The key gas method is a dissolved gas analysis method that utilizes Hydrogen (H₂), hydrocarbons (Methane CH₄, Ethane C₂H₆, Ethylene C₂H₄, Acetylene C₂H₂), and Carbon Monoxide CO.

The Key Gas Method was first explored at Doble laboratories in 1974 [Pugh73, Pugh74]. Early assessments included comparisons with other techniques, notably the Rogers Ratio Method, focusing on their application [Rick78]. Primarily designed for sealed or blanket-covered transformers, careful consideration is needed when applying this method, particularly to free-breathing conservator-type transformers [Griffin88]. 

The Key Gas method focuses on the levels of the individual gases generated from the breakdown of an insulating material after a fault [IEEE C57.104, IEC 60567]. The prevalence of a certain gas infers a corresponding fault diagnosis.

This method associates high levels of the key gas hydrogen with partial discharges or corona in oil as displayed in Figure 1. 

Methane and Ethane are linked to oil overheating at low temperatures. 

Hydrogen (H₂) and Acetylene (C₂H₂) with arcing.

Ethylene (C₂H₄) is associated with high-temperature overheating of oil, Carbon Monoxide (CO), and Carbon Dioxide (CO₂) with overheating of cellulose.

The challenge with this method is that most oil sample profiles do not usually fit into the key gas profiles and this method cannot distinguish between more than one fault mechanism happening at the same time.  

Enter the dissolved gas sample in ppm to get the key gases from the tool below.

Key Gas Input (ppm)

Other dissolved gas analysis methods that may be of interest are:

• Duval’s Triangle 1
• Duval’s Triangle 4
• Duval’s Triangle 5
• LEDT
• Rogers Ratio Method
• Doernenburg’s Method

References

[Pugh73]	D.R. Pugh, “Combustible Gas Analysis”, Minutes of Fortieth International Conference of Doble Clients, 1973, Section 10-401
[Pugh74]	D.R. Pugh, “Advances in Fault Diagnosis by Combustible Gas Analysis”, Minutes of Forty-First International Conference of Doble Clients, 1974, Section 10-1201
[Rick78]	A.L. Rickley, A.E. Baker, G.W. Armstrong, Jr., “Analytical Techniques for Fault-Gas Analysis”, Minutes of Forty-Fifth International Conference of Doble Clients, 1978, Section 10-401
[Griffin88]	P.J. Griffin, “Criteria for the Interpretation of Data for Dissolved Gases in Oil from Transformers (A Review)”, Electrical Insulating Oils, STP 998, H.G. Erdman edited, American Society for Testing and Materials, Philadelphia, 1988, pp.89-106
[C57.104]	ANSI/IEEE, C57.104-2008, Guide for the Interpretation of Gases Generated in Oil Immersed Transformers, Institute of Electrical and Electronic Engineers, Inc., New York, NY, 2008
IEC 60587	Guide for the sampling of gases and of oil-filled electrical equipment and for the analysis of free and dissolved gases. IEC Standard 60567, 2005.