The combustible gases in power transformers are Carbon Monoxide (CO), Hydrogen (H2), Methane (CH4), Ethane (C2H6), Ethylene (C2H4), and Acetylene (C2H2). Gases in the transformer may occur from the external environment or from the chemical reactions internally. An important characteristic of transformer oil is that it allows gases to dissolve within it and store this gas in a dissolved state for a significant amount of time. This situation could have dual outcomes, potentially yielding negative consequences or serving as an opportunity to gather further insights into the transformer’s operations.
Formation
Dissolved gas data are obtained by taking an oil sample and testing [IEC 60567]. The most common dissolved gases in transformer mineral oil are Nitrogen (N2), Oxygen (O2), Carbon Dioxide (CO2), Carbon Monoxide (CO), Hydrogen (H2), Methane (CH4), Ethane (C2H6), Ethylene (C2H4), and Acetylene (C2H2) [Wang1]. All the gases dissolved in transformer oil are measured as “Total Gas” which is measured as a percentage content taken from the oil sample.
The degradation of oil insulation material produces a fair amount of hydrocarbon compounds, which are composed of hydrogen and carbon atoms that are broken up into two distinct classes, namely aliphatic and aromatic hydrocarbons [Arora1]. Aliphatic hydrocarbons are further broken down into three groups, which are alkanes, alkynes, and alkenes. Alkanes are composed of single bonds; alkenes consist of double carbon bonds and alkynes consist of triple carbon bonds [Arora1]. This means that more energy is required for the breakdown of double and triple bonds implying that less energy is required for gases such as methane and ethane and higher energy for ethylene and acetylene.
Hydrocarbons release Carbon Dioxide and H2O as products of combustion [Arora1]. Studies have shown that the thermal decomposition of cellulose insulation produces carbon oxides (CO and CO2), some hydrogen, and methane due to the oil impregnation [Kachler1, Wang1]. The oxygen levels in the transformer arise primarily from the atmosphere. Oxygen causes oxidation of the oil, which sustains the insulation degradation process [Liland1].
The principal degradation product of low-energy electrical discharges found in mineral oil-filled transformers is molecular hydrogen [Rouse1]. The decomposition of methane (CH4) into its elements starts at about 578°C hence competes with its degradation to acetylene [Leroux1]. The formation of ethylene and acetylene has been noted to be specific to high-energy electrical discharges [Arakelian1]. At about 1327°C and higher, acetylene is more stable than other hydrocarbons but decomposes into its elements thus indicating that the conversion or splitting time must be incredibly short (milliseconds) [Leroux1]. The amount of energy needed is very large and in the region of the favorable free energy.
Figure 3 below provides a diagram of the combustible gas generation and their relationship to each other and fault temperature. This is based on laboratory studies and is generally used in the identification of faults.
From experience, it is found that Hydrogen, Methane, and Carbon Monoxide usually start to form in small amounts from around 70-80 ºC. These are considered low-energy dissolved gases and are currently utilized by the LEDT analysis tool which identifies low-energy changes from normal to defective state. If the condition prevails these three gases usually increase in formation.
It is noted that the Hydrogen levels constantly increase with increasing fault temperature. The hydrocarbon gases Methane, Ethane, and Ethylene have an initial increasing trend but as the temperature reaches specific maximum points for each of the gases there is a decrease and tapering point for the very high fault temperatures (arcing zones).
Methane levels start to increase usually for thermal-related faults and tend to maximize at around 250 ºC after which further increases in fault temperature cause a decrease in Methane production due to other higher chemical reactions. It is noted that for temperatures between 200 ºC and 300 ºC, the production of Methane usually exceeds that of Hydrogen
Ethane generally starts to form at temperatures around 250 ºC. This is an indication of the progression of the thermal fault to a medium energy level. The levels of Ethane then increase steadily up to a temperature in the region of 350 ºC which is generally where the further chemical reaction of higher energy takes place to support the production of the higher energy fault gases. The formation of Ethane then starts to decrease. It is noted that from about 275 ºC, the production of Ethane exceeds that of Methane.
At about 350 ºC, the production of ethylene begins. Ethylene is a key gas linked to high thermal faults where the levels increase reaching a maximum production rate at around 700 ºC. Further increases in fault temperature favor the production of Hydrogen and Acetylene where the rate of Ethylene production starts to decrease.
Acetylene production generally begins between 500 ºC and 700 ºC and rapidly increases for temperatures above 700 ºC. Acetylene is the key gas together with Hydrogen, linked to arcing. The high levels of Acetylene are an indication of a definite internal fault condition and the transformer must be taken out of service as soon as possible if it has not already failed.
Analysis using Combustible Gases
From these gases, “Combustible Gases” are identified as gases that can burn if mixed with air. The combustible gases are Carbon Monoxide (CO), Hydrogen (H2), Methane (CH4), Ethane (C2H6), Ethylene (C2H4), and Acetylene (C2H2). The measure “Total Dissolved Combustible Gas (TDCG)” is calculated by the summation of these six combustible gases. The measure “Total Combustible Gas (TCG)” is the percent volume of combustible gas found in the headspace or in the gas space.
The positive aspect of having dissolved gases is that these gases can provide an indication of the health of the transformer and identify the type of faults that may be present in the transformer. Dielectric oil and solid cellulose dielectric materials when degrading under thermal and electrical stresses produce gases of varying compositions and concentrations relative to the severity of the stresses applied to these materials [Griffin1].
The gases dissolve in the oil in a specific nature and concentration thus providing a good representation of the type and severity of the fault in the transformer [Rogers1]. The changes in the gas production rates form key components in the determination of the type of fault(s) involved where some specific gases form profiles for certain types of faults [Gibeault1].
It is found that due to the nature of chemical reactions and stresses within the transformer, Carbon Monoxide is generally the significant gas for normal operating transformers.
The guideline for generally accepted dissolved gas limits for the gases are given in table 1 [IEEE C57-104]
The TDCG generation rates (ppm per day) are also important parameters and can be calculated by taking the difference between the ppm values and the sample date difference in days. The following Table 2 as stipulated by IEEE C57-104 as a guide for the relevant actions to be taken:
The other generally accepted dissolved gas analysis methods are Trending, Key Gas Method, Rogers Ratio, Doernenburg’s Method, Duval’s Triangle 1, Duval’s Triangle 4, Duval’s Triangle 5, Duval’s Pentagon, and newer methods such as LEDT and R-Value Trend.
Use the following link to the “Analysis” section to get the general diagnosis of the oil samples. Enter the oil sample under “Sample 4” to get the diagnosis.
Combustible gases can be used in a health index as applied in the Transformer Age Index Model to assess the health of the transformer
References
| [Arakelian1] | Arakelian, V. G., “Effective Diagnostics for Oil-filled Equipment,” IEEE Electrical Insulation Magazine, Vol. 18, No. 6, pages 26-38, November / December 2002 |
| [Arora1] | Arora, A., “Hydrocarbons (Alkanes, Alkenes, and Alkynes),” Discovery Publishing House, 2006 |
| [Gibeault1] | Gibeault, J. P., Kirkup, J. K., “Early Detection and Continuous Monitoring of Dissolved Key Fault Gases in Transformers and Shunt Reactors,” Electrical Electronics Insulation Conference, Electrical Manufacturing & Coil Winding Conference, IEEE Proceedings, Pointe-Claire, Quebec, Canada, 1995 |
| [Griffin1] | Griffin, P. J., “Criteria for the Interpretation of Data for Dissolved Gases in Oil from Transformers (A Review),” Electrical Insulating Oils, STP 998, American Society for Testing and Materials, Philadelphia, pages 89-106, 1988 |
| [IEC 60567] | IEC 60567, “Oil-filled Electrical Equipment – Sampling of Gases and of Oil for Analysis of Free and Dissolved Gases – Guidance,” 4th Edition, IEC Publication, 2011 |
| [IEEE C57-104] | IEEE guide for the interpretation of gases generated in oil-immersed transformers, IEEE Standard C57.104-2019. |
| [Rogers1] | Rogers, R. R., “IEEE and IEC Codes to Interpret Incipient Faults in Transformers Using Gas in Oil Analysis,” IEEE Transactions on Electrical Insulation, Vol. 13, No. 5, pages 349-354, October 1978 |
| [Kachler1] | Kachler, A. J., Hohlein, I., “Ageing of Cellulose at Transformer Service Temperatures. Part1: Influence of Type of Oil and Air on the Degree of Polymerisation of Pressboard, Dissolved Gases and Furanic Compounds in oil,” IEEE Electrical Insulation Magazine, Vol. 21, No. 2, pages 15-21, 2005 |
| [Leroux1] | Leroux, P. J., Mathieu, P. M., “Kinetics of the Pyrolysis of Methane to Acetylene,” Chem. Eng. Prog. Vol. 57, pages 54-59, 1961 |
| [Liland1] | Liland, K. B., Kes, M., Ese, M. H. G., Lundgaard, L. E., Christensen, B. E., “Study of Oxidation and Hydrolysis of Oil Impregnated Paper Insulation for Transformers Using a Microcalorimeter,” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 18, No. 6, pages 2059-2068, December 2011 |
| [Rouse1] | Rouse, T. O., “Mineral Insulating Oil in Transformers,” IEEE Electrical Insulation Magazine, Vol. 14, Issue 3, Pages 6-16, May-June 1998 |
| [Wang1] | Wang, H., Butler, K. L., “Modeling Transformers with Internal Incipient Faults,” IEEE Transactions on Power Delivery, Vol. 17, No. 2, pages 500-509, April 2002 |