Dissolved Gas Analysis (DGA) measures fault gases dissolved in transformer insulating oil.
It is the most reliable method for early detection of overheating, arcing, and partial discharge — helping utilities avoid catastrophic failures.
DGA stands for Dissolved Gas Analysis — or in full, Oil Dissolved Gas Analyzer. It is a diagnostic technique used to assess the health of oil‑filled power transformers by measuring the types and concentrations of gases dissolved in the insulating oil.
Think of it as a blood test for transformers. Just as a blood test reveals what’s happening inside the human body, DGA reveals the internal condition of a transformer without requiring a shutdown or invasive inspection.
When a transformer experiences abnormal operating conditions — such as overheating, electrical arcing, or partial discharge — the insulating oil and paper begin to degrade.
This degradation produces characteristic gases that dissolve into the oil. By analyzing these gases, maintenance engineers can:
Detect incipient faults before they lead to catastrophic failure
Identify fault types (thermal, electrical, or combination)
Predict remaining life and plan maintenance activities
Avoid unplanned outages and costly emergency repairs
Transformers are among the most critical and expensive assets in any power grid. A single transformer failure can result in millions of dollars in losses, extended outages, and safety risks.
DGA is widely recognized as the most effective method for early fault detection in oil‑filled transformers.
International standards (such as IEEE C57.104 and IEC 60567) provide guidelines for gas interpretation, making DGA a globally accepted practice for transformer condition assessment.
These abnormal cause excessive energy dissipation in the transformer insulation system, it can result in harmful mechanical stress on windings and other internal structures.
| Gas | Primary Fault Indicator |
|---|---|
| Hydrogen (H₂) | Corona, partial discharge, arcing |
| Methane (CH₄) | Low-temperature thermal faults |
| Ethane (C₂H₆) | Low to moderate thermal faults |
| Ethylene (C₂H₄) | High-temperature thermal faults |
| Acetylene (C₂H₂) | High-energy arcing |
| Carbon Monoxide (CO) | Cellulose (paper) degradation |
| Carbon Dioxide (CO₂) | Cellulose degradation |
| Oxygen (O₂) | Seal integrity, oil oxidation |
| Nitrogen (N₂) | Blanket gas integrity |
When energy is diverted, it can affect the insulating liquid or cellulose insulating material, causing the generation of gases. These gases dissolve in the insulating liquid and spread to all available gas spaces in the transformer.
The key gases monitored typically include:
Key takeaways:
DGA detects incipient faults before failure.
Moisture accelerates insulation aging and reduces dielectric strength.
Tailored gas sets match monitoring cost to asset criticality.
While the nine fault gases (hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, carbon dioxide, oxygen, and nitrogen) form the core of traditional DGA, a complete transformer health picture requires attention to two additional dimensions: moisture content and customized gas monitoring strategies.
Moisture (water content) in transformer oil is a critical parameter that directly affects insulation performance and transformer lifespan. Water can enter the oil through several pathways:
Breathing: As transformers heat and cool, they inhale moist air through conservators or breathers
Insulation degradation: Cellulose (paper) insulation breaks down over time, releasing water as a byproduct
Seal failure: Leaking gaskets or seals allow moisture ingress
Why moisture matters:
Reduces dielectric strength: Even small amounts of water dramatically lower the oil’s ability to withstand voltage stress
Accelerates paper aging: Moisture acts as a catalyst for cellulose degradation, shortening transformer life
Lowers breakdown voltage: Wet oil is more susceptible to electrical breakdown, increasing failure risk
Affects gas solubility: Moisture changes how gases dissolve in oil, potentially impacting DGA accuracy
For these reasons, comprehensive online DGA systems typically include moisture measurement alongside gas analysis. Monitoring both gases and moisture provides a complete view of transformer health.
Not all transformers require monitoring of all nine fault gases. Depending on transformer type, capacity, application, and manufacturer specifications, DGA monitoring strategies can be customized to focus on the most relevant gases.
| Configuration | Gases Monitored | Typical Application |
|---|---|---|
| Single‑gas | Hydrogen (H₂) | Distribution transformers; basic early warning |
| Three‑gas (Type A) | H₂ + Methane (CH₄) + Acetylene (C₂H₂) | Medium‑voltage industrial transformers; detects both thermal and electrical faults |
| Three‑gas (Type B) | H₂ + CH₄ + Carbon Monoxide (CO) | Transformers where paper degradation is a primary concern |
| Standard six‑gas | H₂, CH₄, Ethylene (C₂H₄), C₂H₂, CO, Carbon Dioxide (CO₂) | Most transmission and large distribution transformers |
| Full‑spectrum | All 9 gases + moisture | Critical assets (generator step‑up, intertie, large GSU) |
Cost efficiency: Monitoring fewer gases reduces sensor complexity and upfront cost.
Application fit: Different transformer types have different failure modes.
Distribution transformers: often monitored with single‑gas (hydrogen) for basic fault detection.
Medium‑voltage industrial transformers: three‑gas sets provide adequate coverage at lower cost.
Large transmission transformers: full 9‑gas monitoring enables complete diagnostic capability.
Manufacturer guidance: Some transformer manufacturers specify particular gas monitoring sets based on their design and materials.
Risk‑based approach: Higher‑risk assets justify more comprehensive monitoring.
This flexibility allows utilities and industrial facilities to scale their monitoring investment to match asset criticality and application requirements.
Q: What gases does DGA detect?
A: Typically hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, carbon dioxide, oxygen, and nitrogen. Modern systems also measure moisture.
Q: Why is single‑gas hydrogen monitoring not always enough?
A: Hydrogen can originate from non‑fault sources — such as outgassing from steel used in transformer construction — leading to false alarms. Multi‑gas monitoring provides context and reduces misinterpretation.
Q: How often should DGA be performed?
A: For critical assets, continuous online DGA is recommended. For lower‑risk transformers, quarterly or annual laboratory DGA may suffice.
Q: What is the difference between laboratory DGA and online DGA?
A: Laboratory DGA uses gas chromatography (GC) for high‑accuracy spot checks. Online DGA uses technologies like photoacoustic spectroscopy (PAS) for continuous, real‑time monitoring without consumables.
In future articles, we will compare the main DGA technologies — Gas Chromatography (GC), Photoacoustic Spectroscopy (PAS), and infrared methods — and explain why enhanced PAS is enabling consumable‑free online monitoring.
👉 Read the full DGA technology guide → (link to be added)
Dissolved Gas Analysis is the cornerstone of transformer health monitoring. A complete DGA strategy considers not only the nine fault gases but also moisture content, which plays a critical role in insulation integrity. Moreover, the monitoring approach should be tailored to the specific transformer — from single‑gas hydrogen monitors for distribution transformers to full 9‑gas plus moisture systems for critical transmission assets.
By understanding the “blood test” of transformers, utilities can move from reactive repair to true predictive maintenance, protecting critical assets and ensuring grid reliability.
HERTZINNO’s online DGA systems (DGA900, DGA500, DGA300) utilize enhanced MEMS‑based photoacoustic spectroscopy to deliver consumable‑free, maintenance‑free continuous monitoring with flexible gas set options.
Learn more about our online DGA solutions →
