The aim of this paper is to introduce hot-spot and top-oil temperature thermal models for more accurate temperature calculations during transient states based on data received in a normal heat run test (i.e., the top oil in the tank of the transformer and the average winding-to-average oil gradient). Oil viscosity changes and loss variation with temperature are taken into account. The new thermal models will be validated using experimental (fiber-optic test) results obtained at varying load current on a 250-MVA-ONAF-cooled unit, a 400-MVA-ONAF-cooled unit and a 605-MVA-OFAF-cooled unit. The results are also compared with the IEEE-Loading guide (1995) Annex G method.

Dynamic thermal modelling of power transformers

Distributed temperature sensors (DTS) measure temperatures by means of optical fibers. Those optoelectronic devices provide a continuous profile of the temperature distribution along the cable. Initiated in the 1980s, DTS systems have undergone significant improvements in the technology and the application scenario over the last decades. The main measuring principles are based on detecting the back-scattering of light, e.g., detecting via Rayleigh, Raman, and Brillouin principles. The application domains span from traditional applications in the distributed temperature or strain sensing in the cables, to the latest “smart grid” initiative in the power systems, etc. In this paper, we present comparative reviews of the different DTS technologies, different applications, standard, and upcoming, different manufacturers.

Distributed Temperature Sensing: Review of Technology and Applications

With the fast-paced changing technologies in the power industry, new references addressing new technologies are coming to the market. Based on this fact, there is an urgent need to keep track of international experiences and activities taking place in the field of modern transformer design. The complexity of transformer design demands reliable and rigorous solution methods. A survey of current research reveals the continued interest in application of advanced techniques for transformer design optimization. This paper conducts a literature survey and reveals general backgrounds of research and developments in the field of transformer design and optimization for the past 35 years, based on more than 420 published articles, 50 transformer books, and 65 standards.

Transformer Design and Optimization: A Literature Survey

This paper presents a new method for calculating a health index for transformers (for operating voltage 69 kV or less) based on diagnostic tests. The method relies on the use of furan analysis, dissolved gas analysis, and other oil analysis results as a means of calculating the health index using fuzzy set theory. Real field data for 90 working transformers were used to test the proposed method. The results were compared with the results calculated for the same set of transformers by an experienced asset management and health assessment consulting company. The comparison shows that the results are highly reliable.

Calculation of a Health Index for Oil-Immersed Transformers Rated Under 69 kV Using Fuzzy Logic

This paper presents an integrated model based upon the fuzzy approach and evidential reasoning decision-making approach to condition assessment of power transformers. An assessing index system, including the DGA data, electrical testing, and oil testing, is established to facilitate the assessing model. The model is composed of two levels, that is, the fuzzy model and evidential reasoning model. The fuzzy model is proposed for generating the original basic probability assignments for the second-level model. Afterwards, an evidential reasoning decision-making model is utilized to combine all of the evidence and give an overall evaluation conclusion. Based upon this integrated model, a decision-making procedure is put forward to serve as an effective tool for transformer condition assessments. The results show that the assessing model is capable of offering an overall evaluation of the observed transformer condition.

An Integrated Decision-Making Model for Condition Assessment of Power Transformers Using Fuzzy Approach and Evidential Reasoning

Inadvertent failure of power transformers has serious consequences on the system reliability, economics, and the revenue accrual. An accurate estimation of transformer life can, to a very large extent, mitigate the problems, besides satisfying the conflicting requirements of optimum utilization of the equipment and safeguarding the reliability. In this endeavour, the authors have planned long duration aging experiments on scaled-down (prorated) models of a transformer, incorporating all of the essential features of actual equipment under normal operating electric stress and accelerated thermal stress. In continuation of the authors' earlier experimental investigations, an elapsed life assessment study has been instituted by acquiring insulation-aging data under accelerated thermal stresses.

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On the estimation of elapsed life of oil-immersed power transformers

The degradation of electrical insulation in transformers is more often due to thermal stress. In this paper, the authors attempted to work out a method of assessing the magnitude of temperature and the hottest region in the body of a large transformer with a reasonable degree of accuracy. A theoretical model has been developed based on the boundary value problem of heat conduction in transformer winding using finite integral transform techniques. The model requires, in addition to electrical parameters of the transformer, information on the actual design data. The authors believe that this model predicts HST with precision and also indicates a possibility of online data acquisition.

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Prediction of hottest spot temperature (HST) in power and station transformers

The status of insulation in power equipment at any given point in time can be assessed by monitoring properties which are sensitive to the amount of aging. This procedure, called the diagnostic or non-destructive testing, has been in vogue for some time now. The diagnosis, as the name suggests, enables one to disseminate the condition of the insulation by making terminal or enclosure-centric measurements and as applied to transformers, a minimally invasive test procedure involving the examination of the oil phase, for arriving at information on the degradation of the cellulose insulation on the winding is indicated. An examination of the tracks provided by the by-products of chemical dissociation of cellulose, in solution with oil is recognized as a possible approach to condition monitoring. To this end, carefully controlled laboratory aging experiments have been performed on eight, nominally identical and scaled-down (pro-rated) models of single phase transformer units, incorporating all the essential features of actual power transformers under normal operating electric stress and accelerated thermal stress. Acquisition, analysis and validation of experimental data including dissolved gas analysis (DGA), examination of oil phase for the presence of Furans, measurement of the degree of polymerization (DP) of the paper insulation, electrical and certain chemical tests have all been performed to evolve an experimental data-base for assessing condition of insulation.

Assessment of the status of insulation during thermal stress accelerated experiments on transformer prototypes

The degradation of electrical insulation in transformers is traced to thermoelectric processes. Existence of localized hot regions due to thermal insulating properties of electrical insulation would cause thermal runaway around these regions. In an earlier paper , the authors presented analytical methods for estimating the temperature and its distribution at different points of the transformer based on a closed-form mathematical technique using a generalized heat conduction (GHC) model. Certain aspects of the inhomogeniety of the several components of the winding and incorporation of distributed heat source were not addressed. Also, a rigorous treatment involving the changes in winding resistance, which was built into the thermal model as an empirical correction factor, has now been modified and incorporated in the GHC. These considerations have now improved the accuracy of estimation of hottest spot temperature.

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Estimation of the hottest spot temperature (HST) in power transformers considering thermal inhomogeniety of the windings

Non-destructive dielectrometry measurement techniques such as polarisation and depolarisation current (PDC) measurement is gaining popularity in power industry to evaluate the insulation condition of transformers. Due to nature of the polarisation species involved in transformer insulation, a perfect PDC measurement would take a very long time, to allow all the interfacial polarisation species to be relaxed. Test duration of 10,000s charging and 10,000s discharging has been considered a reasonable compromise in the context of power transformer insulation. However, in a practical field situation power utilities usually cannot afford such a long measurement time due to economical factors. The authors of this paper explore the possibility of further reducing the PDC measurement time to meet the preference of power industries. Since polarisation and depolarisation are reverse process to each other; it would be possible to estimate oil and paper conductivity from short duration measurements. An experimental investigation has been performed on a distribution transformer and on a model transformer to estimate and compare the accuracy of conductivity computation with different charging and discharging times. These test data helps to decide on the optimal PDC test duration. To help extrapolation of PDC data from short duration to longer duration (i.e. 10,000s), both the current difference and charge difference methods have been adopted. The results from these methods have been critically analysed in this paper.

Manoj Pradhan

Papers

On the estimation of elapsed life of oil-immersed power transformers

Prediction of hottest spot temperature (HST) in power and station transformers

Assessment of the status of insulation during thermal stress accelerated experiments on transformer prototypes

Estimation of the hottest spot temperature (HST) in power transformers considering thermal inhomogeniety of the windings

Optimal Time Selection for the Polarisation and Depolarisation Current Measurement for Power Transformer Insulation Diagnosis