Suchergebnisse

The assessment of a TiO2 vegetal-based dielectric nanofluid has been carried out, and its characteristics and behavior have been tested and compared with a previously tested maghemite nanofluid. The results obtained reflect a similar affectation of the main properties, with a maximal improvement of the breakdown voltage of 33% at 0.5 kg/m3, keeping the thermal conductivity and the viscosity almost constant, especially the first one. This thermal characterization agrees with the results obtained when applying the TiO2 optimal nanofluid in the cooling of an experimental setup, with a slightly worse performance than the base fluid. Nevertheless, this performance is the opposite to that noticed with the ferrofluid, which was capable of improving the cooling of the transformer and decreasing its temperature. The similarities between the characterizations of both nanofluids, the differences in their cooling performances and their different magnetic natures seem to point out the presence of additional thermomagnetic buoyancy forces to support the improvement of the cooling. ; This research is under BIOTRAFO project—"Raising Knowledge and Developing Technology for the Design and Deployment of High Performance Power Transformers Immersed in Biodegradable Fluids," which has received funding from the European Union Commission's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement H2020-MSCARISE-2018- 823969; 2019-21. The authors of this research wish to thank the Spanish Ministry of Economy for its financial support for the National Research Project: "Improvement of Insulation Systems of Transformers through Dielectric Nanofluids: Thermodynamic Characterizations and Modelling" (DPI2015-71219-C2 1-R). Additionally, they want to thank the Regional Government of Cantabria; more precisely, the Department of Universities, Research, Environment and Social Policy, for its financial support for the Project "Fluidos Biodegradables en Transformadores Eléctricos de Potencia: Impregnación de Dieléctricos Sólidos y Modelado Térmico con THNM." Mr. Olmo would like to acknowledge to the University of Cantabria and the Government of Cantabria for the financial support for the Ph. D. scholarship (CVE-2016-6626).

This contribution presents the early results of a R&D collaboration established between the University of Cantabria and the Power Transformer Manufacturer EFACEC. This paper tests two different techniques of steady state thermal modelling applied to power transformer windings, Computational Fluid Dynamics (CFD) and Thermal-Hydraulic Network Modelling (THNM). The state of the art of thermal modelling demonstrates that these techniques have been used to calculate both average and hotspot winding temperatures by solving the winding temperature and flows profiles within the winding. THNM models have worse accuracy than CFD in the predicted results. The improvement of these THNM models is a topic of study in transformer thermal modelling. The first goal of this paper is to test the accuracy of a new calibrated THNM model of a disc-type winding immersed in mineral oil. Then, this THNM model is tested with ester-based liquids, such as a natural ester and a synthetic ester, to determine if it can be applied to these liquids without further calibrations. Finally, the cooling performance of both type of liquids is compared using only the THNM model results. The results of this work show that the THNM model developed herein gives good estimations of temperatures compared to those obtained with CFD for both types of liquids. Also, the use of alternative fluids leads to lower temperatures when considering the same oil flow rate and temperature as inlet boundary condition. ; This work was supported in part by the European Union's Horizon 2020 Research and Innovation Programme through the Marie Skłodowska-Curie Action-Research and Innovation Staff Exchange (MSCA-RISE) under Agreement 823969, and in part by the Ministry of Economy through the National Research Project: Improvement of Insulation Systems of Transformers through Dielectric Nanofluids under Grant DPI2015-71219-C21-R.

The objective of this work is to study the effect that the addition of magnetic nanoparticles to a natural ester has on its properties and its cooling capacity. Some samples of ferrofluid (natural ester with maghemite) have been prepared using different concentrations. These have been characterized by measuring their thermo-hydraulic and dielectric properties, to find an optimal concentration. Then, the cooling capacities of the optimal nanofluid and the base fluid have been tested in a transformer immersed in these liquids. The experimental platform allowed the measurement of temperatures in different locations at different load levels. Parallel simulations of these tests have been carried out with a Computational Fluid Dynamics model of the experimental platform. The results show an improvement of the insulating capacity of the base fluid with the addition of maghemite nanoparticles, and an enhanced cooling capacity. ; This work was supported in part by the European Union's Horizon 2020 Research and Innovation Programme through the Marie Sklodowska-Curie under Grant 823969, and in part by the Ministry of Economy through the National Research Project: Improvement of Insulation Systems of Transformers through Dielectric Nanofluids under Grant DPI2015-71219-C2 1-R. The work of C. Olmo was supported by the University of Cantabria and the Government of Cantabria through Ph.D. Scholarship under Grant CVE-2016-6626.

Oil-immersed transformers, whose lifespan is defined by cellulose insulation's lifetime, utilize frequently mineral oil. However, this insulating fluid is being replaced by alternative liquids such as natural and synthetic esters. This replacement requires to guarantee a similar behavior of solid insulating materials immersed in them. Although there are different authors who have concluded that Kraft paper reduces its deterioration rate when it is immersed in biodegradable fluids, there are few works that have analyzed the effect of insulation liquids on the mechanical properties of other cellulosic materials such as diamond dotted paper (DDP) during laboratory tests. This paper shows a comparative analysis of four paper/oil specimens (a standard Kraft paper and a diamond dotted paper aged in both mineral oil and natural ester) under controlled laboratory accelerated thermal ageing. This work focuses on changes in mechanical properties such as the energy consumed per unit volume of the failure zone (E R ), rupture strength (σ R ) and strain under ultimate strength (ε cm ). ; This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 823969-BIOTRAFO

The assessment of two vegetal oils as coolant in Low Voltage Winding of a power transformer with zigzag cooling have been analyzed. These dielectric fluids cooling performance has been compared with a typical mineral oil. To make the study, a 2D-axisymmetrical model of a power transformer has been developed to perform a numerical analysis using a Finite Element Method based software, COMSOL Multiphysics®. Some values are obtained in order to establish the comparison, such as hot-spot temperature or hot-spot factor. Moreover, the influence of the increase of the number of passes of the cooling circuit on the hot-spot temperature has been evaluated for all liquids and compared with the initial design. Results obtained in this work show that the hot-spot temperature is lower for the vegetal oils in the initial design. Furthermore, an increase in the number of passes affect more positively to the mineral oil since similar values of the hot-spot temperature for all liquids are obtained. Values of the hot-spot factor indicates that higher number of passes leads to lower efficient cooling circuits owing to the increase of the pressure drop although the hot-spot temperature decreases. ; The author of this work would like to acknowledge to the Spanish Ministry of Science for the financial support to the National Research Project: Performance of the insulating systems in transformers: alternative dielectrics, thermalfluid modelling and post-mortem analysis (DPI2013-43897-P). Also Mr. Santisteban would like to acknowledge to the University of Cantabria and the Government of Cantabria for the financial support for the Ph.D. scholarship (CVE-2015-11149).

This work compares the temperature distribution and hot-spot temperatures obtained in a disc-type winding of a power transformer when using as dielectric liquid a mineral oil or natural esters. The comparison is made with a reference case for mass flow rate and temperature in the inlet and uniform losses for the discs. A further comparison is performed by increasing the mass flow rate at the inlet form 0.78 kg/s to 0.9 and 1.0 kg/s, thus nine case studies are considered in this work. These cases have been analyzed via CFD techniques using the software COMSOL Multiphysics® with a 2D-axysymmetrical model using the Conjugate Heat Transfer module. For the analysis, the hotspot factor H is considered as an indicator of the cooling circuit efficiency since the losses are uniform. Results shows that for the base case, the hot-spot temperature obtained for the mineral oil is 9-11°C higher than the obtained with esters whereas for the increased mass flow rate, hot-spot temperature of mineral oil is equal or even lower than the obtained for natural esters. The analysis of the hot-spot factor shows the dependence of the cooling circuit efficiency on the kind of oil and inlet conditions. ; The research leading to these results has received funding from multiple sources over years, but we would specifically like to acknowledge the support received in the later stages from the Spanish Plan Estatal de I+D under the grant agreement DPI2015-71219-C2-1-R. Also A.Santisteban would like to thank the University of Cantabria and the Government of Cantabria for the supporting of his Ph.D. scholarship.

In many countries, diverse legislations are today asking for more rational use of energy. For this reason, several initiatives are nowadays promoting a reduction of the overall energy consumption. The European Union has published a Directive 2009/125/EC which fix eco-design requirements to energy-related products. One of these products is power transformers which are a basic piece of generation, transmission, distribution and consumption of electrical energy. The operation of these machines implies the transformation of energy, a part of this energy is dissipated in electrical and magnetic circuits. The amount of losses produce by transformers is defined during the design of these machines. So, they must comply with the above EU Directive. In fact, the Official Journal of the European Union has set a list of energy efficiency limits to be fulfilled by new machines sold in Europe (Commission Regulation 548/2014). This article presents three parts. The calculation of the maximum performance is presented in the first part. This is independent of the load percentage, the impedance and the voltage level. A description of the European fleet of power transformer, up to 100MVA is presented on the second section. In the last part of this work, the efficiency of the fleet is analysed using the maximum performance concept and considering the EU Regulation.

The paper analyzes the propagation of voltage sags through the distribution network of an industrial company. For this, power quality is monitoring in high voltage, 44kV, and low voltage, 400V. The theory of M. Bollen is used for the study of the sags recorded by the meters at different points of the installation. It is necessary to know the connection of transformers in order to determine the transmission of the sags. ; The authors would like to thank the support of the Spanish Government under the research project ENE2007-68032-C04-04/CON.

The power transformer lifespan depends on the dielectric capacity of its insulation system, mainly of the paper. Integrity of this material depends significantly on the cooling capacity of the oil used to cool it. Currently, ester-based fluids are being researched with the intention to substitute the traditional cooling liquid in power transformers, the mineral oil. Here, cooling capacity of two different ester-based fluids, a synthetic and anatural esters, is compared with that of a mineral oil. To carry out this task, CFD results of a low voltage winding of a 100 MVA power transformer jointly with the experimental ones get from a heat-run test done in this transformer are used. This winding has an axial cooling system in ON regime. As a result of this study, it can be inferred that alternative liquids could be used with high load levels (rated power or higher) since no significant changes in the temperatures distribution of the winding cooled with these fluids in comparison with that get with mineral oil are appreciated. ; The authors of this research wish to thank for its financial support: - The European Union by means of the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 823969. This grant fund the European project Biotrafo: raising knowledge and developing technology for the design and deployment of high performance power transformers immersed in biodegradable fluids; European Commission, h2020-mscarise-2018- 823969; 2019-21 - The Spanish Ministry of Science, Innovation and Universities by means of the National Research Project Improvement of Insulation Systems of Transformers through Dielectric Nanofluids (DPI2015-71219-C2-1- R). - The Universities and Reseach Council of the Government of Cantabria by means of the grant Fluidos biodegradables en transformadores eléctricos de potencia: impregnación de dieléctricos sólidos y modelado térmico con THNM, VP32, 2019-2.

Power transformers life is limited fundamentally by the insulation paper state, which can be analysed through different techniques such as furanic compound concentration, dissolved gases, methanol concentration, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscope, refractive index of cellulose fibres, degree of polymerisation or tensile strength. The two last techniques provide the best way to evaluate mechanical resistance of insulation paper. This chapter describes briefly the most remarkable studies about post-mortem assessment and thermal ageing tests in which mechanical properties are some of the characteristics evaluated to determine paper degradation. This work also gathers the main relationships developed until now to relate different by-products generated during transformer operation with loss of paper mechanical properties. Finally, this chapter defines the future approaches, which could be used to study paper degradation. ; The authors are grateful for the funding received to carry out this work from the State Scientific and Technical Research and Innovation Plan under the DPI2013-43897-P grant agreement, financed by the Government of Spain.

Vegetal oils are considered a suitable substitute for mineral oil that is widely used in power transformers as insulation and cooling medium. Due to the fact that more and more power transformers containing this alternative insulating fluid are being constructed a mathematical ageing models is needed to help predict transformer failures. The continued performance of power transformers depends on the condition of its paper insulation mainly. In this sense, this paper analyses Kraft paper degradation through the loss of its mechanical strength. Accelerated thermal ageing test of the paper in two different vegetal oils were carried out at three temperatures during diverse periods of time, in order to obtain information on the kinetics of the ageing degradation of the paper. The evolution of the mechanical properties of paper failure are analysed as a function of temperature and ageing time. In addition, the results obtained are compared with the traditional method of degradation analysis, based on the measurement of the degree of polymerisation. ; The authors are grateful for the funding received to carry out this work from the State Scientific and Technical Research and Innovation Plan under the DPI2013-43897-P grant agreement, financed by the Government of Spain.

This research work is devoted to the critical evaluation of the role of the dc-bus in the behavior of threephase adjustable-speed drives with voltage sags. In particular, the dc-link voltage variation under voltage sag and its dependence on bus capacity, dc-link resistance and inductance and source impedance. The analysis can be used to introduce additional capacitance in the dc-link in order to increase the ride-through capability of the ASD. Both an electromagnetic transient model and a set-up facility have been introduced. This test platform can be used in order to study the sensitivity of ASD to different disturbances. ; The authors would like to thank the support of the Spanish Government under the research project ENE2007-68032-C04-04/CON.

The authors would like to thank the support of the Spanish Government under the CICYT research project DPI2002-04416-C04-01.

Voltage sags are nowadays the first power quality problem in the distribution power system. This work evaluates the real implementation of a voltage sag characterization based on a DSP platform. Several classification methods for voltage sags have been presented in the literature. Here the Bollen's classification is tested. Automation of sag assessment is desirable because manual analysis may be quite difficult. Thus, the fuzzy logic technique is chosen for this aim. This logic system generalizes the classical two-valued logic for reasoning under uncertainty. The fuzzy algorithm was first tested by simulation and then it was translated into C code in order to be implemented in a DSP based measurement platform, obtaining a satisfactory result. ; The authors would like to thank the support of the Spanish Government under the CICYT research project DPI2002-04416-C04-01.

The very high-power transformer lifespan depends mainly on the temperature that cellulose insulation reaches during its operation. Traditionally, its cooling has been carried out using mineral oil as coolant. Nowadays, alternative ester-based liquids are under study as substitutes due to their better environmental and re-safety properties. This paper compares the cooling capacity of two ester-based uids with that of a mineral oil using a 3D numerical model of a 100 MVA low voltage winding of a power transformer with axial cooling system and ONAN cooling mode. Heat-run test results with mineral oil have been used to validate this model. As a rst approximation, according to the comparison developed, esterbased uids could replace mineral oils in this type of transformers when they are going to work in a range of powers close to the rated one. ; This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 823969. Also, we acknowledge the support of the Spanish Ministry of Science and Innovation by means of the National Research Project "Asset management of biodegradable-fluid-based transformers'' (PID2019-107126RB-C22) and the Universities and Research Council of the Government of Cantabria by means of the grant "Biodegradable fluids in electrical power transformers: solid dielectric impregnation and thermal modeling with THNM'', VP32, 2019-2.