Dissolved Gas Analysis Limits

Awareness of dissolved gas analysis limits lets you detect minor faults and conditions that can lead to transformer failure. DGAs reduce risk to the unit, system and the personnel who monitor the transformer. You must do a DGA for transformers after high voltage and temperature rise tests, during a commissioning period, before a lapsed warranty and for predictive maintenance.

DGA interpretation is complex, and there are multiple factors to consider when analyzing the results. These include the transformer’s baseline readings and which results are abnormal compared to environmental factors, age and insulation type.

Faults DGA Can Detect

DGA gives you valuable insight into a transformer’s condition through concentration and trend analysis of dissolved gases in transformer oil. Here are examples of faults these tests can detect.

• Overheating: DGA detects overheating from poor cooling, high current loads or faulty components. 
• Partial discharge: A DGA is sensitive to partial discharge activities in transformers. Higher gas levels can indicate electrical faults relating to partial or corona discharges.
• Mechanical stress: These tests can pick up on mechanical faults like core movements, loose connections and winding deformation. 
• Arcing and sparking: Abnormal gas ratios and elevated levels of specific gases can indicate electrical arcing or sparking in a transformer.
• Insulation degradation: Insulation degradation due to aging, chemical reactions or moisture ingress shows up in elevated levels of specific gases like carbon monoxide or methane.

Common Gases in Dissolved Gas Analysis

The presence of gas in oil indicates electrical insulation materials, faults or a chemical reaction in the equipment. Here are gases to look for during a DGA.

• Acetylene: Acetylene indicates the presence of high-energy arcing in a transformer due to a very hot spot, low- or high-energy discharge.
• Hydrogen: A hydrogen amount of over 100 parts per million can indicate the presence of corona, partial discharge or free water electrolysis. It may show up due to thermal faults, power discharges, rust or partial discharge. 
• Methane: Methane gas generally indicates overheating. The source of this gas’ presence may be partial corona discharge or low- and medium-temperature thermal faults.
• Ethane: Ethane often indicates transformer overheating from low- and medium-temperature thermal faults.
• Ethylene: Where ethylene is present, there may be a hot spot or localized overheating from high-temperature thermal faults.
• Carbon monoxide: Carbon monoxide indicates the aging or thermal decomposition of a transformer’s cellulose insulation. Double-check this reading by looking into the furans. This gas may be present due to a thermal fault involving cellulose or from oil oxidation.

Analyzing the concentration and trends of these gases over time helps you identify and mitigate potential transformer faults. It ensures your electrical infrastructure’s longevity and reliability. Concentration levels should be within these ranges.

Less Conservative Levels	More Conservative Levels
Acetylene normal < 1 PPM	Acetylene normal < 1 PPM
Hydrogen normal < 500 PPM	Hydrogen normal < 100 PPM
Methane normal < 120 PPM	Methane normal < 100 PPM
Ethane normal < 65 PPM	Ethane normal < 50 PPM
Ethylene normal < 50 PMM	Ethylene normal < 50 PMM
Carbon monoxide < 1,000 PPM	Carbon monoxide < 350 PPM

Industry Standards and Guidelines

Ensuring consistency and safety in engineering practices includes following industry standards and guidelines, especially regarding dissolved gas analysis limits for transformers. Here is how these standards relate to DGA.

• ASTM D3612: ASTM D3612 is a standard test method for performing DGA on electrical insulating liquids and transformer oils. It outlines guidelines for sampling techniques to interpret and measure the concentrations of dissolved gases.
• IEC 60599: IEC 60599 specifies the interpretation of DGA results for assessing the power transformer’s condition. It categorizes dissolved gas concentrations into fault types to diagnose potential transformer problems like partial discharge or overheating.
• IEEE C57.104: IEEE C57.104 provides guidelines for interpreting DGA results and determining power transformer conditions. It comes with recommendations to establish baseline gas levels, assess changes in gas concentrations and identify abnormal gas ratios that indicate specific faults.
• Manufacturer guidelines: Manufacturers often give guidelines or recommendations for DGA testing and result interpretation. They may include sampling frequencies, acceptable ranges and criteria for assessing the severity of faults you detect.

Factors Influencing Gas Concentration Levels

There are various reasons transformers may start to fail and even more factors that contribute to gas concentration levels. Understanding these variables can help you accurately interpret DGA results and address the underlying concerns to optimize transformer performance and reliability. 

• Temperature: Elevated temperatures accelerate the gas formation rate, potentially leading to higher gas concentrations where there are thermal faults or overloads.
• Load conditions: Changes in transformer loading affect the electrical and thermal stress on insulation materials, influencing the generation and release of gases. It is worth noting that a highly loaded transformer may have higher gas values, so remember each transformer’s particularities during testing.
• Transformer age: Insulation materials degrade as transformers age, which increases gas production and concentrations. Consider that older transformers may have higher baseline gas levels.
• Insulation type: Different insulation materials like paper, oil-impregnated insulation or cellulose can have varying susceptibilities to degradation and gas formation. Again, older or inferior insulation in transformers will produce higher gas concentrations.
• Operating environment: Environmental factors like humidity and pollutant exposure can affect the insulation’s degradation rate and gas formation. Harsh operating environments speed up the aging process to increase these gas concentrations.
• Transformer design and construction: The transformer’s construction and design, including size, insulation system and configuration, can impact gas concentrations.
• Fault severity and type: The severity of the electrical, thermal or mechanical faults occurring in the transformer can influence gas concentration levels in DGA results. Dramatic faults show rapid gas generation and higher concentrations of specific gases.

Learn More With ELSCO Transformers

DGA guidelines are a non-intrusive way to determine if a transformer has an existing incipient fault condition and prevent unexpected outages. Compliance with industry standards ensures accurate DGA practices, allowing engineers to make informed decisions about transformer repair, maintenance and replacement. Moreover, adherence to these guidelines promotes reliable, safe electrical infrastructures to lower the chances of unplanned outages or equipment failure.

The professionals at ELSCO Transformers are here to guide you on DGA analysis limits. Founded in 1912, we have served commercial customers across North America by building, engineering, repairing and selling transformers. Contact us today for your transformer needs or call us at 800-232-9002.

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