Bubble evolution from transformer overload

Oommen, T.V.; Lindgren, S.R.

doi:10.1109/tdc.2001.971223

Cited by 84 publications

(65 citation statements)

References 2 publications

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“…6, the higher the water content in paper (both cellulose and aramid paper), the lower the bubble effect initiation temperature. The results of the measurements for cellulose paper are consistent with information presented in [6,7]. Assuming the same moisture level in both types of paper, it can be concluded that the bubble effect initiation temperature is lower for aramid paper than it is for cellulose paper.…”

Section: Measurement Setupsupporting

confidence: 89%

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A comparison of bubble evolution temperature in aramid and cellulose paper

Przybyłek

2013

2013 IEEE International Conference on Solid Dielectrics (ICSD)

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The paper deals with the problem of the bubble effect in transformer winding insulation. The bubble effect should be understood as the release of water vapor from insulation. The main goal of the research was to compare the initiation temperature of bubble evolution in aramid and cellulose paper depending on the water content. Cellulose and aramid paper samples of different water content were prepared for the research study. The obtained results confirmed that aramid paper is less hygroscopic than cellulose paper. It was also proven that the initiation temperature of bubble evolution is lower for aramid paper than for cellulose paper with the same water content. However, the initiation temperature of bubble evolution is very similar for aramid and cellulose paper conditioned at the same relative humidity.

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Section: Measurement Setupsupporting

confidence: 89%

“…• the higher the content of gases dissolved in mineral oil, the lower the initiation temperature of bubble evolution [6,7],…”

Section: Bubble Effectmentioning

confidence: 99%

A comparison of bubble evolution temperature in aramid and cellulose paper

Przybyłek

2013

2013 IEEE International Conference on Solid Dielectrics (ICSD)

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“…The scientific papers that describe the implementation of the transformer temperature monitoring systems, based either on thermal models or temperature sensor installations, and other power transformer diagnostic methods that consider the key parameters, e.g., the gases, moisture in oil, partial discharge, load current and voltage, insulation power factor and pump-fan operation, are listed in references [4], [8], [9], [10], [16], [17], [19][20], [36], [38][39], [51], [63][64][65][66], [74], [77] and [81][82].…”

Section: Transformer Loading Capacity: Basic Aspectsmentioning

confidence: 99%

Dynamic Thermal Modelling of Power Transformers

Susa¹,

Lehtonen²,

Nordman

2005

IEEE Trans. Power Delivery

284

125

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Power transformers represent the largest portion of capital investment in transmission and distribution substations. In addition, power transformer outages have a considerable economic impact on the operation of an electrical network. One of the most important parameters governing a transformer's life expectancy is the hot-spot temperature value.The classical approach has been to consider the hot-spot temperature as the sum of the ambient temperature, the top-oil temperature rise in tank, and the hot-spot-to-top-oil (in tank) temperature gradient. When fibre optic probes were taken into use to record local hot-spots in windings and oil ducts, it was noticed that the hot-spot temperature rise over top-oil temperature due to load changes is a function depending on time as well as the transformer loading (overshoot time dependent function). It has also been noticed that the top-oil temperature time constant is shorter than the time constant suggested by the present IEC loading guide, especially in cases where the oil is guided through the windings in a zigzag pattern for the ONAN and ONAF cooling modes. This results in winding hottest spot temperatures higher than those predicted by the loading guides during transient states after the load current increases, before the corresponding steady states have been reached.This thesis presents new and more accurate temperature calculation methods taking into account the findings mentioned above. The models are based on heat transfer theory, application of the lumped capacitance method, the thermal-electrical analogy and a new definition of nonlinear thermal resistances at different locations within a power transformer. The methods presented in this thesis take into account oil viscosity changes and loss variation with temperature. The changes in transformer time constants due to changes in the oil viscosity are also accounted for in the thermal models. In addition, the proposed equations are used to estimate the equivalent thermal capacitances of the transformer oil for different transformer designs and winding-oil circulations. The models are validated using experimental results, which have been obtained from a series of thermal tests performed on a range of power transformers. Most of the tested units were equipped with fibre optic sensors in the main windings. Some of them also had thermocouples in the core and structural parts. A significant advantage of the suggested thermal models is that they are tied to measured parameters that are readily available (i.e., data obtained from a normal heat run test performed by the transformer manufacturer). AbstractPower transformers represent the largest portion of capital investment in transmission and distribution substations. In addition, power transformer outages have a considerable economic impact on the operation of an electrical network. One of the most important parameters governing a transformer's life expectancy is the hot-spot temperature value.The classical approach has been to consider the hot-spot temperature as the ...

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“…In order for the transition from dissolved condition into a bubble to occur interfacial tension must be overcome (Oommen, 2000). In oil the increased surface tension of micro bubbles would require an increased internal pressure.…”

Section: Bubblingmentioning

confidence: 99%

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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Bubble evolution from transformer overload

Cited by 84 publications

References 2 publications

A comparison of bubble evolution temperature in aramid and cellulose paper

A comparison of bubble evolution temperature in aramid and cellulose paper

Dynamic Thermal Modelling of Power Transformers

Moisture assessment in transformers including overloading limits

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