The use of vegetable oil (natural ester) in electrical devices like power transformers is increasing due to their high biodegradability and better safety. The lifespan of power trans- formers is mainly defined by cellulose insulation condition, which usually works together with dielectric oil as electrical insulation and also as mechanical winding protector and compactor. That is why the aim and results of this research shows us not only the dielectric parameters evolution, but also the relationship between the mechanical factors and the moisture content of thermal accelerated ageing processes, with commercial vegetable oil, of Kraft paper and Diamond Dotted Paper (DDP). These are two of the most common insulating materials in electric power transformers. In addition, the new tests have been done by a different method of paper ageing analysis. ; 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.
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).
In oil-immersed power transformers, the insulation system is constituted of a dielectric oil-solid combination. The insulation oil generally used is mineral oil; however, this fluid has started to be substituted by natural and synthetic esters due to their higher biodegradability and flash point. The introduction of a new fluid in the insulation system of power transformers requires kinetic models that can estimate the degradation rate of insulation solids. The aim of this work was to go further in quantifying through different kinetic models the deterioration suffered by a commercial cellulose board (PSP 3055), which is one of the solid materials used in the insulation system of oil-filled transformers. The aging study was extended to cellulose board specimens immersed in two different oils (mineral and synthetic ester). It was obtained that there is a lower degradation when synthetic ester is used in the insulation system. Additionally, it can be concluded that the use of mechanical properties to quantify the degradation of the cellulose board through kinetic models provides information about the different behavior shown by PSP 3055 when different fiber direction angles are considered. ; This research was funded by State Scientific and Technical Research and Innovation Plan under the PID2019-107126RB-C22/AEI/10.13039/501100011033 grant agreement, financed by the Government of Spain
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).
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.
This paper develops the comparison of two cogeneration systems that are suitable to perform heat energy recovery of the smoke produced by a casting process. ; The authors would like to thank the financial support of Government of Cantabria and NISSAN under the INVESNOVA research project and to the Council on Research and Technological Development of the University of Cantabria by their administrative assistance.
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.
Power transformers use Kraft paper, thermally upgraded Kraft and other polymeric papers (ex. Nomex) as the main solid insulation between the winding conductors. Dielectric oil used in transformers as an insulating and cooling fluid typically has an operating temperature range of 60–90 °C. These service temperatures can cause slow degradation of both the oil and the insulating paper winding, with a loss of mechanical and dielectric properties. In this sense, this work analyzes paper degradation through Young's Modulus, yield stress, rupture strength and strain under ultimate strength. An accelerated thermal ageing of the paper in mineral oil was carried out at temperatures of 110, 130 and 150 °C over different periods of time, in order to obtain information on the kinetics of the ageing degradation of the paper. The evolution of the mechanical properties and micro mechanisms of paper failure are analysed as a function of temperature and ageing time. Finally, the results obtained are compared with the traditional method of degradation analysis, based on the degree of polymerisation measurement. ; 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.
Oil-immersed transformers use paper and oil as insulation system which degrades slowly during the operation of these machines. Cellulose materials are used generally as insulation solid in power transformers. The degree of polymerization (DP), defined as number of repeating b-glucose residues in the cellulose molecule, is a critical property of cellulosic insulation material used in transformers, since it provides information about paper ageing and its mechanical strength. The fast-developing electric power industry demanding superior performance of electrical insulation materials has led to the development of new materials, as well as different drying techniques performed during transformer manufacturing and service when required. Both developments have caused some practical difficulties in the DP measurement. Moreover, the increasing interest in synthetic dielectric materials replacing cellulose materials requires measuring alternative properties to the DP to quantify the degradation of insulation solids over time. In this sense, this paper proposes the possibility of analyzing paper degradation through fracture toughness. This approach is different from the study of mechanical properties such as tensile strength or strain because it provides a tool for solving most practical problems in engineering mechanics, such as safety and life expectancy estimation of cracked structures and components which cannot to be considered through the traditional assessment of the mechanical resistance of the material. An accelerated thermal ageing of Kraft paper in mineral oil was carried out at 130 ºC during different periods of time, to obtain information on the kinetics of the ageing degradation of the paper. Double-edged notched specimens were tested in tension to study their fracture toughness. The evolution of the load-displacement curves obtained for different ageing times at the ageing temperature of 130 ºC was utilized to the determination of the stress intensity factor. Furthermore, different kinetic models based on this stress intensity factor were applied to relate its evolution over time as a function of the temperature. Finally, the correlation between the DP and stress intensity factor, which depends on the fiber angle, was also defined. ; 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 PID2019-07126RB-C22 grant agreement, financed by the Government of Spain
This paper analyses and compares the impact that maghemite and titania nanoparticles have in the electrical and thermal performance of a commercial natural ester used in power transformers. Vegetal-oil-based nanofluids have been prepared at different concentrations. Once the nanofluids were obtained, the breakdown voltage of the oil samples with and without nanoparticles was measured. It was found that the concentration of nanoparticles influences the breakdown voltage of natural ester. The existence of optimal concentrations has been noticed, and these nanofluids were selected for the upcoming tests. The cooling capacity of these liquids and the base fluid was measured through a thermal analysis in an experimental platform. The experiment developed is based on a 1-phase transformer (800 VA, 230/115 V) immersed in a stainless-steel fulfilled with insulating liquid. The temperatures inside the tank were monitored at the bottom and at the top of the tub to measure the oil temperature. Other sensor measured the hotspot winding temperature, defined as the hottest temperature of winding conductors in contact with solid insulation or insulating liquid. This sensor was located at the top of the winding as the most probable hot-spot location. Ambient temperature was also measured as a reference. A microcontroller (Arduino) and an Integrated Development Environment (IDE) was utilized to record the measurements of the sensors. The results of the thermal study showed that only maghemite based nanofluids exhibited improved heat transfer characteristics in comparison with the natural ester oil. ; This research is under BIOTRAFO project 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. Also it has received financial support from the Spanish National Research Project: "Improvement of Insulation Systems of Transformers through Dielectric Nanofluids: Thermodynamic Characterizations and Modelling" (DPI2015-71219-C2 1-R) and from the Regional Government of Cantabria, Project "Fluidos Biodegradables en Transformadores Eléctricos de Potencia: Impregnación de Dieléctricos Sólidos y Modelado Térmico con THNM."
