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Transformer oil

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Transformer oil which is obtained by fractional distillation of crude oil serves many purposes in a transformer insulation.This report includes the classifications of transformer oil under composition and additives and it also includes the different types contamination that can occur in a transformer.These contamination issues can be tracked by an analysis of transformer oil.This report is consist of different transformer oil testing and also my personal experience in observing few tests at the laboratory of Asset Management Hydro Electrical - CEB.

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Transformer oil

  1. 1. Keywords Contamination, Insulation, Testing, Classification, Significance, Inhibited Abstract Insulating oil used in a transformer is commonly known as transformer oil and it serves many purposes of the transformer by the properties associated with it. Transformer oil can be separated into different classes under accepted classifications. Some classifications are based on additives where as some based on the composition of transformer oil. Transformer oil can get contaminated due to several reasons. These reasons can also be associated with the critical conditions of the transformer. Thus, it is important to regularly check the condition of transformer oil for the maintenance of the transformer. There are plenty of tests associated with transformer oil. This report also includes the tests experienced by myself during the lab session. Transformer Oil Tharuki De Silva 2014/E/051 Faculty of Engineering ,University of Jaffna
  2. 2. 1. INTRODUCTION A transformer is used to step up or down the ac voltages. Thus, it is an essential component in electric power delivery systems. The insulation system of a power transformer consists of mostly hydrocarbon oil and paper. The hydrocarbon oil in an electrical power transformer is commonly known as transformer oil. It is usually obtained by fractional distillation and subsequent treatment of crude petroleum. Transformer oil serves many purposes such as, 2. TYPES OF TRANSFORMER OIL Different types of transformer oils are used to serve different necessities in a transformer by using the endemic properties of each oil type. There are several classifications done categorize transformer oil. The general classification is based on the additives present in transformer oil. Another classification of transformer oil by composition can be listed as follows.  Paraffin based transformer oil This contains Paraffin hydrocarbon, also called alkane, any of the saturated hydrocarbons having the general formula CnH2n+2, where n is any integer.  Naphtha based transformer oil This contains Naphthalene hydrocarbon which has the chemical formula of C10H8. Contents 1. INTRODUCTION……………………………02 2. TYPES OF TRANSFORMER OIL ……..02 3. OIL CONTAMINATION………………….03 4. TESTING ………………………………………03 5. CONCLUSIONS……………………………..06  Cooling  Electrical Insulation  Protection against chemical attack  Prevention of sludge buildup  Inhibited oil - insulating oil with an inhibitor additive added to the level between a minimum of 0.08 and a maximum 0.4 % per litre of oil.  Uninhibited oil - insulating oil without any inhibitor additives but may contain other additives. 4.1 BREAKDOWN VOLTAGE TEST……………………….04 4.2 INTERFACIAL TENSION TEST…………………………04 4.3 FAULT GAS ANALYSIS……………………………………05 3.1 CONTAMINATION BY MOISTURE………………….03 3.2 CONTAMINATION BY PARTICLES OR FIBRES…03 Figure 01 : Chemical structures of Paraffin and Naphtha
  3. 3. Naphtha oil is more easily oxidized than Paraffin oil. But oxidation product (sludge) in the naphtha oil is more soluble than Paraffin oil. Thus sludge of naphtha based oil is not precipitated in bottom of the transformer. But paraffin oil is still used due to easy availability. 3. OIL CONTAMINATION Depending on the design, the transformer oil used is in contact with air and other materials used in construction of the equipment, and therefore can easily be contaminated. In general, presence of a foreign matter in the oil can be considered as contamination. Some of these contaminants may affect the electrical, chemical or physical parameters of the oil. 3.1 Contamination by Moisture Water is soluble in oil within certain temperatures and acidity dependant limits. If these limits are exceeded the water becomes saturated and free water will appear as cloudiness or visible droplets(Pahlavanpour n.d.). Dissolved in the presence of other contaminants or free water can reduce the electrical strength of the oil. In transformers containing paper insulation,the moisture is contained in both oil and the paper.The paper contains about 99% of the total water in comparison with oil. But with change in temperature the water content in oil changes significantly where as the water content in paper will change slightly. This is explained by the Oommen’s curve given in figure 1. Maintaining low moisture content is important in transformers with paper insulation as high moisture in paper may reduce the life of paper insulation. 3.2 Contamination by Particles or Fibres Contamination can occur from particulate or fibrous matter taken up by the oil from the components of the equipment itself, from arcing,fault degradation products in the equipment or accumulated during repair.This contamination can be measured by labortary testing for breakdown voltage which reduces with high particle content and by visual inspection. 4. TESTING It is important to evaluate the condition of oil in a transformer to establish whether the condition of oil is adequate for continued operation. Oil Figure 02 : Oommen’s curve for moisture in paper vs. oil
  4. 4. testing also provides information on the condition on the transformer by the quantity and the quality of contaminants present. The tests for the oil are divided into three groups according to the depth in testing. All the significant tests for transformer oil is given in table 1.The tests done in the lab of AMHE is highlighted for indication . Group 1- Routine tests Colour and Appearance Breakdown Voltage Water Content Acidity Dielectric dissipation factor or Resistivity Inhibitor content Group 2- Complementary tests Sediment and sludge Interfacial tension Particles Group 3-Special investigative tests Dissolved Gas Analysis Oxidation stability Flash point Pour point Density Viscosity Polychlorinated biphenyls Corrosive sulphur From the above given test my personal experience is limited to three important tests which are significant in oil testing and will be discussed in the next section. 4.1 Breakdown Voltage Test The dielectric breakdown voltage is a measure of the ability of oil to withstand a high electric field stress without breaking down.It also proves the presence of moisture and various contaminats.The practiced method of testing in the lab is the spherical electrode system.Where the electrodes have 25mm radius and separated by 1 or 2mm apart. The test procedure for BDV test is as follows.  The mounted electrodes were separated from the container.  The vessel was cleaned thoroughly and filled with sample oil until the level is about 12mm from the top.  The cleaned magnetic stirrer bar as dropped to the vessel so that it is placed at center.  The test vessel was placed at the test chamber, the electrodes were reassembled and the chamber cover was closed.  The test set was switched ON and the following parameters were set and the experiment was started. o Standard- BS 148 : 1984 o Time- 5 min o Stir mode- Magnetic 6 BDV values were measured during the test and the average of these values were taken as the BDV of the sample. The sample was considered safe if the BDV is equal or Figure 03 : Set up for Break down voltage test Table 01 : Different types of Transformer oil tests
  5. 5. greater than 30 kV.Else it was considered fail. 4.2 Interfacial tension Test In this test,the surface tension of oil is measured against water.which is highly polar.The more nearly the liquids are alike in their polarity,the lower value of surface tension will present between them.Thus, if the oxidation in the sample is high,it will result in a lower interfacial tension. The test used in lab was the ring method and the test procedure was as follows.  The platinum ring was first emerged in sample oil up to a depth of about 1cm.The density difference was checked.  Then the ring was cleaned using a cleaning agent and dipped in a distilled water sample up to 1 cm depth, and the up thrust was set to zero.  The sample oil was placed on top of the water sample with the usage of a pipette until the oil layer is of 1 cm thickness.Then the test was started. The test will give 5 interfacial tension values in the units of milliNewtons /meter (mN/m) with a standard deviation value. The result will also illustrate the time taken for the test in seconds and the temperature at the experiment. 4.3 Dissolved Gas Analysis The breakdown of electrical insulating material and components inside the transformer may generate several gases under given temperature conditions. The identification of these gases is very helpful in maintenance of the transformer. Dissolved gas analysis is one of the identification test used for the above process. This method consists of sampling the oil and testing to identify the concentration of gases in each sample.The key gases considered in testing are Hydrogen(H2), Carbon dioxide(CO2), Carbon Monoxide(CO), Methane(CH4), Acetylene(C2H2) ,Ethylene(C2H4), and Ethane(C2H6).The procedure followed is given below.  The sample oil was filled into the glass syringe fully and the stop cock was set.  Next a given quantity of the extracted oil was sent out of the syringe up to the given mark.  The atmospheric air filtered through a carbon filter was filled to the accumulated space in the syringe.  Then the oil was shaken for about 30s thoroughly for the dissolved gases to mix with the air column.Figure 04 : Set up for Interfacial tension test
  6. 6. Then the sample was ready for testing.  Before testing some initial steps were taken. o The 2 chambers were heated to a moderately high temperature (40-60 o C) for about 45 min. To evacuate the chambers. o Then the instrument was tested with the test gas to observe the accuracy. o Next the instrument was checked with the CO2 free atmospheric to see whether the test results indicate a CO2 percentage less than 2% and absence of hydrocarbons.  After the above steps the sample was placed slanted on the column and the syringe was connected to the instrument through an oil trap.  Finally, the sample gas column was ejected with the use of a spring and then it was released after the required gas is extracted.  The test was done automatically to provide all gas percentages present in the sample in a ppm basis. According to the type and the quantity of the gases present we can assume the condition of the transformer(Imad-U- Khan et al. 2007). This can be elaborated as given in table 2. 5. CONCLUSIONS  Inhibited and naphtha based transformer oil can be considered as most suitable.  It is important to maintain an optimum temperature in a transformer to sustain a safe balance between the moisture content in paper and oil.  The test results of BDV test and IFT test can associate errors as the average value of the test is taken to account. As an example, the BDV of a sample may differ after the application of higher voltage.But it is not taken into account in the test result. 6. REFERENCES Imad-U-Khan, I.-U.-K. et al., 2007. Dissolved gas Table 02 : Potential fault type vs. Key gas concentrations Figure 05 : Set up for Dissolved gas analysis
  7. 7. analysis of alternative fluids for power transformers. IEEE Electrical Insulation Magazine, 23(5), pp.2009–2011. Available at: http://www.netaworld.org/sites/default/fi les/public/neta- journals/NWwtr09_Hamrick.pdf. Pahlavanpour, P.B., Revised IEC Standard for Maintenance of In-Service Insulating Oil. , pp.1–14. Available at: http://www.weidmann- solutions.cn/huiyi/Seminar 2004 Sacramento/PahlavanpourPaper2004.pdf.
  • AmirGul4

    Aug. 2, 2018
  • nyna94

    May. 19, 2018

Transformer oil which is obtained by fractional distillation of crude oil serves many purposes in a transformer insulation.This report includes the classifications of transformer oil under composition and additives and it also includes the different types contamination that can occur in a transformer.These contamination issues can be tracked by an analysis of transformer oil.This report is consist of different transformer oil testing and also my personal experience in observing few tests at the laboratory of Asset Management Hydro Electrical - CEB.

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