Fluid-thermal analysis of the cooling capacity of a commercial natural ester in a power transformer

The low fire safety and high environmental impact of the mineral oil of the power transformers are two factors that have a negative influence in the goals of maximizing the return on investment and lowering the total costs associated with power system operation. Thus, many studies about alternative liquids (natural and synthetic esters, silicon oil) are being developed worldwide. This paper studies the cooling characteristics of a commercial natural ester in comparison with a traditional insulating mineral oil. A fluid-thermal simulation is done in a 3D-model of a partial cross-section of a single phase of a three-phase power transformer. The comparison of temperatures, velocities and dimensionless numbers (Re and Pr) of both fluids allows concluding that the direct substitution of the traditional liquid by the new one is not possible since the cooling characteristics of the latter one is quite worse than the first one

Numerical model of a three-phase Busbar Trunking System

The thermal behavior of an industrial Low Voltage non-segregated three-phase busduct is analyzed by means of the comparison of a 3D numerical model with experimental results. This model has been carried out using COMSOL Multiphysics, software based on finite element method. The numerical model replicates the short-circuit test, using the same geometry configuration and the boundary conditions of the laboratory in which this assay is carried out. The standard IEC 61439 is applied, both in test and model, in order to obtain the steady state temperatures in several parts of the busbar system. As a result of the data comparison can be concluded that the experimental test is replicated with sufficient accuracy by the numerical model. In fact, the average error of all the temperatures is smaller than 5%. As a general conclusion, the numerical model developed can be considered accurate enough to use it in the first steps of the busbar design.The authors of this research wish 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, thermal-fluid modelling and post-mortem analysis (DPI2013-43897-P)

Numerical and experimental evaluation of dielectric properties of thermally aged insulating paper used in power transformers

Due to the relevant role of the cellulosic insulation in the dielectric system of a power transformer, it is necessary to know its dielectric properties and its evolution as insulating material. The dependence of the dielectric properties of solid insulation on the moisture content, temperature, or pressure is well-defined. However, it is not clear the effect of the paper degradation on its insulating capacity. To study the impact of ageing on the dielectric properties, samples of Kraft paper were subjected to an accelerated hygrothermal ageing. Degree of polymerization (DP) value was reduced from 1000 to 200, with intermediate study points. All samples were conditioned to the same moisture content in order to only evaluate the effect of the paper degradation. Different dielectric properties, such as loss factor (tanδ) and complex permittivity (ε) were measured using the dielectric spectroscopy technique. These experimental results were used to develop a numerical model with the finite element-based tool Comsol Multiphysics. This model provided some extra information about the material properties, such as the electric field distribution. Results showed that the degradation of Kraft paper modifies its dielectric response.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/AEI/ 10.13039/501100011033). The authors also wish to thank “Fundación Iberdrola” for its financial support for the research project: S090 “Análisis de las Propiedades Dieléctricas de Aislamientos Sólidos Impregnados con Líquidos Dieléctricos”. C. Méndez also wants to acknowledge the Spanish Ministry of Science, Innovation and Universities for the financial support for the FPU grant (FPU19/01849)

Numerical and experimental analysis of the aging impact on the cooling capacity of a natural ester-based oil used in power transformers

An experimental and numerical research was conducted to study the aging effect on the cooling performance of a natural ester-based oil habitually used in power transformers. A monitored experimental platform has been used to observe the temperature increases with the aging of the insulation system (oil and paper). Three samples with three aging levels were tested at three load levels: underload, rated and overload regimes. The two aged samples (6 liters each) were aged in an accelerated way within an oven: 1 week at 150°C. Also, some physicochemical properties of the insulation system (paper and oil) were determined. A 2D numerical model has been developed with COMSOL Multiphysics to determine the temperature and velocity distributions of the different samples in the platform to analyze their differences and to compare their cooling capacity. At first sight, the aging affects negatively to the cooling capacity of the new alternative liquid based on natural esters.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 DPI215-71219-C2-1-R

Titania nanofluids based on natural ester: cooling and insulation properties assessment

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)

Effect of TiO2 nanoparticles on the performance of a natural ester dielectric fluid

Mineral oil has been frequently used in most of high voltage transformers. However, this oil has started to be replaced by vegetable oils. Additionally, different authors have found that mineral oil-based nanofluids have a potential in improving insulating properties of transformer oil. In order to improve the characteristics of a commercial vegetal oil, this work has used TiO2 nanoparticles to obtain a nanofluid whose ageing behavior at 150°C for 300 hours has been studied.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)

Estimating the age of power transformers using the concentration of furans in dielectric oil

Transformers have an insulation system based on dielectric oil and paper. Several factors as shortcircuits, high load, or overheating influence the aging of the insulating system. Thus, these materials are decomposed during transformer operation, and its state determines the status of the transformer. The degree of polymerization of dielectric paper is an indicator of its real condition. The concentration of furanic compounds in oil offer an estimation of paper degradation. Moreover, these compounds are easy to measure from oil samples. This paper review equations which relate one or more properties of the insulating system with the life expectancy of the transformer. These equations were used in a fleet of transformers in order to estimate the accuracy of these mathematical models. Results show that these methods achieve the most reliable estimation when the transformers have operated less than 8 years. Nevertheless, when the transformers are older, the estimation of lifetime based on furanic compounds is quite erratic and should be improved.The research leading to these results has received funding from multiple sources during 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 DPI2013-43897-P

Hydraulic assessment of nanofluids based on mineral oil and natural ester in windings of power transformers

It is common for electric power transformers to be cooled by mineral oil. However, this type of oil has begun to be replaced by oils of natural origin (esters), due to environmental and fire safety reasons. The latter are biodegradable and have an ignition point much higher than that of mineral oils. On the other hand, different authors have found that the dielectric and refrigerant properties of the oils used in transformers can be improved when some types of nanoparticles are added. In order to assess this improvement, this work presents the results obtained by a research in which different nanofluids, produced through commercial dielectric oils (mineral and natural), nanoparticles of titanium (IV) oxide and magnetite, were thermally characterized. The results of this characterization have been used to perform simulations based on computational fluid dynamics. This comparison has allowed to observe the pressure drops and the mass flows in the internal channels of the windings of a real power transformer.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)

The aging impact on the cooling capacity of a natural ester used in power transformers

In this paper, an experimental and numerical research was conducted to study the aging effect on the cooling performance of a natural ester habitually used in power transformers. A monitored experimental platform has been used to observe the temperature increases with the aging of the ester. Three samples with three aging levels (fresh oil, aged oil and aged oil with paper) were tested at three load levels, C = 0.72, C = 1 and C = 1.3 (9 tests). The two aged samples (6 liters each) were aged in an accelerated way within an oven: 1 week at 150 °C. Also, some physicochemical properties of the insulation system were determined. In the case of the liquid, viscosity, moisture, acidity, dissipation factor (90 °C) were measured. The degree of polymerization and the moisture were measured in paper samples. A 2D numerical model has been developed with COMSOL Multiphysics to replicate the thermal-hydraulic behavior of the samples in the platform. That is, the idea is to determine the temperature and velocity distributions of the different samples in order to analyze their differences and to compare their cooling capacity. At first sight, the aging affects negatively to the cooling capacity of the new alternative liquid based on natural esters.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

A comparison of transformer HF models and their application to PQ analysis

This research work is devoted to the comparison of some proposed high-frequency (HF) models of transformers and their application to power quality (PQ) studies. The models are classified according their structure, physical description and experimental methodology and set-up facilities needed to obtain the parameters