Background
Ethylene in Power Transformers is a combustible gas that is produced within a power transformer and may provide an indication of thermal faults.
Chemical Structure
The following is the chemical structure of Ethylene (C2H4)
Ethylene is a hydrocarbon with two Carbon atoms bonded to 2 Hydrogen atoms each. It usually exists as a gas with the formula C2H4.
Formation
What causes the production of Ethylene 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 on d (720 kJ /mol) or C≡C triple bond (960 kJ /mol) occurs [IEC60599].
At about 350 °C, the production of Ethylene (C2H4) usually starts and is usually favored over Ethane and Methane above temperatures of approximately 500 ° C [1]. As the temperature increases further the production rate of Ethylene goes down even though it may still be present in significant but lower quantities.
Fault Analysis
The IEEE guide for the interpretation of gases generated in oil-immersed transformers prescribes a limit of 50 ppm [3].
Ethylene is usually a key gas associated with T2 and T3 type thermal faults with Ethane and Acetylene. It is found from experience that when Ethane and Ethylene are generated with little or no Acetylene it is usually an indication of hot metal faults within the transformer. Ethylene is thus commonly known as a “hot metal gas.”
Some examples may include (but are not limited to) bad contacts on the tap changers, bad connections such as on leads, stray flux on the tank and parts, compromised core grounding, and circulating currents.
Ethylene can be used for fault diagnosis in the following ways:
- Trended over a period of time to identify any increasing patterns
- Rogers Ratio Method
- Doernenburg Ratio Method
- Duval’s Triangle 1 Method
- Duval’s Pentagon Method
- Ethylene 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. |