Background

Methane in Power Transformers is a combustible gas that is produced within a power transformer and may provide an indication of incipient faults.

Chemical Structure

The following is the chemical structure of Methane (CH₄)

Formation

What causes the production of Methane in transformer oil?

As the fault energy increases with higher oil temperatures the scission of the C-C bonds and their recombination into gases with a C-C single bond (607 kJ /mol), C=C double bond (720 kJ /mol) or C≡C triple bond (960 kJ /mol) occurs [1]. Methane is considered a “hot metal” gas together with Ethane (C₂H₆), and Ethylene (C₂H₄). If one of the gases exceeds the IEEE C57.104 – condition 1 limit, then the data indicates that hot metal is in contact with the oil [2]. 

Fault Analysis

The IEEE guide for the interpretation of gases generated in oil-immersed transformers prescribes a Methane limit of 120 ppm [2].

Methane is usually a key gas associated with T1-type thermal faults with Ethane (C₂H₆) and minor amounts of Hydrogen (H₂). At around fault temperatures of 200-300 °C Methane production is found to exceed that of Hydrogen [1].

Methane can be used for fault diagnosis in the following ways:

• Methane can be trended over a period of time to identify any increasing patterns.
• Once the key gas has exceeded the IEEE guide limit use Duval’s Triangle 1
• Use LEDT – Low Energy Degradation Method
• Rogers Ratio Method
• Doernenburg Ratio Method
• Duval’s Pentagon Method
• Methane can be used in a health index as applied in the Transformer Age Index Model to assess the health of the transformer

References

1.	IEC60599, “Mineral oi-filled electrical equipment in service – Guidance on the interpretation of dissolved and free gases analysis,” 2007.
2.	IEEEC57.104, “IEEE guide for the interpretation of gases generated in oil-immersed transformer,” IEEE Power Energy Society, 2019.
3.	Cigre-Technical-Brochure-771, “Advances in DGA interpretation,” JWG D1/A2.47, 2019.