Dielectric properties of magnetic fluid F. HERCHL, P. KOPČANSKÝ, M. TIMKO M. KONERACKÁ, I. POTOČOVÁ, Institute of Experimental Physics Slovak Academy of Sciences, Košice K. MARTON Faculty of Electrical Engineering and Informatics, Technical University, Košice L. TOMČO Faculty of Aviation, Technical University, Košice

Magnetic fluids – - both thermal and dielectric benefits to transformers - improve cooling by enhancing fluid circulation within transformer windings - increase transformer capacity to withstand lightning impulses - minimize the effect of moisture on typical insulating fluids to design smaller, more efficient new transformers, or to extend the life or loading capability of existing units Magnetic fluids in power transformers

HOWEVER – V.Segal, K.Raj, Indian J. Eng. Mater. Sci. 5 (1999) 416. Presence of magnetic particles (magnetite particles coated by Si) in transformer oil (Exon Univolt 60) improved its dielectric breakdown properties for I S < 5 mT. DC impulse breakdown voltage: from 78  108 kV for example. Measuring gap 25.4 mm

Dielectric breakdown strength of magnetic fluids EXPERIMENT Up to 10 kV properly shaped electrodes of a uniform gap of electric field – - Rogowski profile - diameter 1.5 cm; - distance between 0.1 – 1 mm up to 50 mT

The used electrode system

Measurement of breakdown voltage The evaluation was realized according to high-voltage technique: (Kuffel et.al. Oxford 1984) breakdown voltage has to be measured 7 times in the same point the lowest and highest values are omitted the mean value of breakdown voltage is calculated The error of measurements was ± 5 %

Used magnetic fluid: - particles - Fe 3 O 4 - surfactant - oleic acid - liquid carrier - transformer oil TECHNOL US 4000 (  r = 2.15) - volume concentrations of magnetic particles  = saturation magnetizations I s = 1–8 mT - mean magnetic diameter: D m =8.6 nm (VSM magnetization measurements) D TEM =10.2 nm (transmission electron microscopy (TEM)) - observation of the agglomeration processes - a drop of MF sandwiched between two parallel glass cover slips (10-50  m)  placed into magnetic field up to 50 mT (Helmholtz coils, parallel to MF film plane)  observed by microscope equipped with a video camera - time dependencies of the breakdown development - measured by inductive probe and a programmable oscilloscope with its own memory

RESULTS Observation of the agglomeration processes - needle like aggregation; -saturation of average length of aggregates  m (depending on the volume concentration of magnetic particles and applied external magnetic field) Effect of aggregation of magnetic particles in magnetic fluid (  0 H=10mT,  = 0.01) 40  m The average length of needle like clusters vs. time after application of B=10 mT.

The DC and AC dielectric breakdown strengths of magnetic fluid (  = ) and pure transformer oil.

The DC dielectric breakdown strength vs. distance between the electrodes for magnetic fluid (  = and  =0.02  ) transformer oil Technol in B=0 and 31mT Crossover concentration of MPs The crossover from better to worse dielectric properties was found to appear in MF with I s = 4 mT what is in agreement of Segal observations (V. Segal, K. Raj, Ind. J. Eng. Mater. Sci. 5 (1998) 416

Discharge currents in MFs The discharge currents vs. magnetic particles concentration  in B = 0.

The aproximation an average breakdown field as a function of electrode distance by expressions corresponding to Duxbury-Leath model and Weibull model. The breakdown electric field decreases with increasing distance of the electrodes what corresponds to the theory. Breakdown electric field vs. electrode distance d by DC conditions.

Electric conductivity of MFs The AC electric conductivity vs. magnetic particles concentration  in B = 0, B  E and B  E.

Permittivity of MFs Permittivity vs. electric field intensity in B = 0, B  E and B  E. J. Phys.: Condens. Matter Acta Physica Polonica A

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