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).

Over the last few decades the insulating performance of transformer oils has been broadly studied under the point of view of nanotechnology, which tries to improve the insulating and heat dissipation performance of transformer oils by suspending nanoparticles. Many authors have analyzed the thermal and dielectric behavior of vegetable oil based-nanofluids, however, very few works have studied the evolution of these liquids during thermal aging and their stability. In this paper has been evaluated the performance of aged vegetable oil based-nanofluids, which have been subjected to accelerated thermal aging at 150 °C. Nanoparticles of TiO2 and ZnO have been dispersed in a commercial natural ester. Breakdown voltage, resistivity, dissipation factor and acidity of nanofluid samples have been measured according to standard methods, as well as stability. Moreover, it has been analyzed the degradation of Kraft paper through the degree of polymerization (DP). The results have showed that although nanoparticles improve breakdown voltage, they increase the ageing of insulation liquids and dielectric paper. ; This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 823969.The authors of this research wish to thank the Ministry of Economy for its financial support for the National Research Project: Improvement of Insulation Systems of Transformers through Dielectric Nanofluids (DPI2015-71219-C2 1-R).

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.

Transformers are installed in power distribution systems to perform changes in supply voltage. Large consumers often have several transformers installed in parallel to ensure continuity of supply in the event of failure. These machines can achieve very high efficiency, but their efficiency is not constant since it depends on the power demanded at each time. Therefore, the level of efficiency that correspond to the operation of a specific transformer depends on two factors: machine technical characteristics and electrical load. In this work, the authors have proposed a methodology which shows the optimal number of transformers to be connected at each period in the substations of a large Spanish hospital, in order to achieve the maximum seasonal efficiency of these machines. The results of the energy saving are determined with respect to the current situation, in which all the transformers are permanently connected. On the other hand, the European Union has established a new regulation that sets the minimum energy efficiency requirements for new power transformers. This efficiency improvement is proposed to be applied gradually in two stages, a first limit came into force in 2015, while a more restrictive approach will appear in the year 2021. This work has also studied the potential energy savings that would occur when the substations of the hospital have more efficient transformers complying with the new European Regulation 548/2014.

Nanofluids based on vegetal oils remain almost unexplored, especially when prepared with non-ferric oxide nanoparticles. This work has analysed the effect of three different nanoparticles on properties of a commercial vegetal oil. The first aim was to evaluate if there is an optimal concentration for dielectric improvement, and if the effect of different nanoparticles is similar. Secondly, other key properties were considered. The evaluated dielectric properties have been AC breakdown voltage, relative permittivity, volumetric resistivity and dissipation factor. The results are quite promising because dielectric properties have shown a considerable improvement. On the other hand, thermal conductivity or acidity test results, although adverse, were not definitive to refuse to continue this research. ; The authors of this research wish to thank the Ministry of Economy for financial support to the National Research Project: Improvement of Insulation Systems of Transformers through Dielectric Nanofluids (DPI2015-71219-C2 1-R). Also 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).

In this paper, an experimental research was conducted to study the effect of the presence of Fe 2 O 3 nanoparticles over cooling and dielectric performance of a natural ester used in power transformers. Different concentration samples of nanofluid have been characterized to find an optimal one, focusing on viscosity, thermal conductivity and dielectric strength. A monitored experimental platform has been used to observe the temperature increases during operation while being cooled. This includes a single-phase distribution transformer, working at three different load levels, C=0.7, C = 1 and C=1.3. Both natural ester base fluid and optimal nanofluid have been used as cooling fluid. At first sight, the nanoparticles seem not to affect neither thermal conductivity nor viscosity at the concentrations used. On the contrary, breakdown voltage of base fluid experiments an enhancement at some of them. The cooling capacity of the nanofluid has also shown an improved behavior. ; The present investigation has been supported by Spanish Ministry of Economy (DPI2015-71219-C2 1-R), the University of Cantabria and the Government of Cantabria (CVE-2016-6626).

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.

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.

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.

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.