Dissolved gas analysis makes it possible to detect transformer defects at an early stage of their development, determine the expected nature of the defect, and assess the extent of existing damage. The transformer condition is evaluated by comparing the quantitative data obtained during the analysis with the permissible gas concentration limits, as well as by analyzing the growth rate of gas concentration in oil. For transformers rated at 110 kV and above, this analysis should be performed at least once every six months.
Why dissolved gas analysis matters
Transformer oil serves as an effective diagnostic medium for evaluating the transformer condition. During normal operation of a transformer, small amounts of gas dissolve in the insulating oil as a result of internal insulation aging and other operational phenomena.
Developing faults in power transformers are diagnosed with the use of several criteria, including ultimate gas concentration values, gas concentration growth rates, and gas pair and group ratios. The ultimate concentration method is based on identifying when gas concentration levels exceed defined threshold values, indicating the presence of faults that may potentially lead to equipment failure.
The rate of increase in gas concentration also provides insight into the severity and urgency of a developing fault. If the relative increase in gas concentration exceeds 10% per month, the fault is considered to be rapidly developing. Fault types are further classified based on the ratios of specific gas pairs, which help distinguish between thermal and electrical faults.
Thermal faults include issues such as short circuits, overheating of insulation, contacts, bushings, and other metallic components within the transformer tank. Electrical faults are associated with discharges of varying intensity. In practice, fault development may involve a combination of both thermal and electrical phenomena.
Globally recognized standards, including IEC 60599:2015 and IEEE C57.104:2019, define the accepted methods for interpreting dissolved gas analysis results. These standards are based on extensive historical datasets and expert validation to establish reliable benchmarks for transformer fault diagnostics. The fundamental principle of these methods is the use of gas ratios, which involve comparing the relative concentrations of key diagnostic gases.
Diagnostic methods for dissolved gas analysis
Diagnostic methods for identifying transformer faults through dissolved gas analysis have distinct characteristics and applications. The most popular options include the Key Gas Method (KGM), the Doernenburg Ratio Method (DRM), the Rogers Ratios Method (RRM), the IEC 60599 Ratio Method (IRM), the Duval Triangle Method (DTM), and the Duval Pentagon Method (DPM). Each of these methods relies on interpreting dissolved gases in dielectric fluid to detect and classify potential issues.
The Key Gas Method focuses on identifying faults by analyzing the concentrations of specific gases. Each gas serves as an indicator of a particular type of fault within the transformer.
For instance, hydrogen (H₂) is commonly associated with electrical discharges, such as partial discharges or sparking, which may result from insulation breakdown or poor electrical connections.
Carbon monoxide (CO) is a key indicator of overheating of paper insulation, typically caused by excessive heat generated during transformer operation.
Ethylene (C₂H₄) usually forms during the thermal degradation of transformer oil when it is exposed to elevated temperatures. This may occur due to hot spots inside the transformer caused by overloading or localized heating.
Acetylene (C₂H₂), on the other hand, is a clear indicator of high-energy electrical discharges, such as arcing, which can severely damage insulation and other internal components.
The Doernenburg Ratio Method improves diagnostic accuracy by examining the ratios between specific gas pairs. By comparing hydrogen, methane (CH₄), ethane (C₂H₆), and ethylene, this method differentiates between thermal decomposition and electrical discharges, making it useful for distinguishing between fault types.
The Rogers Ratios Method further enhances this analysis by evaluating gas ratios across three specific gas pairs. It identifies faults associated with thermal degradation in different temperature ranges—below 572°F, between 572°F and 1294°F, and above 1294°F—while also detecting aging processes and discharge events.
Similarly, the IEC 60599 Ratio Method relies on gas ratios to classify faults such as low- and high-energy discharges and thermal defects within the same temperature ranges. This method provides a standardized approach, making it particularly suitable for international diagnostic practices.
The Duval Triangle Method simplifies fault interpretation by using the concentrations of methane, ethylene, and acetylene. By plotting these values on a triangular diagram, the method visually distinguishes between faults such as overheating, partial discharges, and arcing. It is widely regarded for its reliability in diagnosing transformer insulation issues.
The Duval Pentagon Method builds on this concept by incorporating two additional gases—hydrogen and methane—to provide a more comprehensive analysis. This method accounts for natural insulation aging, offering deeper insight into overall transformer condition. However, it still requires further validation and practical refinement before achieving widespread adoption.
Among these methods, the Duval Triangle is generally considered the most reliable for identifying insulation faults. While the Duval Pentagon offers a broader diagnostic perspective, including aging effects, its practical application is still being evaluated.
Duval Triangle
One of the most commonly used diagnostic methods in dissolved gas analysis is the Duval Triangle, which focuses on three gases: methane (CH₄), acetylene (C₂H₂), and ethylene (C₂H₄). In order to apply the Duval Triangle, gas concentrations are first converted into percentages that together sum to 100%. These values are then plotted on a triangular diagram, where the position of the plotted point indicates the nature of the fault.
The diagram is divided into distinct zones, each corresponding to a specific fault type, such as thermal overheating or electrical discharges. Each gas is represented along an axis of the triangular diagram.
Key Gases and Fault Indicators
Certain gases and their combinations serve as indicators of specific fault types:
- Hydrogen (H₂): a primary indicator of partial discharges.
- Methane (CH₄): associated with low-energy faults.
- Acetylene (C₂H₂): indicates high-energy arcing.
- Ethylene (C₂H₄): indicates overheating of insulating oil or electrical conductors.
- Carbon monoxide (CO) and carbon dioxide (CO₂): indicators of solid insulation degradation, particularly cellulose.
For example, elevated concentrations of CO and CO₂ often indicate thermal degradation of cellulose insulation, especially when their combined concentration exceeds 1%.
