Dissolved Gas Analysis (DGA) of Mineral Oil Used in Transformer

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					International Journal of Application or Innovation in Engineering & Management (IJAIEM)
       Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 1, Issue 2, October 2012                                         ISSN 2319 - 4847


    Dissolved Gas Analysis (DGA) of Mineral Oil
                  Used in Transformer
                                             Rahul Pandey1, M.T.Deshpande2
                      1
                       IEEE Student Member & M.E. student (Power System) at SSCET Bhilai (C.G.) INDIA
                          2
                           IEEE Member & H.O.D Electrical Department at SSCET Bhilai (C.G.) INDIA




                                                          ABSTRACT
Transformer oil is one of the most common materials used for transformers. The oil has two important functions. The oil need
to provide cooling and electrical insulation for the transformer. Any deterioration in the oil can lead to the premature failure of
the transformer. When the mineral oil is subjected to high thermal and electrical stress, gases are generated from the
decomposition of the mineral oil. Different type of faults will generate different gases, and the analysis of these gases will
provide useful information about the condition of the oil and the identification of the type of fault in the transformer. The
chemical analysis of these gases is called dissolved gas analysis or DGA. The DGA will require the removal of an oil sample
from the transformer and this can be done without de-energization of the transformer. The oil sample is analysed in the
laboratory using gas chromatography technique.
Keywords: Transformer, Dissolved Gas Analysis (DGA), Fault types, Gas Chromatograph

    1. INTRODUCTION
Power generation industry is most important in any country all over the world. By and large entire economy of the
country depends upon its capacity to generate power. During the last decade a lot of accidents took place in this
industry and causes of these accidents/explosions remained unexplained. Recently with the advent of latest technology
in the field of analysis, it has been possible to pin point the possible cause of explosion in the power stations.
In oil-filled equipment, such as power transformers, which is mainly a mixture of hydrocarbons, failures are inevitable
if proper care is not taken. With the use of transformer, the oil starts degrading due to various factors such as ageing of
the oil, overvoltage, environmental condition, overheat and numerous unknown factors. In the process of degradation
lot of lower hydrocarbons like methane, ethane, acetylene, ethylene etc. are produced along with some permanent gases
like CO, CO2 and H2. These mixtures in sealed environment can cause an explosion [3]. The gases depend upon the
condition of the oil and hence, it is extremely vital to monitor the concentration of these explosive gases in the running
transformer.

     2. MECHANISM OF GAS GENERATION
The cause for the gas generation in the mineral oil is the breaking of the chemical bonds in the hydrocarbon molecules
of the mineral oil. Energy is needed for the breaking of the chemical bonds and this comes from the energy contained
in the fault of the transformer. The gases generated include hydrogen (H2), methane (CH4), ethane (C2H6), ethylene
(C2H4), acetylene (C2H2), carbon dioxide (CO2) and carbon monoxide (CO).




Figure1 Chemical Structure of Gases                                        Figure 2 Formations of Radicals
These generated gases will initially dissolve in the oil and cannot be seen by the naked eye. As the volume of generated
gases increases, more of these gases will dissolve into the oil. There will come a point when the oil will be totally
saturated with the dissolved gas, and any further increase in gases cannot be contained as dissolved gas in the oil. The
excess gases will come out as free gas.


Volume 1, Issue 2, October 2012                                                                                       Page 208
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
       Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 1, Issue 2, October 2012                                         ISSN 2319 - 4847

                                           Table1: Recombination of Radicals




     3. CLASSIFICATION OF FAULT
There are four basic types of faults [1], which can occur in the transformer:
      Arcing or high current break down
      Low energy sparking or partial discharges.
      Localized overheating or hot spots and general overheating due to inadequate cooling or sustained overloading
Each of the faults results in thermal degradation of the oil either alone or in combination with paper insulation. This
gives rise to the evaluation of various hydrocarbon gases, hydrogen and oxides of carbon, in quantities depending on
the type of fault [4].
      Heavy current arcing is characterized by the evolution of significant quantities of hydrogen and acetylene
         (C2H2). If the arcing also involves paper insulation, the oxide of carbon will also be present.
      Partial discharge usually results in evolution of hydrogen and lower order hydrocarbons.
      Localized heating or hot spot gives rise to methane and ethane in appreciable amount.
      Prolonged overloading or impaired heat transfer can cause CO and CO2 to be generated due overheating paper
         insulation.
IEC-599[7] is the guide to the interpretation of dissolved gas analysis in mineral oil. The faults are broadly divided into
thermal and electrical type.
                                                   Table 2: Types of Fault
                                                           Faults
                                            Thermal        Electrical Type
                                            Type
                                            Thermal        Partial Discharge
                                            Fault (T1)
                                            Thermal        Discharge of Low
                                            Fault (T2)     energy
                                            Thermal        Discharge of High
                                            Fault (T3)     energy
Fault of low energy will favour the breaking of C-H molecular bonds. More energy or higher temperature is needed to
break the C - C single bonds, C = C double bonds and C ≡ C triple bonds in ascending order of greater energy or higher
temperature. Acetylene gas will require very high temperature of at least 800 to 1200 degree Celsius to form because it
has a C ≡ C triple bond. Ethylene gas will form at a lower temperature of more than 500 degree Celsius because of the
C = C double bond. Ethane and methane will form at lower temperature because of the C - C single bond.
For thermal fault T1 of less than 300 degree Celsius, the paper insulation of the transformer will turn brownish. For
thermal fault T2 between 300 to 700 degree Celsius, the paper insulation will carbonize. For thermal fault T3 of more
than 700 degree Celsius, the oil will carbonize, metal will colorize of fuse.
For electrical fault of partial discharge is nature, wax may form in the oil. Electrical fault of low energy discharge and
high energy can be due to discharge through the oil, discharge through the paper insulation or discharge at the surface
of the paper insulation, or degradation of the surface of the paper insulation to form conducting paths or small arcs.

  4. NORMAL VALUES OF DISSOLVED GAS
The mineral oil will contain normal values of dissolved gas which will indicate no incipient fault in the transformer.
Figure shows the normal values of dissolved gases in the oil. When the DGA results of all the 7 key gases are less than
the values of figure it can be concluded there was no incipient fault in the transformer.

                                    Table 3: Normal Values of Dissolved Gas in Oil

                                                  Gas           ppm
                                                  H2            100
                                                 CH4             50
                                                 C2H6            50

Volume 1, Issue 2, October 2012                                                                                Page 209
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
       Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 1, Issue 2, October 2012                                         ISSN 2319 - 4847

                                               C2H4           50
                                               C2H2           5
                                               CO2           5000
                                                CO           200

    5. STANDARD GAS




                                               Figure 4 Standard Gas
Figure shows the standard gas cylinders are used for calibration of these gases. The cylinder contains approximately
100 ppm of hydrocarbons, 200 ppm of H2 and 5000 ppm of each CO and CO2.

                                  Table 4: Concentration levels of Standard Gas
                              RT     Peak Area            Area      Heigh Heigh
                              (min) name (mV*sec) %                 t (mV) t %
                              0.91     CO       625.914     34.41    80.6     53.463
                              1.28     CH4      163.738     9.003    23.408   15.527
                              4.04     CO2      439.803     24.18    23.193   15.384
                              5.45     C2H4     226.796     12.47    10.703    7.099
                              6.93     C2H6     202.329     11.12    7.939     5.266
                              10.6     C2H2     156.659     8.614    4.272     2.834




                                                Figure5 Sample Gas


                                     Table 5: Concentration levels of Sample Gas




Volume 1, Issue 2, October 2012                                                                          Page 210
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 1, Issue 2, October 2012                                         ISSN 2319 - 4847

                                RT          Peak      Area       Area      Height      Height
                                (min)       name      (mV*sec)   %         (mV)        %
                                0.91        CO        307.637    22.66     31.393       36.612
                                1.29        CH4       144.512    10.64     16.83        19.628
                                4.18        CO2       383.266    28.23     17.896       20.871
                                5.71        C2H4      197.47     14.54     9.043        10.547
                                7.3         C2H6      180.775    13.31     6.723        7.841
                                 11.1       C2H2      143.55     10.57     3.78         4.413




                                            Figure 6 Corona or Partial Discharge
Figure shows large quantities of hydrogen can indicate heavy current arcing. Oxides of carbon may also be found if the
arcing involves paper insulation. Partial discharge usually results in evolution of hydrogen and lower order
hydrocarbons.
                                       Table 6: Concentration levels of Gases


                         RT            Peak        Area          Area        Height          Height
                         (min)         name        (mV*sec)      %           (mV)            %
                         0.05          H2          120.894         71.78     13.413             71.85
                         0.4           H2          47.521          28.21     5.255              28.14




                                                      Figure 7 Overheating

Figure shows Localized heating or hot spot gives rise to methane and ethane in appreciable amount.

                                            Table7: Concentartion levels of Gases

            RT (min)     Peak name           Area (mV*sec)        Area %             Height (mV)        Height    %
            0.98         CO                  14.305               7.915              1.094                      12.362
            1.3          CH4                 5.297                2.931              0.548                       6.194
            4.01         CO2                 143.223              79.24              6.48                       73.202
            5.42         C2H4                6.339                3.507              0.296                        3.34
            6.9          C2H6                2.183                1.208              0.126                       1.427
            10.3          C2H2                9.39                5.19                0.308             3.474


Volume 1, Issue 2, October 2012                                                                                          Page 211
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
       Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 1, Issue 2, October 2012                                         ISSN 2319 - 4847

     6. APPLICATION
In a new transformer, typical hydrocarbon gases concentration for good new oil after vacuum filtration would be within
5 p.p.m. DGA shall be repeated once a month after commissioning and then at increasing intervals as found necessary.
In an overhauled and repaired transformer, the transformer should be subjected to DGA a week after decommission and
again after about three months in service.

     7. CONCLUSION
In interpretation of the results obtained for a particular transformer, due regard should be given to the following factors
before arriving at a specific conclusion:
      Date of commissioning of the transformer
      Loading cycle of the transformer
      Date on which the oil was last filtered
Early diagnosis and periodic monitoring of the condition of oil-filled electrical equipment used in generation and
distribution of power can improve reliability and availability of power. DGA is an unique condition monitoring tool and
has given significant benefits in the form of major reductions in unscheduled transformer failure, reduced maintenance,
and reduced cost of repairs.
Some inaccuracy is always associated with laboratory DGA measurements of transformer oil, which may affect the gas
ratios, concentration differences and other calculations upon which transformer condition assessment and fault
diagnosis by DGA are based.

REFERENCES
  [1] J.J Kelly, “Transformer fault diagnosis by dissolved–gas analysis” IEEE Trans. On Industry Applications, vol. 16,
    no.pp.777-782, Dec.1980.
  [2] R.Rogers, “IEEE and IEC codes to interpret incipient faults in transformer, using gas in oil analysis,” IEEE
    Trans. On Electr.Insul. vol.13, no.5, pp.349-354, October 1978.
  [3] M. Duval, “Dissolved gas analysis: It can save your transformer,” IEEE Electrical Insulation Magazine, Vol 5,
    No.6, pp. 22-27, 1989.
  [4] IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers, IEEE Standard C57-104-
    2008, Sep.2008.
  [5] M.Duval, “New technologies for dissolved gas in oil analysis,” IEEE Electrical Insulation Mag., vol.19, no.2 pp.
    6-15, 2003.
  [6] R.R. Roger, “IEEE and IEC Codes to Interpret Incipient Faults in Transformers, Using Gas-In-Oil Analysis,”
    IEEE Tran. Elect. Insul. Vol. EI-13, No. 5, pp. 348-354, 1978.
  [7] IEC Publication 599, “Interpretation of the Analysis of the Gases in Transformers and Other Oil-Filled Electical
    Equipment in Service,” 1978.
  [8] Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers, IEEE Std C57. 104-1991, 1991.
  [9] J. Singh, Y. R. Sood, and R. K. Jarial, “Condition monitoring of power transformers – bibliography survey,”
    IEEE Electr. Insul. Mag., vol. 24, no. 3, pp. 11–24, 2008.

                     Rahul Pandey presently pursuing his M.E in power system engineering from Shri
                     Shankaracharya College of Engineering & Technology.




                     M.T Deshpande presently working as Head of Department (Electrical) in Shri Shankaracharya
                     College of Engineering & Technology, IEEE member.He is M.Tech in Power System, having 31
                     years of experience in steel industry and 9 years in teaching,




Volume 1, Issue 2, October 2012                                                                                Page 212

				
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