Research and development of ester filled power transformers

Ester-based insulating liquids are preferred for power transformers due to their fire safety features, minimal environmental impact, as well as the ability to prolong the lifetime of insulation paper compared to conventional mineral oil. The number of ester-filled high voltage power transformers has been increasing in recent years but remains low due to requirement of design modification and lack of experience with long-term operating performance. This article introduces some of the research contributions on the application of ester liquids in large power transformers made by the members of the University Transformer Research Alliance (UTRA) over the past two decades

Improvement of transformer liquid insulation using nanodielectric fluids: a review

During the last 20 years, the search for new applications for nanotechnology has become one of the busiest in science, engineering, and manufacturing. New nanotechnology-based materials with superior properties have been developed and are already used in many everyday products and processes. The application of nanotechnology to high-voltage engineering has been mainly oriented towards the development and characterization of the so-called nanodielectric materials [1]. In 1994, Lewis [2] suggested that "a major field of study in the future development of dielectrics will concern their properties when relatively few molecules are involved. Such smallness arises naturally at interfaces of nanometric thickness and will occur also when dielectrics are employed in the nano-technical devices of the future." The physical phenomena that govern the behavior of materials at sub-microscopical scale are outlined in this publication [2]. The term "nanodielectric" was introduced by Frechette [3], [4], who defined nanodielectrics as "multicomponent dielectrics possessing nanostructures, the presence of which results in the change of one or several of its dielectric properties".This work has been supported by the Spanish Ministry of Economy and Competitiveness within the project DPI2015-71219-C2-2-R.Publicad

High Voltage Insulating Materials-Current State and Prospects

Studies on new solutions in the field of high-voltage insulating materials are presented in this book. Most of these works concern liquid insulation, especially biodegradable ester fluids; however, in a few cases, gaseous and solid insulation are also considered. Both fundamental research as well as research related to industrial applications are described. In addition, experimental techniques aimed at possibly finding new ways of analysing the experimental data are proposed to test dielectrics

Investigation of CeO2 nanoparticles on the performance enhancement of InsulatingOils

Integrating nanotechnology in dielectric fluid significantly inhibits losses and boosts overall dielectric fluid performance. There has been research done on the effects of introducing various nanoparticles, such as titania, alumina, silica nanodiamonds, etc. In this paper, a novel nanoparticle, Ceria (CeO2), has been used, and its properties were examined using the FTIR (Fourier Transform Infrared) spectrum, the XRD (X-ray Diffrac-tion) spectrum, the SEM (Scanning Electron Microscopy), and the TEM (Transmission Electron Microscopy). This paper illustrates an efficient dielectric fluid prepared by the successful dispersion of Cerium Oxide (CeO2) nanoparticles in various concentrations into four commercial oils, namely mineral oil, rapeseed oil, synthetic ester oil, and soybean oil, to enhance and improve their dielectric characteristics. The performance investigation emphasises breakdown strength enhancement and other dielectric properties of the colloidal solution comprising different nanoparticle (NP) concentrations. Various commercial oils are used as a base in nano-oil to diversify their applicability as dielectric fluids by measuring the correlation in dielectric parame-ters and statistically assessing their applicability with normal and Weibull distributions. The obtained ex-perimental data sets were analysed using the Statistics and Machine Learning Toolbox in MATLAB. The aging measurement has been done only on mineral oil, and results were matched using a predictive model of statistics and the Machine Learning Toolbox in MATLAB. Well-dispersed CeO2 NPs in the insulating oils lead to a significant increase in AC breakdown strength. The effect of ageing on the dielectric properties of nano oils yields better results than conventionally aged oil. It has been observed that the breakdown voltage is enhanced by up to 30 % for mineral oil at an optimal concentration of 0.01 g/L, 9% for synthetic ester oil at 0.03 g/L, 18% for rapeseed oil at 0.02 g/L, and 19% for soybean oil at 0.03 g/L nanoparticle concentration. Following the dispersion of CeO2 nanoparticles, the dielectric constant of all insulating oils has also signifi-cantly improved. The overall experimental results are promising and show the potential of the CeO2 NPs-based nano oil as an efficient and highly performing dielectric oil for different power applications.publishedVersio

Partial Discharge Phenomena in Converter and Traction Transformers: Identification and Reliability

After the development of power electronics converters, the number of transformers subjected to non-sinusoidal stresses (including DC) has increased in applications such as HVDC links and traction (electric train power cars). The effects of non-sinusoidal voltages on transformer insulation have been investigated by many researchers, but still now, there are some issues that must be understood. Some of those issues are tackled in this Thesis, studying PD phenomena behavior in Kraft paper, pressboard and mineral oil at different voltage conditions like AC, DC, AC+DC, notched AC and square waveforms. From the point of view of converter transformers, it was found that the combined effect of AC and DC voltages produces higher stresses in the pressboard that those that are present under pure DC voltages. The electrical conductivity of the dielectric systems in DC and AC+DC conditions has demonstrated to be a critical parameter, so, its measurement and analysis was also taken into account during all the experiments. Regarding notched voltages, the RMS reduction caused by notches (depending on firing and overlap angles) seems to increase the PDIV. However, the experimental results show that once PD activity has incepted, the notches increase PD repetition rate and magnitude, producing a higher degradation rate of paper. On the other hand, the reduction of mineral oil stocks, their relatively low flash point as well as environmental issues, are factors that are pushing towards the use of esters as transformer insulating fluids. This PhD Thesis also covers the study of two different esters with the scope to validate their use in traction transformers. Mineral oil was used as benchmark. The complete set of dielectric tests performed in the three fluids, show that esters behave better than mineral oil in practically all the investigated conditions, so, their application in traction transformers is possible and encouraged

A Novel Oil-immersed Medium Frequency Transformer for Offshore HVDC Wind Farms

In this project, a design of an oil type medium frequency transformer for offshore wind farm applications is proposed. The design is intended for applications when series coupling of the output of the DC/DC converters of the wind turbine on their secondary side is done to achieve a cost-effective high voltage solution for collecting energy from offshore wind parks. The focus of the work is on the insulation design of the high voltage side of a medium frequency transformer where the magnetic design constraints should also be satisfied.Above all, a proof of concept is made demonstrating a possible solution for the design of the transformer for such a DC/DC converter unit. The transformer suggested is using oil/paper as insulation medium. Furthermore, characterisation of an eco-friendly biodegradable transformer oil for this type of HVDC transformer application is made. Moreover, an introduction of reliable high frequency characterisation test methods to medium frequency transformer designers is made. In addition, the Non-Linear Maxwell Wagner (NLMW) relations are further developed to form a method for the development of an HVDC MFT transformer. All in all, the DC series concept has been further developed one step closer to pre-commercialization, i.e. from TRL 1 to about 2

Mobility of charge carriers, particle charging and electro-hydrodynamic processes in dielectric liquids and nanofluids

For over a century, different dielectric liquids are used as insulators and coolants in high voltage systems. Currently, with stricter environmental regulations and developments of more compact and elevated voltage apparatuses, the power and pulsed power industries require environmentally friendly dielectric liquids with better insulation and cooling ability, and higher adaptability. Among these liquids, ester liquids, including synthetic and natural esters, are introduced in the past decades: first synthetic ester liquids were introduced in the 1970s and now are utilised in high voltage power systems as liquid insulators. Apart from the pure dielectric liquids, nanofluids, which are developed by adding nanoparticles into dielectric liquids, started to generate significant interest among researchers and practitioners in high voltage technology. This is because the nanofluids may have greater dielectric strength and better heat conduction properties than pure dielectric liquids. However, physical mechanisms which result in this potential increase in breakdown strength of nanofluids are not fully understood and require further investigation. In the present work field, both the dielectric liquids and the nanofluids hosted by these dielectric liquids were studied experimentally. Three types of dielectric liquids were studied in this thesis: mineral oil, synthetic ester, and natural ester. The investigation is focused on the mobility of charge carriers in these dielectric liquids stressed with an external electric field with different magnitudes. The obtained results show that the mobility of charge carriers in all tested dielectric liquids has the same order of magnitude under the same electric field. Thus, it is concluded that the space charge density in the ester liquids, which have greater electrical conductivity than the conductivity of the mineral oils, is significantly higher than that in the mineral oil. Nanofluids have been developed using these three types of dielectric liquids and two types of nanoparticles: TiO2 and BN nanoparticles. This study included developing an analytical model of field charging processes in nanofluids stressed with the external electric field and experimental investigation of the mobility of charge carriers and electrohydrodynamic (EHD) behaviours in nanofluids. The analytical modelling is based on the Maxwell-Wagner relaxation theory. The surface charge distribution across the surface on a nanoparticle placed in an insulating liquid stressed with a step external electric field has been analytically obtained. The obtained results show that the surface charge density is governed by the electric conductivity and the dielectric permittivity of the dispersed particles and the hosting liquid. Furthermore, the Coulomb force between two particles immersed in a liquid was obtained analytically, enabling the analysis of the force acting between particles suspended in the host liquid, laying the foundation for further investigation of the EHD effects based on the experiment results. The experimental investigation of the EHD effects in the nanofluids demonstrated that both the TiO2 and BN nanoparticles acquired a net negative charge in both ester liquids when stressed with the external electric field. However, in the case of mineral-oil-based nanofluid, TiO2 the BN particles become only polarised under the action of the external electric field, leading to the formation of ‘particle chains’ in the host liquid. The appearance of the ‘particle chains’ observed in the experiments was explained by the mathematical model developed in this work. The results obtained in this work will be of interest to researchers and practitioners working in the field of insulation liquids and their practical applications in high voltage power and pulsed power systems.For over a century, different dielectric liquids are used as insulators and coolants in high voltage systems. Currently, with stricter environmental regulations and developments of more compact and elevated voltage apparatuses, the power and pulsed power industries require environmentally friendly dielectric liquids with better insulation and cooling ability, and higher adaptability. Among these liquids, ester liquids, including synthetic and natural esters, are introduced in the past decades: first synthetic ester liquids were introduced in the 1970s and now are utilised in high voltage power systems as liquid insulators. Apart from the pure dielectric liquids, nanofluids, which are developed by adding nanoparticles into dielectric liquids, started to generate significant interest among researchers and practitioners in high voltage technology. This is because the nanofluids may have greater dielectric strength and better heat conduction properties than pure dielectric liquids. However, physical mechanisms which result in this potential increase in breakdown strength of nanofluids are not fully understood and require further investigation. In the present work field, both the dielectric liquids and the nanofluids hosted by these dielectric liquids were studied experimentally. Three types of dielectric liquids were studied in this thesis: mineral oil, synthetic ester, and natural ester. The investigation is focused on the mobility of charge carriers in these dielectric liquids stressed with an external electric field with different magnitudes. The obtained results show that the mobility of charge carriers in all tested dielectric liquids has the same order of magnitude under the same electric field. Thus, it is concluded that the space charge density in the ester liquids, which have greater electrical conductivity than the conductivity of the mineral oils, is significantly higher than that in the mineral oil. Nanofluids have been developed using these three types of dielectric liquids and two types of nanoparticles: TiO2 and BN nanoparticles. This study included developing an analytical model of field charging processes in nanofluids stressed with the external electric field and experimental investigation of the mobility of charge carriers and electrohydrodynamic (EHD) behaviours in nanofluids. The analytical modelling is based on the Maxwell-Wagner relaxation theory. The surface charge distribution across the surface on a nanoparticle placed in an insulating liquid stressed with a step external electric field has been analytically obtained. The obtained results show that the surface charge density is governed by the electric conductivity and the dielectric permittivity of the dispersed particles and the hosting liquid. Furthermore, the Coulomb force between two particles immersed in a liquid was obtained analytically, enabling the analysis of the force acting between particles suspended in the host liquid, laying the foundation for further investigation of the EHD effects based on the experiment results. The experimental investigation of the EHD effects in the nanofluids demonstrated that both the TiO2 and BN nanoparticles acquired a net negative charge in both ester liquids when stressed with the external electric field. However, in the case of mineral-oil-based nanofluid, TiO2 the BN particles become only polarised under the action of the external electric field, leading to the formation of ‘particle chains’ in the host liquid. The appearance of the ‘particle chains’ observed in the experiments was explained by the mathematical model developed in this work. The results obtained in this work will be of interest to researchers and practitioners working in the field of insulation liquids and their practical applications in high voltage power and pulsed power systems

A Review on Properties, Opportunities, and Challenges of Transformer Oil-Based Nanofluids

The mineral oil or synthetic oil in conjunction with paper is mainly being applied as dielectric medium in many of the high voltage apparatus. However, the advent of high voltage levels such high voltage alternating current (HVAC) and high voltage direct current (HVDC) has prompted researchers to direct their focus onto an insulation system which can bear the rising high voltage levels. The modern insulating liquid material development is guided by various factors such as high electrical insulation requirements and other safety and economic considerations. Therefore transformer manufacturer companies have to design transformers with these new specific requirements. The transformer oil-based nanofluids with improved dielectric and thermal properties have the potential to replace mineral oil base products in the market place. They are favorable because they function more superior than mineral oil and they contribute definite insulating and thermal gains. This paper reviews recent status of nanofluids use as transformer oils. The nanofluids used as transformer oils are presented and their advantages are described in comparison with mineral oil. The multiple experimental works carried out by different researchers are described, providing an overview of the current research conducted on nanofluids. In addition scope and challenges being confronted in this area of research are clearly presented