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 IS < 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 - Fe3O4

- surfactant - oleic acid - liquid carrier - transformer oil TECHNOL US 4000 (r = 2.15)

- volume concentrations of magnetic particles = 0.0025-0.02 - saturation magnetizations Is= 1–8 mT

- mean magnetic diameter: Dm=8.6 nm (VSM magnetization measurements)

DTEM=10.2 nm (transmission electron microscopy (TEM))

- observation of the agglomeration processes - a drop of MF sandwiched between two parallel glass cover slips (10-50m) 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 - 100-300 m (depending on the volume concentration of magnetic particles and applied external magnetic field)

Effect of aggregation of magnetic particles in magnetic fluid (0H=10mT, = 0.01)

40 m

0 50 100 150 200

0

50

100

150

200

250

300

Ave

rag

e L

en

gth

[m

]

Time [s]

= 0.01

= 0.02

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

( = 0.0025) and pure transformer oil.

0,0 0,2 0,4 0,6 0,8 1,0

2

3

4

5

6

7

8

9

10E

[M

Vm-1

]

d [mm]

DC, MF, = 0.0025 DC, Oil AC, MF, = 0.0025 AC, Oil

(B = 0 T)

The DC dielectric breakdown strength vs. distance between the electrodes for magnetic fluid ( =0.0025 and =0.02) transformer oil Technol in B=0 and 31mT

Crossover concentration of MPs

0 0,2 0,4 0,6 0,8 1,05

6

7

8

9

10 MK =0.0025 B=0mT MK =0.0025 B||E MK =0.0025 BE Technol B=0mT MK =0.02 B=0mT MK =0.02 B||E MK =0.02 BE

E [

MV

m-1]

d [mm]

The crossover from better to worse dielectric properties was found to appear in MF with Is = 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

0 0,01 0,02 0,030

3

6

9

12

B=0mT d[mm]

0,05 0,1 0,2 0,3 0,4 0,5

I/

I 0.00

24

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

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5

50

100

150

200

250

300

350 MK na baze ITO 100 B=0mT B=40mT B||E B=40mT BE

.1

09 [S

.m-1]

The AC electric conductivity vs. magnetic particles concentration in B = 0, B E and B E.

Permittivity of MFs

0,5 1,0 1,5 2,0 2,52,10

2,12

2,14

2,16

2,18

2,20MF = 0.0019

B=0mT B=40mT B||E B=40mT BE

r

E[MV/m]0,5 1,0 1,5 2,0 2,5

2

3

4

5MF = 0.019

r

E[MV/m]

B=0mT B||E B=40mT BE B=40mT

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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