Background

Acetylene is a combustible gas that is produced within a power transformer and may provide an indication of high energy faults.

Chemical Structure

The following is the chemical structure of Acetylene (C₂H₂)

Formation of Acetylene

What causes the production of Acetylene 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 [1].

Acetylene usually forms at temperatures of at least 700 °C to 1200 °C with quick quenching to lower temperatures which is why it is formed in significant quantities mainly in arcs where the conductive ionized channel is at several thousands of degrees Celsius, and the interface with the surrounding liquid oil necessarily below 400 °C [1].

It is also identified that acetylene may still be formed at lower temperatures (<700 °C), but in very small quantities [1].

Fault Analysis using Acetylene

The IEEE guide for the interpretation of gases generated in oil-immersed transformers prescribes a limit of 1 ppm [3].

Due to arcing being a condition of high energy discharge usually because of a breakdown of the insulation the presence of acetylene is taken seriously. Usually, when the arcing condition is present the fault has already progressed to a serious state and will require drastic actions to isolate the transformer if it has not already failed.

The source of arcing can come from insulation breakdown in the windings, (coil to coil or coil to ground) where the insulation is damaged and cannot maintain the dielectric capability. Another source of arcing can be from winding shorts (turn to turn, or phase to phase, or phase to ground) and loose connections, usually in the high voltage areas which can result in high energy discharges.

Acetylene 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
• Acetylene 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.