Transformers play a crucial role in facilitating the efficient transmission and distribution of electricity across cities, industries, and grids. But like any asset under constant stress, they are prone to deterioration. Early detection of incipient faults in transformers is highly cost-effective, as it significantly reduces the likelihood of unplanned outages.
One of the most precise techniques for diagnosing emerging issues in oil-filled electrical equipment is Dissolved Gas Analysis (DGA).
In this article, we’ll explore the science behind DGA, its evolving practical applications, and how it empowers utilities and engineers to protect high-value assets.
What is Dissolved Gas Analysis (DGA)?
Dissolved Gas Analysis (DGA) is a diagnostic method used to evaluate the health and condition of oil-filled transformers. This process involves sampling the insulating oil and testing it for dissolved gases that are produced as a result of thermal and electrical stresses within the transformer.
The Key Gases Analyzed
DGA focuses on identifying and quantifying several critical gases, each associated with particular fault types:
Furthermore, there are various interpretation methods used in DGA.
Methods of DGA Interpretation
There are multiple standardized approaches to interpreting DGA results:
- Key Gas Method: Focuses on individual gas levels and compares them to alarm thresholds.
- Gas Ratio Methods: Uses defined ratios (Rogers Ratio, IEC Ratio) to categorize fault types.
- Duval Triangle: A graphical method that uses three gas concentrations to identify fault zones.
- Trend Analysis: Used to monitor gas levels over time, allowing for the early detection of developing issues.
Additionally, modern DGA tools are also leveraging AI-based fault recognition models, which combine traditional interpretation techniques with predictive analytics to offer even earlier warnings.
Benefits of Dissolved Gas Analysis (DGA)
When used consistently, DGA empowers operators with otherwise invisible insight, reducing the guesswork in transformer maintenance and operation. Here are the key benefits of incorporating DGA into your transformer monitoring program:
- Early Fault Detection: DGA detects faults early, allowing for prompt repair and preventative detection.
- Condition-Based Maintenance: DGA provides a clear picture of the equipment’s health and prevents unnecessary maintenance.
- Improved Equipment Reliability: Regular DGA monitoring ensures the safety and reliability of electrical systems.
- Cost Savings: DGA contributes to significant cost savings by preventing unexpected failures and optimizing maintenance schedules.
- Historical Trend Insights: Routine DGA testing enables operators to track gas evolution over time.
Practical Applications of Dissolved Gas Analysis (DGA)
While transformers are the primary focus of dissolved gas analysis, their diagnostic power extends across a wide range of oil-insulated or oil-lubricated electrical equipment. Any component using insulating or lubricating oil as a dielectric or cooling medium can benefit from gas monitoring.
Below is a breakdown of where DGA delivers real-world value:
1. Transformers: Both power transformers and distribution transformers use insulating oil for insulation and cooling. DGA is important for monitoring transformer health, detecting faults early (such as thermal, partial discharge, or arcing), and extending their operational life.
2. Reactors: Reactors are used to limit short-circuit currents and filter harmonics in electrical systems. DGA helps in assessing the condition of reactors by monitoring gas levels that indicate insulation degradation or overheating.
3. Cables: High-voltage cables often use oil-impregnated paper as insulation. DGA can be used to monitor the condition of this insulation by detecting gases that indicate overheating or degradation of the paper.
4. On-load Tap Changers (OLTCs): OLTCs are used in transformers to adjust the turns ratio while the transformer is energized. DGA can help detect faults in OLTCs, such as overheating or insulation breakdown.
5. Switchgear and Circuit Breakers: Some types of switchgear and circuit breakers use insulating oil. DGA can be used to monitor these types of equipment for insulation degradation or faults indicated by abnormal gas levels.
6. Bushings: High-voltage bushings provide electrical insulation for conductors entering transformers or other electrical equipment. DGA can be used to monitor bushings for potential issues indicated by dissolved gas levels.
7. Wind Turbines: Many wind turbines use gearboxes to increase the rotational speed of the rotor to that of the generator. These gearboxes contain oil for lubrication and cooling. DGA can monitor the condition of the gearbox by analyzing gases dissolved in the lubricating oil. It can detect early signs of gear wear, overheating, or potential failures, allowing for proactive maintenance to prevent costly downtime.
8. Industrial Machinery: Generators that may use insulating oil. DGA can be applied to monitor the condition of the generator insulation, detecting faults such as overheating or insulation breakdown indicated by abnormal gas levels.
How Dissolved Gas Analysis (DGA) Works
DGA is a multi-step laboratory process that transforms a simple oil sample into actionable insight about internal transformer conditions. Each phase must be executed with precision to ensure reliable results. Here’s a summary of the process:
- Sampling: A representative sample of insulating oil is collected from the electrical equipment using precise procedures to ensure the sample is uncontaminated and accurately represents the oil in the equipment. When collecting the sample, it is important to prevent air from entering the transformer, especially if the headspace is under vacuum. The valve used for sampling should also be flushed with approximately 2 liters of oil to prevent contaminants that may have settled in the valve from affecting the sample.
- Gas Extraction: The extracted gases dissolved in the oil are extracted using techniques such as vacuum extraction or headspace analysis. This step ensures that gases dissolved in the oil are accurately measured.
- Gas Analysis: The extracted gases undergo analysis under ASTM D3612 (latest revision) and IEC 60567, which use analytical techniques such as Gas Chromatography (GC). Initially, the gas is injected into the GC system. Inside the column, the gas sample then undergoes separation based on its molecular properties. Different gas components exit the column at varying times and are subsequently detected by a sensor. The resulting signals are then transmitted to a computer, where they are plotted on a chromatogram. This chromatogram displays peaks (see blue peaks in the image above) that represent different gas components, allowing for comparison with established standards and accurate quantification.
- Interpretation: The results of the gas analysis are interpreted based on established norms and standards specific to the type of electrical equipment and the gases detected. Interpretation may involve comparing gas ratios, absolute gas concentrations, or using diagnostic tools like the Duval Triangle to identify potential faults or abnormalities.
- Reporting: A detailed report is generated summarizing the findings of the DGA analysis. This report typically includes the concentrations of the key gases, interpretation of the results, recommended actions (if any), and historical trends if available.
- Follow-up: Based on the analysis results, appropriate actions such as further monitoring, maintenance, or repairs may be recommended to ensure the continued reliable operation of the electrical equipment.
When to Conduct Dissolved Gas Analysis (DGA) for Transformers
The frequency for DGA in transformers typically depends on several factors, including the size of the transformer, its criticality to operations, environmental conditions, and regulatory requirements. Here are some general guidelines:
- Manufacturer Recommendations: Transformer manufacturers often provide guidelines on the recommended frequency of DGA testing. This is usually based on the type and size of the transformer, its design, and expected operational stresses.
- New Transformers: For new transformers, initial DGA testing may be conducted to establish a baseline. Depending on the results, subsequent testing intervals can be determined.
- Industry Standards: Standards such as IEEE (Institute of Electrical and Electronics Engineers) and IEC (International Electrotechnical Commission) may also provide recommendations on the frequency of DGA testing for transformers based on best practices and experience.
- Operating Conditions: Transformers operating under more severe conditions, such as high loads, frequent switching operations, or harsh environments (e.g., coastal areas with salt air), may require more frequent DGA testing compared to those under more benign conditions.
- Historical Data: Historical performance data and trends from previous DGA tests can also influence the frequency of testing. If previous tests have indicated stable conditions with low gas levels, testing intervals may be extended.
- Regulatory Requirements: In some industries or regions, regulatory authorities may mandate specific intervals for DGA testing as part of preventive maintenance and safety protocols.
Event-Driven Testing Recommendations
DGA should be immediately conducted in response to events such as:
- Transformer protection relay trips
- Sudden or severe load changes
- Cooling system failures
- Visual or audible signs of distress (e.g., leaks, pressure alarms)
- After repairs, oil processing, or relocation
With proper scheduling and responsive testing, DGA can shift transformer maintenance from reactive firefighting to a predictive, proactive practice.
TJ|H2b Laboratory Testing Services
At TJ|H2b, we offer accurate analytical testing at competitive prices, ensuring our clients receive top-quality service. Our expertise also extends to unique, special, and R&D projects, so if you don’t see the test you need, give us a call. We may be able to assist you!
Explore our full range of testing services here. Contact us today!
