Effect of TiO2 and ZnO nanoparticles on the performance of dielectric nanofluids based on vegetable esters during their aging

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)

Health indexes for power transformers: A case study

It seems essential that a health index for a power transformer should take into account the age of the transformer and its loading in service.The authors gratefully acknowledge funding of this work from several sources, particularly support in the later stages from the Spanish Plan Estatal de I+D under the grant agreement DPI2013-43897-P

Evaluation of the optimal connection of power transformers in the substations of a hospital

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

Numerical analysis of the hot-spot temperature of a power transformer with alternative dielectric liquids

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)

Comparative study of techniques used in the generation expansion planning

At the present time the generation expansion planning (GEP) has become a problem very difficult to solve for multiple reasons: many objectives, high uncertainties, very great planning horizon, etc. Its resolution by means of the exact traditional techniques, in numerous occasions, is not viable by the excessive time that is needed. For that reason, technical modern that allow the resolution in smaller time and with smaller accuracy in the solution are applied. Most of the approximate techniques are included inside a wider concept that is denominated Artificial Intelligence. In this article the more promising techniques of IA are studied indicating their applications in the PEG as well as their advantages and disadvantages

Thermal analysis of natural esters in a low-voltage disc-type winding of a power transformer

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

Thermal modelling of electrical insulation system in power transformers

Temperature is one of the limiting factors in the application of power transformers. According to IEC 60076-7 standard, a temperature increase of 6 C doubles the insulation ageing rate, reducing the expected lifetime of the device. Power losses of the transformer behave as a heating source, and the insulating liquids act as a coolant circulating through the windings and dissipating heat. For these reasons, thermal modelling becomes an important fact of transformer design, and both manufacturers and utilities consider it. Different techniques for thermal modelling have been developed and used for determining the hot-spot temperature, which is the highest temperature in the winding, and it is related with the degradation rate of the solid insulation. First modelswere developed as a first estimation for modelling the hot-spot temperature and the top-oil temperature. These models were based on thermal-electric analogy and are known as dynamic models. Other two different kinds of models are widely used for thermal modelling, known as Computational Fluid Dynamics (CFD) and Thermal Hydraulic Network Models (THNMs). These two techniques determine the temperature and velocity fields in the winding and in the insulating fluid. In this chapter, the different techniques for transformer thermal modelling will be introduced and described

CFD study of the impact of the deviation on the mass flow inlet on winding hot-spot temperature of a power transformer using mineral and ester oils

This work presents a study where a deviation on the mass flow rate in adjacent sectors of the winding is considered. A CFD model was developed for this study, where heat transfer between sectors is enabled due to the lack of thermal symmetry, and the goal of this study is to determine whether this effect can be neglected or not. To carry out this study, a Low Voltage Winding of a 100 MVA power transformer was selected. This transformer has been manufactured and tested by BEST Transformers, and the results obtained from the heat run test were used to validate the CFD model. The study also considered an alternative dielectric liquid, natural ester, in order to estimate if the different thermal and hydraulic conditions can affect to the relevance of the studied effect. The CFD simulations have been carried out with ANSYS Fluent. The results showed a deviation of 3-4% of the heat dissipated due to the non-symmetric conditions with a 10% of mass flow deviation, which is the heat transfer between the adjacent sectors. This effect is identical when considering mineral oil or natural ester. The results shows there is a small impact of the winding temperatures due to this effect, and that it can be considered negligible.Part of the work was performed during secondments between BEST Transformers and the University of Cantabria executed in the framework of the BIOTRAFO project “Raising knowledge and developing technology for the design and deployment of high-performance power transformers immersed in biodegradable fluids”, H2020-MSCA-RISE2018- 823969, 2019-21. 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/AEI/ 10.13039/501100011033

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)