DGA Interpretation: From Data to Diagnosis
Measuring gas concentrations is only the first step. The real value of DGA lies in interpreting those numbers to identify what type of fault is developing inside the transformer, how severe it is, and how urgently action is needed.
Several standardized diagnostic methods exist, each with different strengths. The best practice is to apply multiple methods and look for consistent findings — multi-method integration achieves 89–92% diagnostic accuracy compared to 42–62% for any single method alone.
Method 1: Duval Triangle 1
Standard: IEC 60599 Annex B
Gases used: CH₄, C₂H₄, C₂H₂ (three hydrocarbon gases only)
Best for: Distinguishing thermal faults from electrical faults
How It Works
The Duval Triangle plots the relative proportions of three gases:
- %CH₄ = CH₄ / (CH₄ + C₂H₄ + C₂H₂) × 100
- %C₂H₄ = C₂H₄ / (CH₄ + C₂H₄ + C₂H₂) × 100
- %C₂H₂ = C₂H₂ / (CH₄ + C₂H₄ + C₂H₂) × 100
Each point in the triangle corresponds to a specific gas ratio combination. The triangle is divided into 7 fault zones:
| Zone | Fault Type | Gas Signature |
|---|---|---|
| PD | Partial Discharge (corona) | High %CH₄, very low %C₂H₂, low %C₂H₄ |
| D1 | Low-energy discharge | Moderate %C₂H₂, moderate %CH₄ |
| D2 | High-energy discharge (arcing) | High %C₂H₂, low %CH₄ |
| T1 | Thermal fault <300°C | High %CH₄, very low %C₂H₂ |
| T2 | Thermal fault 300–700°C | Moderate %CH₄, higher %C₂H₄ |
| T3 | Thermal fault >700°C | Low %CH₄, high %C₂H₄, some %C₂H₂ |
| DT | Combined thermal + electrical | Mixed signature — both C₂H₄ and C₂H₂ elevated |
Example Calculation
If your DGA results are:
- CH₄ = 120 ppm, C₂H₄ = 80 ppm, C₂H₂ = 10 ppm
- Total = 210 ppm
- %CH₄ = 57.1%, %C₂H₄ = 38.1%, %C₂H₂ = 4.8%
This point falls in zone T2 — a thermal fault in the 300–700°C range.
When to Use
Duval Triangle 1 is the most widely used DGA diagnostic method globally. Use it first for any transformer showing elevated hydrocarbon gases. It is particularly effective for separating thermal faults (requiring maintenance planning) from electrical faults (requiring urgent action).
Method 2: Duval Pentagon 2
Standard: CIGRE TB 771
Gases used: H₂, CH₄, C₂H₆, C₂H₄, C₂H₂ (all five hydrocarbons)
Best for: Detecting stray gassing and distinguishing oil vs. paper involvement
How It Works
The Duval Pentagon uses all five hydrocarbon gases, normalized to percentages and plotted in a pentagon coordinate system. It identifies 7 fault zones plus an additional stray gas zone (S) that Duval Triangle 1 cannot detect.
| Zone | Fault Type |
|---|---|
| PD | Partial discharge |
| D1 | Low-energy discharge |
| D2 | High-energy discharge |
| T1 | Thermal fault <300°C |
| T2 | Thermal fault 300–700°C |
| T3 | Thermal fault >700°C |
| S | Stray gassing (catalytic reactions, NOT a fault) |
Key advantage: The stray gas zone (S) is critical. Some healthy transformers generate H₂, CH₄, and C₂H₆ through catalytic reactions between oil and certain metals (particularly stainless steel, zinc, and some coatings) at normal operating temperatures. The Pentagon can identify this non-fault condition, preventing unnecessary outages.
When to Use
Use Duval Pentagon 2 when:
- Duval Triangle 1 gives borderline results between zones
- You suspect stray gassing (new transformer with rising H₂ but no other fault indicators)
- You need maximum diagnostic accuracy (Pentagon uses all available hydrocarbon data)
Method 3: IEC Basic Gas Ratio Method
Standard: IEC 60599
Gases used: H₂, CH₄, C₂H₂, C₂H₄, C₂H₆
Best for: Quick classification using standardized ratio codes
How It Works
Three gas ratios are calculated and coded:
| Ratio | Formula | Gas Pair |
|---|---|---|
| R1 | CH₄ / H₂ | Methane-to-Hydrogen |
| R2 | C₂H₂ / C₂H₄ | Acetylene-to-Ethylene |
| R3 | C₂H₄ / C₂H₆ | Ethylene-to-Ethane |
Each ratio is assigned a code (0, 1, or 2) based on its value range, and the three-digit code maps to a fault type:
| R1 Code | R2 Code | R3 Code | Fault Diagnosis |
|---|---|---|---|
| 0 | 0 | 0 | Normal (no fault) |
| 1 | 0 | 0 | Partial discharge (low energy) |
| 0 | 1 | 0 | Low-energy discharge (D1) |
| 0 | 2 | 0 | High-energy discharge (D2) |
| 0 | 0 | 1 | Thermal fault <300°C (T1) |
| 0 | 0 | 2 | Thermal fault 300–700°C (T2) |
| 2 | 0 | 2 | Thermal fault >700°C (T3) |
Limitations
The IEC ratio method is simpler than Duval methods but has two known limitations:
- “No decision” cases — some ratio combinations don’t map to any fault (approximately 15–20% of cases)
- Sensitivity to measurement uncertainty — small changes near code boundaries can flip the diagnosis
Method 4: Rogers Ratio Method
Standard: IEEE C57.104 (historical), IEC 60599
Gases used: Same as IEC — CH₄/H₂, C₂H₂/C₂H₄, C₂H₄/C₂H₆
Best for: North American utilities familiar with legacy IEEE methods
How It Works
Rogers uses four ratios (adding C₂H₆/CH₄ to the three IEC ratios) with a more detailed coding system:
| Ratio | Formula | Code 0 | Code 1 | Code 2 |
|---|---|---|---|---|
| R1 | CH₄/H₂ | ≤0.1 | 0.1–1.0 | ≥1.0 |
| R2 | C₂H₆/CH₄ | <1.0 | ≥1.0 | — |
| R3 | C₂H₄/C₂H₆ | <1.0 | 1.0–4.0 | ≥4.0 |
| R4 | C₂H₂/C₂H₄ | <0.5 | 0.5–3.0 | ≥3.0 |
The four-digit code maps to 12 fault types, providing finer granularity than the IEC method.
When to Use
Rogers Ratio is still widely used by utilities that maintain legacy IEEE-aligned diagnostic procedures. For new installations and modern asset management programs, Duval methods are generally preferred due to higher accuracy and fewer “no decision” results.
Method 5: Key Gas Method
Standard: IEEE C57.104
Gases used: All 7 fault gases
Best for: Rapid first-pass screening; non-specialist interpretation
How It Works
The Key Gas Method uses simple rules of thumb based on which gases dominate the DGA results:
| Dominant Gas(es) | Likely Fault | Action Priority |
|---|---|---|
| H₂ only | Partial discharge or corona | Monitor trending |
| H₂ + CH₄ (low C₂H₄, no C₂H₂) | Low-temperature thermal (<300°C) | Schedule inspection |
| C₂H₄ dominant (with CH₄, C₂H₆) | High-temperature thermal (>500°C) | Plan short-term outage |
| C₂H₂ present (any amount) | Arcing or severe electrical fault | Immediate investigation |
| CO + CO₂ elevated (CO₂/CO <3) | Cellulose/paper degradation | Assess remaining life |
| H₂ + C₂H₂ + C₂H₄ all elevated | Major internal fault in progress | Emergency outage |
When to Use
The Key Gas Method is the simplest entry point and can be applied by non-specialists for initial triage. However, it should never be the only method used for critical decisions — always confirm with Duval or IEC ratio analysis.
Multi-Method Integration Strategy
The recommended diagnostic workflow for online DGA data:
Step 1: Triage with Key Gas Method
Is C₂H₂ present? → If yes, priority is immediately elevated.
Are any gases above IEEE 90% norms? → Initiate full analysis.
Step 2: Classify with Duval Triangle 1
Plot CH₄/C₂H₄/C₂H₂ percentages → Identify fault zone (PD/D1/D2/T1/T2/T3/DT).
Step 3: Verify with IEC or Rogers Ratios
Calculate ratio codes → Confirm or challenge the Duval diagnosis.
If methods disagree → suspicious case, escalate to laboratory GC confirmation.
Step 4: Refine with Duval Pentagon 2 (if available)
Check for stray gas zone (S) if results are ambiguous.
Use pentagon coordinates for maximum diagnostic confidence.
Step 5: Trend Analysis
The most important data point is not the absolute gas concentration — it is the rate of change. A gas concentration rising rapidly through normal ranges is more concerning than a stable concentration above a threshold.
Step 6: CO₂/CO Ratio Check
If cellulose involvement is suspected, evaluate CO₂/CO ratio and rate-of-change for both gases.
Diagnostic Accuracy Comparison
| Method | Accuracy (Published Studies) | “No Decision” Rate | Input Gases |
|---|---|---|---|
| Key Gas | 42–55% | N/A (always gives answer) | All 7 |
| Rogers Ratio | 55–65% | ~15% | 5 |
| IEC Ratio | 60–70% | ~18% | 5 |
| Duval Triangle 1 | 70–82% | <5% | 3 |
| Duval Pentagon 2 | 80–89% | <3% | 5 |
| Multi-Method Integration | 89–92% | <1% | All 7 |
Bottom line: No single method is perfect. The best diagnostic approach combines at least three methods (Duval Triangle 1 + IEC Ratios + trend analysis) for maximum confidence.
Interactive DGA Diagnostic Tool
Manual calculation of Duval coordinates is tedious. Use our DGA Diagnostic Calculator (coming soon) to input your gas concentrations and receive instant fault zone classification with Duval Triangle and Pentagon coordinates.
Next Steps
- Fault Types & Gas Signatures — Detailed physical mechanisms, photos, and real case examples of each fault type
- DGA Fundamentals — Review the 7 key fault gases and their generation chemistry
- Online vs Offline DGA — Choosing the right monitoring strategy for your transformer fleet