds, but the sampling frequency should be increased to daily or continuous.

High-Energy Discharge / Arcing (D2)

Temperature range: >1000°C (power arc)
Primary gas: C₂H₂ (acetylene) — massive production
Secondary gases: H₂, C₂H₄, CH₄ — all sharply elevated
Key ratio: C₂H₂/C₂H₄ > 3.0
Duval zone: D2

Physical Mechanism

High-energy arcing is the most destructive transformer fault. A sustained power arc between windings, from winding to core/tank, or across a tap changer contact can:

• Vaporize copper conductors
• Carbonize hundreds of liters of oil
• Generate explosive gas pressures (tank rupture risk)
• Destroy the transformer within seconds to minutes

Arcing temperatures exceed 1000°C and can reach 3000°C at the arc root. At these temperatures, all hydrocarbon bonds break and recombine, producing large quantities of acetylene, hydrogen, and other gases.

DGA Signature — Emergency Pattern

Emergency Response

Any transformer with rapidly rising acetylene >10 ppm should be considered at imminent risk of catastrophic failure. The recommended response:

1. Immediately increase to continuous monitoring (if not already)
2. De-energize if possible — the arc will not self-extinguish
3. Do not rely on Buchholz relay alone — gas accumulation may lag behind arc development
4. Prepare for complete failure — fire suppression, oil containment, spare transformer mobilization

Thermal Fault — Low Temperature (T1, <300°C)

Temperature range: <300°C
Primary gas: CH₄ (methane) — dominant
Secondary gases: H₂, C₂H₆
Key ratio: C₂H₄/C₂H₆ < 1.0 (indicating lower temperature)
Duval zone: T1

Physical Mechanism

T1 faults involve mild overheating from:

• Slightly overloaded conductors
• Early-stage bad contacts or loose connections
• Circulating currents in core clamping structures
• Improperly seated bushings

At these temperatures, oil molecules thermally crack into smaller fragments, with methane as the primary product.

DGA Signature

Action Plan

T1 faults are generally not urgent. They indicate a developing problem that should be investigated at the next scheduled maintenance opportunity. Continue monitoring for transition to T2 (increasing C₂H₄).

Thermal Fault — Medium Temperature (T2, 300–700°C)

Temperature range: 300–700°C
Primary gas: C₂H₄ (ethylene) becomes significant
Secondary gases: CH₄, C₂H₆, H₂
Key ratio: C₂H₄/C₂H₆ = 1.0–4.0
Duval zone: T2

Physical Mechanism

T2 faults represent significant overheating — typically from:

• Bad contacts at bolted connections (common after maintenance)
• Strand-to-strand short circuits in windings (few strands affected)
• Core lamination short circuits (localized)
• Blocked oil cooling ducts (localized hot spot)

DGA Signature

Action Plan

T2 faults require planned intervention within weeks to months. Load reduction can temporarily slow gas generation. Schedule internal inspection at next opportunity.

Thermal Fault — High Temperature (T3, >700°C)

Temperature range: >700°C
Primary gas: C₂H₄ (ethylene) + C₂H₂ appears
Secondary gases: All gases elevated
Key ratio: C₂H₄/C₂H₆ > 4.0; C₂H₂ begins to appear
Duval zone: T3 (bordering D2 as temperature rises)

Physical Mechanism

T3 faults are severe overheating approaching the carbonization temperature of oil:

• Multiple strand short circuits in windings
• Severe core faults with large circulating currents
• Tank wall heating from magnetic flux impingement
• Advanced OLTC contact deterioration

At these temperatures, oil carbonizes into conductive carbon particles, accelerating degradation and risking flashover.

DGA Signature

Action Plan

T3 faults require urgent action. The transformer should be removed from service as soon as practical. If C₂H₂ is rising concurrently, treat as potential D2 — consider immediate de-energization.

Cellulose / Paper Degradation

Temperature range: All temperatures (chemical process)
Primary gas: CO (carbon monoxide)
Secondary gas: CO₂ (carbon dioxide)
Key ratio: CO₂/CO < 3 (pyrolysis); CO₂/CO = 3–7 (oxidation); CO₂/CO > 7 (normal aging)
IEEE C57.104 90% norms: CO = 900 ppm, CO₂ = 9,000 ppm

Physical Mechanism

Paper insulation (cellulose) degrades through two distinct mechanisms:

Pyrolysis (Thermal Decomposition)

• Occurs at hot spots >105°C
• Produces CO as the primary carbon oxide
• CO₂/CO ratio drops below 3
• Irreversible — paper cannot regenerate
• Degree of polymerization (DP) decreases — mechanical strength lost

Oxidation (Normal Aging)

• Occurs slowly at normal operating temperatures (60–90°C)
• Produces more CO₂ than CO
• CO₂/CO ratio typically >7
• Accelerated by moisture and oxygen

DGA Signature

Risk Assessment

Cellulose degradation is the most serious long-term condition because paper insulation cannot be replaced without a complete transformer rewind. Rising CO with a falling CO₂/CO ratio indicates active paper pyrolysis — the transformer’s solid insulation is being consumed.

Action: If CO₂/CO < 3 with rising CO trend, assess remaining transformer life. Consider derating to reduce hotspot temperatures. Prepare replacement strategy.

Fault Summary Matrix

References

• IEC 60599:2022 — Mineral oil-filled electrical equipment — Interpretation of dissolved and free gases
• IEEE C57.104-2019 — Guide for Interpretation of Gases in Mineral Oil-Immersed Transformers
• CIGRE TB 771 (2019) — Advances in DGA Interpretation (330,000+ case database)
• Duval, M. — “A Review of Faults Detectable by Gas-in-Oil Analysis in Transformers” (IEEE Electrical Insulation Magazine, 2002)

Next Steps

• DGA Diagnostic Methods — Apply the Duval Triangle, IEC Ratios, and multi-method integration
• DGA Fundamentals — Review the chemistry and properties of the 7 key fault gases
• Online vs Offline DGA — Selecting the right monitoring strategy for your assets