Dissolved Gas Analysis(DGA) of Power Transformer Oil Shivaji choudhury
1.Introduction Transformer is one of the most important but complex component of electricity generation and transmission system. Much attention is needed on maintenance of transformers in order to have fault free electric supply and to maximize the lifetime and efficacy of a transformer.
2. Gases in oil filled transformers The detection of certain gases generated in an oil-filled transformer in service is frequently the first available indication of a malfunction that may eventually lead to failure if not corrected.
2.1.Benefits of DGA Assesses the internal condition of the transformer Helps calculate probability of failure and end of life Identifies degradation before it leads to failure Essential for effective maintenance and replacement strategies Low cost test process
4.Cause of gas formation The two principal causes of gas formation within an operating transformer are 4.1.Cellulosic Decomposition 4.2.Oil Decomposition
4.1.Cellulosic Decomposition The thermal decomposition of oil- impregnated cellulose insulation produces carbon oxides (CO, CO2) and some hydrogen or methane.
4.2.Oil Decomposition Mineral transformer oils are mixtures of many different hydrocarbon molecules, and the decomposition processes for these hydrocarbons in thermal or electrical faults are complex.
4.3.Oil Decomposition -some of these gases will be formed in larger or smaller quantities depending on the energy content of the fault. -for example, low energy faults such as corona partial discharges in gas bubbles, or low temperature hot spots, will form mainly H2 and CH4.
5.Interpretation of Gas Analysis Thermal Faults Electrical Faults—Low Intensity Discharges Electrical Faults—High Intensity Arcing
5.0.Halsteads Thermal Equilibrium PartialPressures as a Function of Temperature
5.1.1.Thermal faults The decomposition of mineral oil from 150 °C to 500 °C produces relatively large quantities of the low molecular weight gases, such as hydrogen (H2) and methane (CH4), and trace quantities of the higher molecular weight gases ethylene (C2H4) and ethane (C2H6).
5.1.2.Thermal faults At the upper end of the thermal fault range, increasing quantities of hydrogen and ethylene and traces of acetylene (C2H2) may be produced. In contrast with the thermal decomposition of oil, the thermal decomposition of cellulose and other solid insulation produces carbon monoxide (CO), carbon dioxide (CO2), and water vapor at temperatures much lower than that for decomposition of oil.
5.1.3.Thermal faults The ratio of CO2/CO is sometimes used as an indicator of the thermal decomposition of cellulose. As the magnitude of CO increases, the ratio of CO2/CO decreases. This may indicate an abnormality that is degrading cellulosic insulation.
5.2.Electrical Faults—Low Intensity Discharges Low intensity discharges such as partial discharges and very low level intermittent arcing produce mainly hydrogen, with decreasing quantities of methane and trace quantities of acetylene. As the intensity of the discharge increases, the acetylene and ethylene concentrations rise significantly .
5.3.Electrical Faults—High Intensity Arcing As the intensity of the electrical discharge reaches arcing or continuing discharge proportions that produce temperatures from 700 °C to 1800 °C, the quantity of acetylene becomes pronounced.
6.Interpretation of Dissolved GasAnalysis (DGA) Key gas Method- IEEE Type of faults –IEC 60599 IEC Gas ratio method Duval Triangle Rogers ratio method flow chart Deornenburg method flow chart
6.1.1.Thermal -oil Decomposition products include ethylene and methane ,together with smaller quantities of hydrogen and ethane .traces of acetylene may be formed if the fault is severe or involves electrical contacts. Principal gas - ethylene
6.1.2.Thermal -cellulose Large quantities of carbon dioxide and carbon monoxide are evolved from overheated cellulose .hydrocarbon gases ,such as methane and ethylene ,will be formed if fault involves an oil impregnated structure Principal gas—carbon monoxide
6.1.3.Electrical - relative corona Low energy electrical discharges produce hydrogen and methane ,with small quantities of ethane and ethylene . Principal gas –hydrogen
6.1.4.Electrical –arcing Large amounts of hydrogen and acetylene are produced ,with minor quantities of methane and ethylene .carbon dioxide and carbon monoxide may also formed if fault involves cellulose. Oil may be carbonized. Principal gas- acetylene
6.2.Type of faults –IEC 60599 1. PD- Partial Discharges (corona) 2. D1- Discharges of low energy Electrical 3. D2- Discharges of high energy 4. T1 - Thermal faults < 300° 5. T2 - Thermal faults > 300°< 700 Thermal 6. T3 - Thermal faults > 700°
6.2.1-Partial discharges of thecorona-type (PD).- Typical examples are discharges in gas bubbles or voids trapped in paper, as a result of poor drying or poor oil-impregnation.
6.2.2.Discharges of low energy(D1) -Typical examples are partial discharges of the sparking-type, inducing pinholes or carbonized punctures in paper.-or low-energy arcing, inducing carbonized perforations or surface tracking of paper, or carbon particles in oil.
6.2.3.Discharges of high energy (D2) -Typical examples are high energy arcing, flashovers and short circuits, with power follow through, resulting in extensive damage to paper, large formation of carbon particles in oil, metal fusion, tripping of the equipment or gas alarms .
6.2.4.Thermal faults of temperatures< 300 °C (T1) Faults T1 are evidenced by paper turning: -brown (> 200 °C). -black or carbonized (> 300 °C). Typical examples are overloading, blocked oil ducts, stray flux in beams.
6.2.5.Thermal faults of temperaturesbetween 300 and 700°C (T2) Faults T2 are evidenced by : -carbonization of paper. -formation of carbon particles in oil. Typical examples are defective contacts or welds, circulating currents.
6.2.6.Thermal faults of temperatures> 700°C (T3) Faults T3 are evidenced by : -extensive formation of carbon particles in oil. -metal coloration (800 °C) or metal fusion(> 1000 °C). Typical examples are large circulating currents in tank and core, short circuits in laminations.
7.Evaluation of Transformer Condition UsingIndividual and TDCG (total dissolved combustible gas) Concentration A four-level criterion has been developed to classify risks to transformers. Condition 1 TDCG below this level indicates the transformer is operating satisfactorily . Condition 2 TDCG within this range indicates greater than normal combustible gas level. Condition 3 TDCG within this range indicates a high level of decomposition. Condition 4 TDCG within this range indicates excessive decomposition. Continued operation could result in failure of the transformer.
8.Sampling ASTM D3613 requires that transformer oil sampling be taken via a syringe and stopcock system from a mineral-oil insulated transformers drain point to ensure no oil contact with air. To minimise air ingress, it is important that the syringe not be pulled forcefully, i.e. the transformer oils natural gravity flow should be allowed to work the oil into the syringe .