A refined mathematical model for prediction of bubble evolution in transformers

Fessler, W.A.; Rouse, T.O.; McNutt, W. J.; Compton, O.R.

doi:10.1109/61.19229

Cited by 56 publications

(39 citation statements)

References 13 publications

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“…The measurements reveal deviations to earlier results (Fessler, et al, 1989;Scala, Buchgraber, Seitlinger, 2004) derived from previously documented results. The earlier formulas (Scala, Buchgraber, Seitlinger, 2004) were adapted as follows:…”

Section: Properties Of Cellulosecontrasting

confidence: 91%

Moisture assessment in transformers including overloading limits

Scala

2008

Elektrotech. Inftech.

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A moisture model determines the moisture distribution in cellulose and oil. The model focuses on oil flow, moisture vapour pressure and the saturation properties of oil and cellulose.Measurements of material properties form the basis of each computation model. Direct measurements were performed. Samples of paper and pressboard were aged in oil. The measurements of moisture absorption were performed in air. For given moisture content the moisture vapour pressure reaches lower values than in previous results. Consequently, moisture values in oil reach lower values and the risk of bubbling at rising temperatures reduces.It has, for the first time, been established that bubbling requires a contribution of dissolved gas like air to reach critical vapour pressure levels to obtain bubbling at 150 C hotspot and with 3% average moisture content and 1.2% in the hotspot respectively. Here oil must absorb on atmosphere air at 70 C. Without dissolved gas an average moisture content of 4% and a hot spot temperature of about 165 C would be necessary to come close to a risky condition.The low tendency to bubbling is also favoured by the reduced moisture content in the winding hotspot insulation which is caused by the temperature distribution due to axial oil gradient and winding gradient.The general calculation for individual spots in the transformer insulation like the hotspot is based on the moisture vapour pressure of cellulose and oil as the driving force of moisture migration. The axial temperature distribution, however, generates an axial gradient of moisture vapour pressures in oil. The cellulose insulation reaches the moisture pressure level of its neighbouring oil.The average cellulose moisture content provides basic information about the moisture condition. The calculation model is required to determine the moisture values in a transformer under operation conditions. Aging and bubbling considerations are based on the moisture content of the hotspot.Filling of a transformer with oil often happens at a moisture content of 5 ppm. At low temperatures and slow moisture diffusion into the bulk material the cellulose surface can develop extremely high moisture values. Therefore, a high temperature during filling is recommended.Furthermore, a transformer can, in principle, absorb moisture from the atmosphere or vice verse. The direction of migration is determined by the difference of local moisture vapour pressure between oil and atmosphere which depends on temperature and moisture. The slow migration velocity at low temperatures and tightness of the tank limits the value to below 7% moisture content in cellulose. Moisture vapour pressureWater evaporates -a well-known fact. The vapour pressure is determined by the temperature of water. In equilibrium and with water and vapour at the same temperature the vapour pressure saturates. At increasing water temperatures the vapour pressure p 0 ðT water Þ increases exponentially.The moisture vapour pressure p 0 ðT water Þ remains unchanged in spite of heating of the atmosphere above th...

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Section: Properties Of Cellulosecontrasting

confidence: 91%

Moisture assessment in transformers including overloading limits

Scala

2008

Elektrotech. Inftech.

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“…At constant temperature, thermodynamic equilibrium exists between the oil and paper. The Fessler equation was used to calculate the water content of paper from the water activity of the oil [15,16]. This method has the advantage that although the solubility of the oil to water may change as the oil oxidizes, the solubility of the oil is not required for the calculation.…”

Section: Investigation and Methodsmentioning

confidence: 99%

Aging rate of grade 3 presspaper insulation used in power transformers

Lelekakis

Wijaya

Martín

et al. 2014

IEEE Trans. Dielect. Electr. Insul.

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Paper materials are used as the insulation of power transformers. Over time these materials slowly degrade, until they reach a point when they no longer function effectively as transformer insulation, and the transformer has then reached its end of life. The aging rate of paper is affected by temperature, water, oxygen and acids. Investigations have been performed previously on Kraft and Kraft thermally upgraded types of paper. However, a different type of paper used in transformer insulation, Grade 3 presspaper (which contains cotton), has not been extensively tested and compared to Kraft paper. In these experiments we studied the aging rate of Grade 3 presspaper and compared it to our previous studies of Kraft paper. Traditionally, the aging rate of paper has been studied in sealed vessels. The problem with this approach is that the chemical environment within the vessel will change during aging, and so the aging rate will be affected. In our experiment setup we controlled the water and oxygen content to more accurately determine the aging rate. Similarly to Kraft paper, the aging rate of Grade 3 presspaper with the same water content increased with oxygen content in the oil. Life curves were developed based on the water content of the paper and the oxygen content of the oil.

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“…Water causes three dangerous effects: it decreases the dielectric withstand strength, accelerates cellulose aging, and causes the emission of bubbles at high temperatures [17][18][19][20]. The knowledge about the actual moisture concentration is required to control transformer condition.…”

Section: Water Influencementioning

confidence: 99%

Determination of transformer oil quality by the acoustic method

2017

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Abstract. Large power transformers are the most expensive and strategically important components of any power generation and transmission system. Their reliability has crucial importance for the energy system operation. Insulation breakdown can generate a serious failure of a large power transformer, which causes substantial costs to repair and financial loss due to power outage. Most power utilities are, therefore, highly motivated to assess the actual condition of the transformer insulation system. The majority of transformer condition check methods are only available for off-line periodic diagnostics. The acoustic emission (AE) method as a complex online diagnostics technique to identify evolving failures is proposed. Results of measuring the acoustic signals generated by the presence of impurities in transformer oil and determining the impurities influence on its condition as an indicator of the transformer condition is presented.

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A refined mathematical model for prediction of bubble evolution in transformers

Cited by 56 publications

References 13 publications

Moisture assessment in transformers including overloading limits

Moisture assessment in transformers including overloading limits

Aging rate of grade 3 presspaper insulation used in power transformers

Determination of transformer oil quality by the acoustic method

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