Long before a fault becomes visible in a transformer, it leaves a trace in the insulating oil. Things like dissolved gases are produced by thermal and electrical stress inside the unit. Catching those traces early is one of the most cost-effective ways to avoid an unplanned outage.
That’s the job of Dissolved Gas Analysis (DGA) — one of the most precise diagnostic tools available for oil-filled electrical equipment. In this article, we’ll discuss how DGA works, what it can tell you, and where it delivers the most value for utility maintenance.
What Is Dissolved Gas Analysis?
DGA is a diagnostic method that evaluates the health of oil-filled transformers by sampling the insulating oil and testing it for gases produced by internal heat and electrical stress.
The Key Gases Analyzed
DGA focuses on identifying and quantifying several critical gases, each associated with particular fault types:
| Type of Gas | Gas Indication |
| Hyrodgen*: H2 | Indicates partial discharge, arcing, and overheating |
| Methane*: CH4 | Indicates low-energy arcing and thermal faults |
| Ethane*: C2H6 | Points to overheating and thermal faults |
| Ethylene*: C2H4 | Points to overheating and thermal faults |
| Acetylene*: C2H2 | A marker for high-energy arcing and severe thermal faults |
Note: * denotes combustible gas
How the Results Get Interpreted
Once the gas levels are measured, they’re run through standardized interpretation methods:
- Key Gas Method — compares individual gas levels to alarm thresholds
- Gas Ratio Methods — use defined ratios (Rogers Ratio, IEC Ratio) to categorize fault types
- Duval Triangle — a graphical method using three gas concentrations to pinpoint fault zones
- Trend Analysis — tracks gas levels over time to catch developing issues early
Modern DGA also increasingly pairs these methods with AI-based fault recognition models for even earlier warnings.
A note on fluid type: DGA works across mineral oil, ester, and silicone-filled transformers, but the interpretation standards aren’t interchangeable. Ester fluids, for example, generate different gas ratios than mineral oil under the same fault, so applying mineral-oil thresholds to an ester-filled unit can misread fault severity. TJ|H2b interprets results against the standard built for your fluid type (IEEE C57.104 for mineral oil, C57.155 for esters, C57.146 for silicone) so your results are read against the right baseline.
Why DGA Matters
Thermal, electrical, and partial-discharge activity all show up in gas levels long before they show up as downtime, which is why DGA catches faults early enough to act on. Because it gives such a clear read on equipment health, maintenance can happen when it’s actually needed.
Routine testing builds a trend line, not just a single snapshot, so you can track gas evolution over time in a way that’s often more insightful than any one result. All of that adds up to real savings: preventing even one unplanned failure typically outweighs years of routine testing costs.
Learn more in-depth information in our Transformer Testing Decision Guide: When, Why & How to Test Insulating Fluid.
Where DGA Delivers Value Beyond Transformers
Any equipment that uses oil for insulation or lubrication can be monitored with DGA. The most common applications:
Core Electrical Equipment
- Transformers — power and distribution units alike, for early detection of thermal, partial discharge, or arcing faults
- Reactors — monitored for insulation degradation or overheating
- On-Load Tap Changers (OLTCs) — checked for overheating or insulation breakdown during operation
- Bushings — monitored for insulation issues at the point conductors enter equipment
Extended Applications
- Cables — oil-impregnated paper insulation checked for overheating or degradation
- Switchgear & circuit breakers — oil-insulated units monitored for abnormal gas levels
- Wind turbine gearboxes — lubricating oil analyzed for early signs of gear wear or overheating
- Generators — insulating oil checked for early signs of degradation
How DGA Works
Image from: Microbe Notes, “Gas Chromatography: Principle, Parts, Steps, Procedure, Uses”
- Sampling — a representative oil sample is drawn from the equipment using procedures that keep air and contaminants out.
- Gas Extraction — dissolved gases are pulled from the oil via vacuum extraction or headspace analysis.
- Gas Analysis — gases are separated and measured using Gas Chromatography under ASTM D3612 and IEC 60567, producing a chromatogram of gas concentrations.
- Interpretation — results are compared against established norms, gas ratios, and tools like the Duval Triangle to identify fault type.
- Reporting — a report summarizes gas concentrations, interpretation, recommended actions, and historical trends.
- Follow-Up — based on the findings, next steps might include continued monitoring, further testing, or repair.
Pair DGA With TJ|H2b Diagnostics
DGA is most powerful alongside other tests. Microscopy and Forensic Analysis examine particle evidence in the same oil sample, and particle size often correlates directly with the arc energy DGA detects.
For a full picture of transformer health, DGA is also a core component of TJ|H2b’s Transformer Condition Assessment (TCA®) program.
Ready to Start Testing?
TJ|H2b offers accurate DGA and related diagnostic testing at competitive prices. We also offer support for R&D or unique projects if you don’t see the test you need.
Explore our full range of diagnostic testing services or contact us today to start your testing program.
