Dielectric oil and solid cellulose dielectric materials when degrading
under thermal and electrical stresses produce gases of varying compositions and
in concentrations relative to the severity of the stresses applied to these
materials [Griffin1]. These gases dissolve in the oil where the nature and
concentrations of the dissolved gases sampled provides 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].
Dissolved gas data are obtained from oil sample testing [IEC 60567]. The most common gases analysed
from oil tests are hydrogen (H2), methane (CH4),
acetylene (C2H2), ethylene (C2H4), ethane
(C2H6), carbon monoxide (CO), carbon dioxide (CO2),
oxygen (O2) and nitrogen (N2) [Wang1].
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.
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.
The principle 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 favourable free energy.
All 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].
All 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].
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
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[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
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[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
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[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
|
[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
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[Leroux1]
|
Leroux, P. J., Mathieu, P. M., “Kinetics of the Pyrolysis
of Methane to Acetylene,” Chem. Eng. Prog. Vol. 57, pages 54-59, 1961
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[Liland1]
|
Liland, K. B., Kes, M., Ese, M. H. G., Lundgaard, L. E.,
Christensen, B. E., “Study of Oxidation an 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
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[Rouse1]
|
Rouse, T. O., “Mineral Insulating Oil in Transformers,” IEEE
Electrical Insulation Magazine, Vol. 14, Issue 3, Pages 6-16, May-June 1998
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[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
|
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