Substitution of transformer oil as insulator medium by magnetic fluid in transformers requires observation of
electric properties of magnetic fluids at temperatures higher than 20 °C. That´s why important physical quantities
were measured at temperature in interval from 20 °C to 80 °C. The magnetodielectric anisotropy was studied at
the same temperature region. Two important quantities have been measured: specific electric conductivity and
dielectric breakdown strength of magnetic fluids at volume concentrations from 0,185 % to 2 %. So the behavior
of magnetic fluids as insulator medium could be observed at working conditions of transformers

Cimbala, Roman

Kalcunová, Iraida

Király, Jozef

Kopčanský, Peter

Marton, Karol

Molčan, M.

Timko, Milan

Tomčo, Ladislav

DSpace at University of West Bohemia

    Ročník 2011   Číslo VI    Magnetodielectric anisotropy in magnetic fluids in temperature interval from 20 °C to 80 °C K. Marton1, R. Cimbala1, I. Kolcunová1, J. Király1, L. Tomčo2, M. Timko3, P. Kopčanský3, M. Molčan3,  1 Department of Electric Power Engineering, Faculty of Electrical Engineering and Informatics, TU in Košice, Mäsiarska 74, Košice 2  Department of Aerodynamics and Simulations, Faculty of Aeronautics, TU in Košice, Rampová 7, Košice 3  Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Watsonova 47, Košice E-mail : Karol.Marton@tuke.sk, Roman.Cimbala@tuke.sk, Iraida.Kolcunova@tuke.sk, Jozef.Kiraly@tuke.sk, Ladislav.Tomco@tuke.sk, timko@saske.sk, kopcan@saske.sk, molcan@saske.sk   Abstract Substitution of transformer oil as insulator medium by magnetic fluid in transformers requires observation of electric properties of magnetic fluids at temperatures higher than 20 °C. That´s why important physical quantities were measured at temperature in interval from 20 °C to 80 °C. The magnetodielectric anisotropy was studied at the same temperature region. Two important quantities have been measured: specific electric conductivity and dielectric breakdown strength of magnetic fluids at volume concentrations from 0,185 % to 2 %. So the behavior of magnetic fluids as insulator medium could be observed at working conditions of transformers.  Abstrakt Náhrada transformátorového oleja ako izolačného média magnetickou kvapalinou v transformátoroch si vyţaduje skúmanie elektrických vlastností magnetických kvapalín pri teplotách vyšších ako 20 °C. Z toho dôvodu odporúčame merať fyzikálne veličiny v interval teplôt 20 °C do 80 °C. Magnetodielektrická anizotropia bola skúmaná v tej istej teplotnej oblasti. Zamerali sme sa na dve veličiny: mernú elektrickú vodivosť a elektrickú pevnosť magnetickej kvapaliny o objemovej koncentrácii 0,185 % do 2 %. Takţe správanie magnetickej kvapaliny ako izolačného media mohlo byť pozorované pri pracovných podmienkach transformátora.      INTRODUCTION The application of magnetic fluids as insulator medium in transformers of high voltage has not been done sufficiently deep so far. The structure of magnetic fluids themselves shows that magnetic fluids are complex material consisting of three components: nonpolar component – inhibited transformer oil as carrier liquid, polar component – oleic acid as surfactant and solid magnetite nanoparticles of average diameter 10 nm. The still completed measurements of electric properties have showed fair-sized differences of observed fluid particularly when observed medium is placed into combined electric and magnetic fields at two different orientations of used fields (E∥H, E⊥H). The goal of this work was observation of magnetodielecric anisotropy of both dielectric breakdown strength (Eb) and specific electric conductivity [1] of magnetic fluids based on transformer oil and observation temperature dependences of important quantities characterizing electric properties of magnetic fluids. The experiments have proved the presence of electrophoretic conductivity in magnetic fluids, too.     THEORY Complex physico-chemical character of the magnetic fluids requires investigation of their properties:  in DC and AC electric fields,  in weak (below 106 V.m-1) and strong electric fields (above 107 V.m-1),  in "clean" - nonpolar insulating liquids, in polar liquids with surfactant and in pigmented liquids by colloidal nanoparticles.  Based on the elementary Ohm's law in differential form  dEEγ=di , after a detailed analysis we get this equation:  kTWkTυqδn=qbn=γ ai exp6200  (1) where bi is the ion mobility, that is also exponentially dependent on temperature T and it can be expressed as:  kTWkTυqδ=Eυ=b aii exp62 (2)       where δ is the distance of potential holes, υ is the frequency (eg. 1012-1013 s-1) and Wa is the activation energy. The coefficient bi in mineral oils, when weak electric fields are applied, reaches value of 10-8 m2.s-1.V-1 and in strong fields it increases to 10-7 m2.s-1.V-1 (mobility of negatively charged ions). When DC field (voltage) is applied on a magnetic fluid containing nanometer sized particles of ferrites then it is expected that electrophoretic conductivity occurs in the inter-electrode space, which is defined as follows: πηrnεξ=γ kk62 (3) where  is the electrokinetic potential, η is the dynamic viscosity, ε is the electric permittivity and r is the particle radius. Electrical conductivity of liquid insulating material is often associated with the viscosity η of the liquid, that is dependent on temperature, which can be  expressed as: kTWυγkT=η aexp63 (4) The equation (4) is a part of Walden's law that is applicable on nonpolar (or weakly-polar) liquid insulators in the form: γ η= const.  (5) Relationship between the specific electrical conductivity and electrical stability can be found from modelling of heat transition, when we come out from the model of a dielectric (insulator) placed between two plane parallel electrodes. After creation of differential equations that corresponds to balance state of energy, we get the equation [3]: dzγE=|dzdT+λ|dzdTλ zdz+z2  (6) where λ is the coefficient of thermal conductivity of dielectric material (oil), γ is the equivalent conductivity of liquid media. The maximum intensity of electric field at a generated temperature can be reached by another solution of the differential equation (6).  EXPERIMENT AND RESULTS The specific conductivity measurements have been carried out with help of closed small container that was armed with permanent magnets that were source of homogeneous magnetic fields of value from 0 to 40 mT with possibility to change orientation of electric and magnetic fields (E∥H, E⊥H). The comparing of voltage decrement on measured resistor (magnetic fluid) with normal resistor on base of Ohm´s law was used for determination of specific conductivity. The experimental set up is illustrated in fig.1 [1].   Fig. 1 Experimental set up for specific conductivity measurements  The experiments have been carried out with magnetic fluids of volume concentration of magnetite particles from 0,185 % to 2 % at DC voltage from 200 V to 1000 V. The course of dependencies γ = f(T) with U as parameter showed the validity of equation (1). The magnetodielectric anisotropy was more distinct at higher values of voltage.  Fig. 2 The dependencies of specific conductivity on temperature at voltage of 200 V  The measurements of dielectric breakdown strength, i.e. dielectric stability of magnetic fluids were carried out on the base on the STN norm. The sample of magnetic fluids was placed into small container that was armed by Rogowski´s electrodes and permanent magnets (NdFeB). Magnetic fluids temperature was controlled by ultra thermostat.          Fig. 3 Experimental set up for dielectric breakdown strength measurements [4]  Electric field intensities were higher than 107 V.m-1, i.e. experiments were carried out in strong electric field. The course of dependencies in fig.4 corresponds to the same dependency for pure mineral oil that contains small amount of water (0,02 %) at low temperature. Water at higher temperature changes from state of emulsion solution to molecular state and so dielectric breakdown strength reaches lower values. This decrease is caused by increasing of magnetic fluid conductivity. The observed maximum of dependency at perpendicular orientation of magnetic and electric fields shifts to lower temperatures.    Fig. 4 Dielectric breakdown strength vs. temperature for magnetic fluid with a low volume concentration of magnetite particles (0,25 %)   CONCLUSIONS It is interesting that on dependencies of conductivity on temperatures at various volume concentrations (mainly at higher values) of magnetite particles in magnetic fluids could be observed stair-like formations that were loaded down on exponential dependency of specific conductivity for observed medium. It could be assumed that observed formation are caused by arrangement effect of magnetite particles in magnetic fluids that is dependent on temperature. This hypothesis is supported by courses of specific conductivity dependencies on temperatures in magnetic fluids that have been measured at constant voltage of 100 V, during 200 sec  at given temperatures.    ACKNOWLEDGEMENTS This work was supported by the projects VEGA MŠ SR  1/0368/09 and 2/0077/09, Centre of Excellence of SAS Nanofluid.  REFERENCES [1] Marton K., Tomčo L., at al.: Zborník z medzinárodnej konferencie Diagnostické metódy v diagnostike trakčných zariadení, ŢU v Ţiline, 2008. [2] Franz W.:  Elektrischer Durchschlag, Springer Verlag, Berlín, 1956. [3] Kučinskij G. S.: Razrjady v tverdych a ţidkich dielektrikach, Leningrad, 1981. [4] Marton K., Tomčo L., at al.: X. Sympozjum „ Problemy eksploatacji ukladow izolacyjnych wysokiego napecia“, Krynica, 2005. [5] Kopčanský P., Tomčo L., et al.: Journal of Magnetism and Magnetic Materials., Vol. 289, 2005.        

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Magnetodielectric anisotropy in magnetic fluids in temperature interval from 20 °C to 80 °C

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Magnetodielectric anisotropy in magnetic fluids in temperature interval from 20 °C to 80 °C

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