Background

Carbon Monoxide is a combustible gas that is usually produced with Carbon Dioxide within a power transformer and may provide an indication of faults related to the paper insulation.

Chemical Structure of Carbon Monoxide

The following chemical structure of Carbon Monoxide is CO and Carbon Dioxide is CO₂

Formation of Carbon Oxides

What causes the production of Carbon Monoxide and Carbon Dioxide in transformer oil?

Carbon particles usually form at 500 °C to 800 °C and are also observed after arcing in oil or around hotspots. It is found that the formation of CO₂ and CO from oil-impregnated paper insulation increases rapidly with temperature increases [IEC60599].

It is also identified that the polymeric chains of solid cellulosic insulation (paper, pressboard, wood blocks) are composed of several anhydroglucose rings, weak C-O molecular bonds, and glycosidic bonds which break up at temperatures higher than 105 °C with complete decomposition and carbonization above 300 °C [1].

The oxidation reaction in oil may result in the formation of small quantities of CO and CO₂, which can accumulate over long periods of time into more substantial amounts [1].

Carbon Monoxide, Carbon Dioxide, and Water are formed together with minor amounts of hydrocarbon gases, furanic, and other compounds during this process. Another point to note is that CO and CO₂ formation increases with increasing temperature and high levels of Oxygen and Moisture content of oil and paper [1].

Fault Analysis

As an assessment, the CO₂/CO ratio may provide some indication of possible problems in the transformer [1].

Generally, high values of CO (> 1000 ppm) and CO₂/CO ratios less than 3 are an indication of probable paper involvement in a fault, with possible carbonization, in the presence of other fault gases [1].

It must also be noted that in some cases with closed and free breathing transformers CO can accumulate in the oil with the CO₂/CO ratio being less than 3. It is thus cautioned that this must be looked at in conjunction with other gases like H₂ and hydrocarbons. CO₂ however can accumulate more rapidly than CO in open transformers operating at changing loads because of their different solubilities in oil [1].

Other experiences have identified that high values of O₂ (>10000 ppm) and high CO₂/CO ratios (>10) can indicate mild (<160 °C) overheating of paper or oil oxidation, especially in open transformers [1].

The IEEE guide for the interpretation of gases generated in oil-immersed transformers prescribes a limit of 350 ppm for Carbon Monoxide and 2500 ppm for Carbon Dioxide [3].

Carbon Monoxide and Carbon Dioxide can be used in fault diagnosis in the following ways

• Trended over a period of time to identify any increasing patterns
• LEDT Method
• CO₂/CO ratio
• CO₂ and CO 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.