The key gases monitored typically include:
| 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 |
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.
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 →
