Dissolved Gas Analysis (DGA) Explained

Dissolved Gas Analysis is the most powerful diagnostic tool available for assessing the condition of oil-filled transformers. By measuring the gases dissolved in transformer oil, DGA can detect incipient faults months or even years before they would otherwise be discovered — providing valuable time to plan maintenance and avoid catastrophic failures.

The Science Behind DGA

Transformer mineral oil is a hydrocarbon mixture. When the oil is subjected to thermal or electrical stress, the hydrocarbon molecules break down and recombine to form smaller gas molecules. The specific gases produced — and their relative proportions — are characteristic of the type and severity of the fault.

Similarly, the cellulose paper used for conductor insulation decomposes under thermal stress, producing carbon monoxide and carbon dioxide. By monitoring both oil-derived and paper-derived gases, DGA provides insight into the condition of the transformer’s entire insulation system.

The Fault Gases

Interpretation Methods

Once gas concentrations are measured, the next step is interpretation. Four main methods are used, and the best practice is to apply all four and look for consensus:

1. Key Gas Method

The simplest approach: identify which gas is dominant and correlate it with the most likely fault type. For example, a transformer with primarily acetylene is almost certainly experiencing arcing.

2. Gas Ratio Methods (Rogers, IEC, Dornenburg)

Calculate ratios between specific gas pairs (e.g., CH4/H2, C2H2/C2H4, C2H4/C2H6) and compare against standard tables. Different ratio combinations indicate different fault types.

3. Duval Triangle

Plot the relative percentages of CH4, C2H4, and C2H2 on a triangular diagram. The position within the triangle indicates the fault type: PD (partial discharge), T1, T2, T3 (thermal faults of increasing severity), D1 (low-energy discharge), D2 (high-energy discharge).

4. Standard Limits (IEC 60599, IEEE C57.104)

Compare individual gas concentrations and Total Dissolved Combustible Gas (TDCG) against standard-defined condition levels. Condition 1 is normal; Condition 4 requires immediate action.

Gassing Rate: The Critical Metric

While absolute concentrations are important, the rate at which gas levels are increasing (gassing rate, in ppm/day) is often more diagnostically significant. A transformer with stable, slowly rising gas levels may be monitored safely for years. But a transformer with rapidly increasing gas levels demands urgent attention — even if absolute concentrations are still within “normal” ranges.

Online DGA monitors are particularly valuable for gassing rate analysis because they provide frequent, consistent measurements that reveal trends which bi-annual or annual laboratory samples would miss.

Online vs. Laboratory DGA

Laboratory GC testing provides the most accurate absolute measurements and remains essential. However, for critical transformers, online monitoring fills the gap between laboratory samples, providing early warning of developing faults. Many utilities now combine both approaches: online monitors for continuous surveillance of critical assets, supplemented by annual or semi-annual laboratory testing across the fleet.

This article was prepared by the VA-TEK engineering team. Visit our DGA Interpretation Guide for more details, or browse our VT-DGA Series monitors.