http://ieeexplore.ieee.org/iel7/7569814/7569815/07569878.pdf

Electromagnetic Shielding

Deutsche Forschungsgemeinschaft (Project No. BE 2464/10-3), the EU-FP7 project “NANOPYME” (310516), and the EU Horizon-2020 project “AMPHIBIAN” (720853). M. R. E. was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award No. DE-SC0005051. F. B. thanks Andreas Ulbricht for fruitful discussions over the many years. C. L. was funded by the U.S. Department of
Energy through the University of Minnesota Center for Quantum Materials under DE-FG02-06ER46275 and DESC-0016371. S. D. acknowledges financial support from the
German Research Foundation (DFG Emmy Noether Grant No. DI 1788/2-1).

https://link.aps.org/accepted/10.1103/RevModPhys.91.015004

Magnetic small-angle neutron scattering

Synthetic esters have become more and more popular in last few decades, explaining the increasing number of units filled with this liquid year by year. They have been investigated under different aspects, both from the fundamental point of view and breakdown mechanisms, well as from the application point of view. However, their use in high voltage equipment is always a challenge and deeper knowledge of the various aspects that can be encountered in their exploitation is needed. The intent of this review paper is to present the recent research progress on synthetic ester liquid in relation to the selected issues, most important for ester development in the authors’ opinion. The described issues are the breakdown performance of synthetic esters, lightning impulse strength and pre-breakdown phenomena of synthetic esters, synthetic esters-based nanofluids, combined paper-synthetic ester based insulating systems, application of synthetic ester for retro-filling and drying of mineral oil-immersed transformers, DGA(dissolved gas analysis)-based diagnosis of synthetic esters filled transformers as well as static electrification of synthetic esters. The different sections are based both on the data available in the literature, but above all on the authors’ own experience from their research work on synthetic ester liquids for electrical application purposes.

A Review on Synthetic Ester Liquids for Transformer Applications

Experimental study on viscosity of spinel-type manganese ferrite nanofluid in attendance of magnetic field

Doped ceramics of indium oxides for negative permittivity materials in MHz-kHz frequency regions

An experimental study of dielectric permittivity and dissipation factor of transformer oil-based magnetic nanofluid is reported in this paper. The investigated nanofluid consists of a commercial transformer oil and iron oxide nanoparticles (0.93 % solid volume fraction) coated with oleic acid as a stabilizing agent. Both, the transformer oil and the nanofluid were subjected to measurements of dielectric permittivity and dissipation factor under 1 kV at a power frequency of 50 Hz in the temperature range from 303 to 373 K. The compared results show that the magnetic nanofluid exhibits approximately 4 orders of magnitude greater dielectric losses than the transformer oil. This raised a question about the suitability of such a nanofluid for cooling applications in a transformer. Therefore, the cooling effectiveness of the nanofluid was tested in a model single-phase power transformer under 4.3 kVA load. The prototype transformer was designed with the aim to monitor its temperature at various locations during the load test. It was found that the cooling with the lossy nanofluid does not result in the deterioration of the thermal conditions of the transformer as compared to the cooling with the transformer oil. On the contrary, the tested nanofluid provides a slight decrease in the average transformer temperature rise. Possible reasons accounting for the slight enhancement in the transformer cooling are discussed. The increased thermal conductivity and development of thermomagnetic convection are considered as primary reasons of the cooling effect.

Transformer oil-based magnetic nanofluid with high dielectric losses tested for cooling of a model transformer

Progress in electrical engineering puts a greater demand on the cooling and insulating properties of liquid media, such as transformer oils. To enhance their performance, researchers develop various nanofluids based on transformer oils. In this study, we focus on novel commercial transformer oil and a magnetic nanofluid containing iron oxide nanoparticles. Three key properties are experimentally investigated in this paper. Thermal conductivity was studied by a transient plane source method dependent on the magnetic volume fraction and external magnetic field. It is shown that the classical effective medium theory, such as the Maxwell model, fails to explain the obtained results. We highlight the importance of the magnetic field distribution and the location of the thermal conductivity sensor in the analysis of the anisotropic thermal conductivity. Dielectric permittivity of the magnetic nanofluid, dependent on electric field frequency and magnetic volume fraction, was measured by an LCR meter. The measurements were carried out in thin sample cells yielding unusual magneto-dielectric anisotropy, which was dependent on the magnetic volume fraction. Finally, the viscosity of the studied magnetic fluid was experimentally studied by means of a rheometer with a magneto-rheological device. The measurements proved the magneto-viscous effect, which intensifies with increasing magnetic volume fraction.

Roman Cimbala

Papers

Transformer oil-based magnetic nanofluid with high dielectric losses tested for cooling of a model transformer

Magnetic Field Effect on Thermal, Dielectric, and Viscous Properties of a Transformer Oil-Based Magnetic Nanofluid

Thermally Stimulated Acoustic Energy Shift in Transformer Oil

Experimental study of AC breakdown strength in ferrofluid during thermal aging

Structure and viscosity of a transformer oil-based ferrofluid under an external electric field