DGA Interpretation Guide
DGA Interpretation Guide
Dissolved gas analysis results must be interpreted correctly to identify fault types and determine appropriate actions. This guide covers the four primary interpretation methods used by the industry, all of which are supported by VA-TEK DGA monitors and software.
1. Key Gas Method
The simplest approach. Certain gases are strongly associated with specific fault types:
| Key Gas | Indicated Fault |
|---|---|
| Hydrogen (H2) | Partial discharge (corona) |
| Acetylene (C2H2) | Arcing / high-energy discharge |
| Ethylene (C2H4) | Thermal fault > 500 C in oil |
| Ethane (C2H6) | Thermal fault < 500 C in oil |
| Methane (CH4) | Low-temperature thermal fault |
| Carbon Monoxide (CO) | Cellulose (paper) overheating |
| Carbon Dioxide (CO2) | Cellulose aging / overheating |
2. Duval Triangle Method
The Duval Triangle (IEC 60599) uses the relative proportions of three hydrocarbon gases — CH4, C2H4, and C2H2 — plotted on a triangular diagram to classify faults into six zones: PD (partial discharge), T1 (thermal < 300 C), T2 (thermal 300-700 C), T3 (thermal > 700 C), D1 (low-energy discharge), and D2 (high-energy discharge). This method is widely used because it provides clear, unambiguous fault zones.
3. Rogers Ratio Method
The Rogers Ratio method calculates three gas ratios (CH4/H2, C2H2/C2H4, C2H4/C2H6) and compares them against defined ranges to classify faults. While useful, it can produce “no-match” results in some cases where the gas pattern does not fit neatly into the defined ratio ranges. Our software automatically flags no-match cases for further investigation.
4. IEC 60599 / IEEE C57.104 Standards
These standards define concentration limits for individual gases and Total Dissolved Combustible Gas (TDCG), categorized into four condition levels:
| Condition | Description | Recommended Action |
|---|---|---|
| Condition 1 | Normal operation | Continue routine monitoring |
| Condition 2 | Gas levels above normal | Increase sampling frequency; investigate load history |
| Condition 3 | High gas levels — probable fault | Immediate investigation; consider outage |
| Condition 4 | Critical gas levels — active fault | Remove from service; perform internal inspection |
Gassing Rate Analysis
Beyond absolute concentrations, the rate of gas increase (ppm/day or ppm/year) is often more indicative of fault severity than the concentration itself. A transformer with stable, slowly rising gas levels may be monitored safely for years, while one with a rapidly increasing trend requires urgent attention even if absolute levels are moderate. VA-TEK software automatically calculates gassing rates and provides visual trend charts.