Dissolved Gas Analysis (DGA) is a crucial method for detecting early failures in oil-based transformers. DGA works by identifying dissolved gases formed due to thermal or electrical disturbances within the transformer. In practice, there are several approaches to interpreting DGA results, the two most widely used being the Duval Triangle and IEC 60599.
This article discusses a comparison between the two methods, including their advantages, disadvantages, and practical applications.
1. Duval Triangle: A Simple Visual Representation
The Duval Triangle is a graphical method developed by Michel Duval of Hydro-Québec. It uses the relative concentrations of three main gases:
- Methane (CH4)
- Ethylene (C2H4)
- Acetylene (C2H2)
These three gases are plotted in a triangle divided into zones, each representing a type of fault, such as:
- PD (Partial Discharge)
- T1, T2, T3 (Low to High Level Thermal Discharges)
- D1, D2 (Discharges/Electric Arcs)
Advantages of the Duval Triangle:
- Fast and easy to use
- No ratio calculations required
- Provides a visual indication of faults
Disadvantages:
- Only uses three types of gases
- Interpretation can be ambiguous if gases are present at zone boundaries
2. IEC 60599: Formal Standard Based on Gas Ratios
IEC 60599 is an international standard that provides guidelines for interpreting DGAs through combinations of gas ratios, such as:
- CH4/H2
- C2H4/C2H6
- C2H2/C2H4
This method defines various types of disturbances based on specific ratio values, taking into account gas history and trends.
Advantages of IEC 60599:
- Systematic and documented
- Suitable for long-term monitoring and gas trends
- Widely used in maintenance management systems
Disadvantages:
- More complex, requires a deep understanding of ratios
- Can be inaccurate if gas concentrations are very low
3. When to Use Each?
Duval Triangle Situation IEC 60599
Rapid field evaluation Yes Less suitable
Official reporting for audits or regulators Less accurate Yes
Monitoring of high-risk transformers Can be combined Highly recommended
Interpretation by non-expert technicians Simple Requires training
4. Combining the Two: Best Practice
In the real world, many practitioners use a combination of the Duval Triangle and IEC 60599 to improve interpretation accuracy. The Duval Triangle visualization can provide initial indications, while IEC 60599 provides ratio- and trend-based insights.
Both the Duval Triangle and IEC 60599 have their own advantages. Duval is faster and more visual, suitable for quick inspections, while IEC 60599 is more systematic and suitable for documentation and technical decision-making. For best results, use both in a complementary manner, supported by regular monitoring and adequate technical training.
