DGA

Dissolved Gas Analysis of Transformer Oil

1. D issolved Gas Analysis By : Rahul Buswala, Chemical Engineer Email: [email protected]

2. INTRODUCTION N 2 C 2 H 6 H 2 CH 4 C 2 H 4 C 2 H 2 C 3 H 8 C 3 H 6 CO 2 CO O 2 Transformer oil sample analysis for dissolved gases is a useful, predictive, maintenance tool for determining transformer health. The breakdown of electrical insulating materials and related components inside a transformer generate gases within the transformer. The amount and relative distribution of these gases depend upon the type & severity of the fault . When gassing occurs in transformers enough useful information can be derived from following 09 gases so the additional gases (propane & propene) are usually not examined: Hydrogen Oxides of carbon: Carbon dioxide & Carbon monoxide Hydrocarbons: Methane, Ethane, Ethylene & Acetylene Atmospheric gases: Oxygen & Nitrogen. Dissolved Gas Analysis

3. INTRODUCTION Principle causes of gas formation: Electrical disturbances Partial or Disruptive discharge through the insulation Thermal decomposition Excessive temperature rise in insulation due to insufficient cooling, excessive current circulating through the metal parts or insulation, overheating of internal winding or bushing connection lead, or overloading. These two fault may occur in combination. Dissolved Gas Analysis

4. INTRODUCTION The DGA procedure consists of essentially four steps: Dissolved Gas Analysis

5. INTRODUCTION REFERENCE STANDARDS Dissolved Gas Analysis

6. SAMPLING OF TRANSFORMER OIL Dissolved Gas Analysis Precaution & Place of sampling Make sure that the oil in energized transformer is not under a negative pressure when taking an oil sample. Since this could introduce air bubbles in the oil. Precautions should be taken during sampling of oil to deal with any sudden release of oil and avoid oil spillage. Much care is required during sampling normally oil sample should be taken from a point where it is representative of the bulk of the oil in the transformer, like from the bottom oil drain valve or the oil sample valve. However, sometime sampling shall be done where it is not representative of bulk to locate the site of fault like tap changer, selector switch or gas relay.

7. Dissolved Gas Analysis Cleaning of sampling point To prevent from contamination of oil samples, clean the sampling equipment carefully and flushing of the sampling point shall be done. The drain valve is flush with a sufficient quantity of oil under turbulent flow to remove any contaminants like water and particles, that might have deposited / accumulated in the drain valve and at its orifice. Sample Container Syringes --------------------------> Glass Hypodermic Flexible bottle --------------------> Metal (tin) can Bottle ------------------------------> Glass ( Amber colour ) Bottle--------------------------------> Stainless Steel Plastic bottles should not be used for DGA as the dissolved may diffuse through the wall of plastic bottle. SAMPLING OF TRANSFORMER OIL

8. analysis FOR DISSOLVED GASES There are three methods for gas extraction: Method A (Extraction by vacuum) Toepler and Partial Degassing Method B (Stripping) Displacement of dissolved gases by bubbling the carrier gas through the oil Method C (Headspace) Partition of gases between the oil sample & a small volume of carrier gas The dissolve gases extracted then will be analysed in Gas Chromatograph for composition. Dissolved Gas Analysis

9. Dissolved Gas Analysis Method A is usually require mercury to compress the extracted gas under vacuum and mercury affects health and environment. This method is more laborious and require continuous involvement of operator. However, it is having better sensitivity & accuracy. Method B is not widely in use for DGA. This method having minimum detection limit for hydrogen is 20 ppm which is higher than method A & C. This could affect interpretation of results when low levels of gases are present. Method C is having advantage of mercury free extraction & automatic sampling. Require very sensitive equipment to analyse at lower concentration. Care shall be taken to select the headspace temperature and use the respective partition coefficients which is a function of temperature. analysis FOR DISSOLVED GASES

10. Dissolved Gas Analysis Gases Acceptance Test ( ul /l) Service Test ( ul /l) Hydrogen 2 5 Hydrocarbons 0.1 1 Carbon monoxide 5.0 25 Carbon dioxide 10 25 Oxygen 500 500 Nitrogen 2000 2000 analysis FOR DISSOLVED GASES

11. GAS CHROMATOGRAPHY Dissolved Gas Analysis After extraction of gases they shall be analysed over an Gas Chromatograph which shall have following facilities: TCD Detector to detect atmospheric gases & hydrogen FID Detector to detect hydrocarbon gases Methanizer to convert CO & CO 2 to CH 4 so that they can detect over FID. Columns to separate the constituent gases from mixture. Argon: Use as a carrier gas in some cases helium can also be used but care shall be taken for hydrogen determination. Air & Hydrogen: These gases are required for flame of FID detector

12. GAS CHROMATOGRAPHY Dissolved Gas Analysis Thermal Conductivity Detector (TCD) is used to determine the concentration of Hydrogen & atmospheric gases. Basically it is based on wheatstone bridge circuit. This detector senses changes in the thermal conductivity of the column effluent and compares it to a reference flow of carrier gas . Temperature changes leads to the change in the resistance. Thermal conductivity of some carrier gases is given in this table.

13. GAS CHROMATOGRAPHY Dissolved Gas Analysis Flame Ionization Detector (FID) The effluent from the column is mixed with hydrogen and air, and ignited. Organic compounds burning in the flame produce ions and electrons which can conduct electricity through the flame. A large electrical potential is applied at the burner tip, and a collector electrode is located above the flame. The current resulting from the pyrolysis of any organic compounds is measured.

14. Mechanism of gas formation OIL DECOMPOSITION DECOMPOSITION OF CELLULOSIC INSULATION STRAY GASSING OF OIL OTHER SOURCES Dissolved Gas Analysis

15. OIL DECOMPOSITION Oil decomposition products are Hydrogen & Hydrocarbon gases . Some solid particles of carbon & hydrocarbon polymer (X wax) are other possible product of oil decomposition. However oil may oxidize with formation of small quantities of CO & CO 2 which may accumulate over long period of time. Low Energy faults like corona discharge favours the formation of Hydrogen gas while other hydrocarbon gases can be formed if the energy of discharge & temperature is high enough to break the C-C, C=C or C≡C bond. Ethylene is favoured above temperature 500°C while Acetylene favoured above temperature 800°C. Dissolved Gas Analysis Mechanism of gas formation

16. CELLULOSE DECOMPOSITION Cellulosic insulation are made up of large no. of anhydrous glucose rings having weak C-O bonds glycosidic bonds. This decomposes at lower temperature than oil. Carbon monoxide and dioxides, water vapour forms with minor quantity of hydrocarbon gases & furan compounds. Decomposition occurs at 105°C or above with complete decomposition or carbonization occurs at 300°C or above. Dissolved Gas Analysis Mechanism of gas formation

17. STRAY GASSSING OF OIL It is formation of gases in oil heated to temperature <200°C. In general Hydrogen , methane, & ethane forms at this temperature or due to oxidation of oil. It is a non-damage fault. Dissolved Gas Analysis Mechanism of gas formation

18. OTHER SOURCES OF GAS Gases can be formed in the equipment not as a result of fault but through rusting, chemical reaction involving steel, uncoated surface or protective paints. Hydrogen can be formed due to reaction of moisture with steel/galvanised steel surface in presence of oxygen from oil, or due to moisture with special coatings on metal surface . Hydrogen & acetylene can be formed in new steel surface which absorbed during manufacturing process or welding and releases slowly into the oil. Internal transformer paints may also form gases. High value of hydrogen with absence of hydrocarbon gases indicates this source & even sometime gases form in a transformer which is never been energized due to this reason. Dissolved Gas Analysis Mechanism of gas formation

19. interpretation Dissolved Gas Analysis The major (minor) fault gases can be categorized as follows by the type of material that is involved and the type of fault present: LT: Low T emperature, HT: High Temperature

20. interpretation Dissolved Gas Analysis The solubilities of the fault gases in mineral oil as well as their temperature dependence are also important factors for consideration in fault gas analyses. Table below lists the saturation solubilities for the fault gases. Solubility of gases in transformer oil Static Equilibrium at 760 mm Hg and 25°C (by Volume). Hydrogen Nitrogen Carbon monoxide Oxygen Methane Carbon dioxide Ethane Ethylene Acetylene 7.00 % 8.60% 9.00 % 16.0 % 30.0 % 120 % 280 % 280 % 400 %

21. interpretation Dissolved Gas Analysis When rates of gas generation are being followed it is important to take into account the solubilities of these gases as a function of the oil temperature. Over a temperature range of 0 to 80°C some gases increase in solubility up to 79% while others decrease their solubility up to 66%.

22. interpretation Dissolved Gas Analysis

23. Case study-1 Dissolved Gas Analysis Sl.No Components Unit Concentration of gas 1 CH 4 ppm(V ) 98.64 2 C 2 H 6 “ 15.15 3 C 2 H 4 “ 109.5 4 C 2 H 2 “ 1.10 5 H 2 “ 80.66 6 CO “ 2.92 7 CO 2 “ 16.34 8 O 2 “ 1962.18 9 N 2 “ 6484.01 10 TGC % 1.0 Type of Sample: Transformer Insulating Oil (after over excitation) Details: 80 MVA, 765 KV, Reactor Date of Analysis: 20-07-2017

24. Case study-1 Dissolved Gas Analysis DIAGNOSIS

25. Case study-1 Dissolved Gas Analysis DIAGNOSIS

26. Case study-1 Dissolved Gas Analysis DIAGNOSIS

27. Case study-2 Dissolved Gas Analysis Sl.No Components Unit Concentration of gas 1 CH 4 ppm(V ) 41.0 2 C 2 H 6 “ 1.80 3 C 2 H 4 “ 32.0 4 C 2 H 2 “ 44.0 5 H 2 “ 161.0 6 CO “ 9.40 7 CO 2 “ 11.0 8 O 2 “ 2262.87 9 N 2 “ 6196.70 10 TGC % 1.0 Type of Sample: Transformer Insulating Oil (after short circuit) Details: 315 MVA, 400 KV, GT Date of Analysis: 09-01-2018

28. Case study-2 Dissolved Gas Analysis DIAGNOSIS

29. Case study-2 Dissolved Gas Analysis DIAGNOSIS

30. Case study-2 Dissolved Gas Analysis DIAGNOSIS

31. Case study-2 Dissolved Gas Analysis DIAGNOSIS