research article a novel nanomodified cellulose insulation ...research article a novel nanomodified...

Research ArticleA Novel Nanomodified Cellulose InsulationPaper for Power Transformer

Yuan Yuan1,2 and Ruijin Liao1

1 State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University,Shapingba District, Chongqing 400044, China

2 School of Materials Science and Engineering, Chongqing University, Shapingba District, Chongqing 400044, China

Correspondence should be addressed to Yuan Yuan; [email protected]

Received 6 February 2014; Accepted 10 March 2014; Published 31 March 2014

Academic Editor: Fan Dong

Copyright © 2014 Y. Yuan and R. Liao.This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A novel cellulose insulation paper handsheet has successfully been modified with various contents of montmorillonite (MMT).Relative permittivity and breakdown strength were investigated. The microstructure of MMT in Kraft paper was observed withscanning electron microscopy (SEM) and X-ray diffraction. The relative permittivity of the immersed oil Kraft-MMT handsheets(K-MMT) initially decreased with the increasing amount of MMT. For MMT concentration of 9 wt%, K-9% MMT possessed thelowest relative permittivity of approximately 2.3 at 50Hz.Thebreakdown voltage of the paper-oil-paper composite insulation systemincreased from 50.3 kV to 56.9 kV. The tensile strength of the paper handsheet was also measured.

1. Introduction

Kraft paper is widely used as a form of cellulose insulationin oil-filled transformer equipment [1–5]. For very long time,it has been made from wood fiber. However, Kraft paperis the preferred insulation for all oil-filled transformers forits low price and reasonably good performance. The relativepermittivity of immersed oil Kraft is about 4.4 or morethan twice that of oil (about 2.1 at 50Hz). Thus, the oilgap shares higher electric field strength [6]. The electricfield strength of the oil gap would become lower, when therelative permittivity of Kraft paper was reduced. Uniformelectric field distributions can be achieved in paper-oil-paper composite insulation systems.Therefore, the insulatingdistance in transformers can also be decreased, which meansthe miniaturization of transformer and the reducedamountof cellulose insulation paper.

In my previous work of our group, low relative permittiv-ity polyimide (PI)-SiO

2films andKraft-SiO

2paper handsheet

were successfully prepared using SiO2hollow spheres with

different weight percentages [7–9]. The relative permittivityof all the composites has been decreased, and electric fieldstrength has been improved at the same time.

In the present work, Kraft-montmorillonite insulationpaper handsheets (K-MMT) were obtained successfully withdifferent contents of MMT. The distribution of the MMT inthe handsheet was observed by scanning electronmicroscopy(SEM). The effect of the content of the MMT on the relativepermittivity of the oil-immersed handsheet was investigatedby broadband dielectric spectroscopy. Breakdown tests of thepaper-oil-paper composite insulation system with differentrelative permittivity papers were performed. The tensilestrength of the paper handsheet was also measured.

2. Material and Methods

The pulp was coniferous wood pulp from Russia, purchasedbyQingdao Xinhaifeng Co., Ltd. (Qingdao, China). A naturalmontmorillonite (Nanomer I.31PS, Nanocor) clay surfacemodified with octadecylamine and silane coupling agent wasused as the reinforcement filler. The nanocomposite wasprepared by physical blending.

The dielectric property was measured by a Novo ControlBroadband Dielectric Spectrometer with the films dipped inoil over the frequency varying from 1Hz to 10MHz at room

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2014, Article ID 510864, 6 pageshttp://dx.doi.org/10.1155/2014/510864

2 Journal of Nanomaterials

(a) (b)

Figure 1: SEM images of the cross-sectional surface of K-MMT composites (a) the dispersed condition of MMT and (b) MMT nanoparticle.

temperature. The morphology of the cross-sectional surfaceof K-MMT paper handsheet was observed on an NOVA400field emission scanning electron microscope (SEM) (FEI,USA) with a working voltage of 10 KV. The compositeswere fractured first in liquid nitrogen and mounted onconductive glass by means of a double-sided adhesive tape;then, a thin layer of gold is sputtered onto the cross-sectionalsurface before SEM observation. The condensed structure isanalyzed from the figure on a Empyrean X-ray DiffractionEquipment (PANalytical Corporation, Almelo, Netherland),with 2𝜃 changes from 2∘ to 30∘, a scan speed of 2∘/min, andCuK𝛼 radiation (𝜆 = 0.154 nm) as the X-ray source. Thethermal property was measured by Q50 thermogravimetryanalysis instrument with heating rate 10∘C/min. Meanwhile,the tensile strength was characterized by AT-L-1 tensilemachine (ANMTCorporation, Jinan, China) using ISO 1924-2:1994 method.

3. Results and Discussion

3.1. Preparation of Kraft-Montmorillonite (K-MMT) PaperHandsheet. The MMT were dissolved in deionized water(1 : 100wt%) and the slurry was homogenized by vigorousagitation with a magnetic stir bar for 10min. MMT powderwith different weight percentages were added to Kraft pulps.The mixtures were stirred for 3min at 3000 r/min in a fiberdisintegrating device andwere used to prepare the handsheet.Each wet handsheet was pressed at 15MPa for 5min at 80∘Cand dried at 105∘C for 7min under a vacuum. Handsheetwith a target basis weight of 120 g/m2 was produced. MMTwith a low content were uniformly dispersed with increasedconcentration (Figure 1(a)). The MMT locally aggregated asalmost individual particles in the matrix (Figure 1(b)).

The structures of the Kraft, K-MMT, and MMT werecharacterized by X-ray diffraction. Figure 2 presented the X-ray diffraction spectrum of Kraft, K-MMT, and MMT. Fromthe figure, both K-MMT and MMT have obviously peaks atabout 4∘, and Kraft did not have any sharp peak but a broadpeak. This clearly proved the successful modification of theK-MMT composite.

4.32

4.14c

b

a

2 6 10 14 18 22 26 30

2𝜃 (deg)

Inte

nsity

Figure 2: X-ray diffraction spectrum for (a) Kraft; (b) K-MMT; (c)MMT.

10

9

8

7

6

Tens

ile st

reng

th (k

N/m

)

0 2 4 6 8 10 12

Content (wt. %)

Figure 3: Effect of MMT content on the tensile strength of oilimmersed K-MMT handsheet.

Journal of Nanomaterials 3

10−3 10−2 10−1 100 101 102

100 101 102

103 104 105 106 107

7

6

5

4

3

2

3

2𝜀𝛾

𝜀 𝛾

KraftK-3% MMT

K-6% MMTK-9% MMT

Frequency (Hz)

Frequency (Hz)

Figure 4: The relation of relative permittivity with different frequency and content of MMT in the oil immersed K-MMT handsheet.

The contents of MMT nanoparticles in the handsheetswere 0%, 3%, 6%, 9%, and 12%; they were thus designatedas Kraft, K-3% MMT, K-6% MMT, K-9% MMT, and K-12%MMT, respectively.

3.2. The Tensile Strength of Kraft-Montmorillonite (K-MMT)Paper Handsheet. The tensile strength of Kraft (immersedoil) modified ofMMTnanosheet wasmeasured following theISO 1924-2:1994 method. Figure 3 contained the mechanicalproperty of the series of modified K-MMT. From the figure,the tensile strength had a tiny decrease with the increaseof the MMT content from 0 to 9%. However, the tensilestrength exhibited a dramatic reduction when the MMTcontent exceeded the 9%. So K-12% MMT would not bediscussed for its poor mechanical property.

3.3. The Relative Permittivity of Kraft-Montmorillonite (K-MMT) Paper Handsheet. The relative permittivities of Kraft(immersed oil) modified of MMT nanosheet was tested atdifferent frequencies ranging from 10−2Hz to 107Hz at 25∘C.Figure 4 possessed the relative permittivity spectrum of theK-MMT. It showed that the variation trends of the four sam-ples were similar. The changes in the relative permittivitiesdecreased moderately in the range from 1Hz to 107Hz anddramatically from 0.01Hz to 1Hz.

The relative permittivity of the K-MMT (K-3% MMT, K-6% MMT, and K-9% MMT) composite was lower than thatof Kraft at different frequencies. The relative permittivity ofair is 1.0. Thus, air voids stored in MMT nanosheet were acause of reduced relative permittivity [10]. The existence ofMMT nanosheet improves the distance of fiber chains. Theoil content of handsheet increased due to air voids in thecomposite [11]. Therefore, the relative permittivity decreased[12–15]. Noticeably, the relative permittivity (at 50Hz) ofthe handsheet decreased from 2.55 (Kraft) to 2.30 (K-9%MMT). K-9%MMTexhibited the lowest relative permittivity.

b

a

2

1

3

3

5 4

(1) HV electrode(2) Ground electrode

(4) Paperboard(5) Mineral oil

(3) Handsheet

Figure 5: Computational domain.

A low content of MMT nanosheet ( K-3% MMT > K-6%MMT > K-9%MMT.

3.4. Breakdown Electrical Strength of a Paper-Oil-Paper Com-posite Insulation System. A uniformly distributed electricfield between two test electrodes was proven by simulationanalysis. Figure 5 depicts the computational domain for thedielectric test electrode. Line ab is the symmetry axis of elec-trode.Thedevelopedmodelwas built under two-dimensionalaxial symmetry configurations and implemented using


Electrode

60

50

40

30

20

10

Fiel

d str

engt

h (k

V/m

m)

25 26 27 28

Displacement (mm)

(a)

Electrode

60

55

50

45

40

35

30

25

Fiel

d str

engt

h (k

V/m

m)

−30 −20 −10 0 10 20 30

Displacement (mm)

(b)

Figure 6: Electric field distribution of oil gap (a) symmetry is 𝑥-axis, the bottom center of ground electrode is origin; (b) vertical symmetryaxis is 𝑥-axis, the center of oil gap is origin.

a COMSOLMultiphysics package based on the finite elementmethod. A sinusoidal voltage of 50Hz with a peak value of100 kV was applied to the dielectric test setup.

The electric field distribution between the two testelectrodes was shown in Figure 6. The two test electrodes(Figure 5) had a range from 25mm to 28.3mm (Figure 6(a))and −9.5mm to 9.5mm (Figure 6(b)). The electric field ofpaper or oil was uniform along the direction of line abbetween the two test electrodes (Figure 6(a)). The curve(Figure 6(b)) shows the electric field distribution of oil alongthe vertical direction of line ab. The electric field distributionof paper along the vertical direction of line ab was inaccordance with that in Figure 6(b).Thus, the electric field ofpaper or oil was also uniform in the vertical direction of lineab between the two test electrodes.Therefore, the electric fielddistribution between the two test electrodes was uniform.

The diagram of the electrodes for measuring the break-down voltage of the handsheet was shown in Figure 7. Thediameter and height of the high-voltage (HV) and groundelectrodes were both 25mm. A copper bar was used toconnect the HV electrode with the HV AC current power. Inthis test, mineral oil was used for the dielectric in the stainlesssteel box.

The focus of the experiment was the effect of the relativepermittivity on the breakdown voltage of the compositeinsulation system. Therefore, the thickness of the oil gap wasonly 3mm. The oil gap was formed in the 3 cm diameterhole of the 3mm thick paperboard. The external diameterof the paperboard was 6 cm. The thickness of the four kindsof experimental handsheet papers (Kraft, K-3% MMT, K-6%MMT, and K-9% MMT) were 0.15mm in this experiment.Their relative permittivity at 50Hz was 2.55, 2.51, 2.48, and2.30, respectively. The handsheet paper was cut into 4 cmdiameter circles. All samples were put into the vacuumchamber and were dried at 90∘C for 48 h, and then themineral oil at 40∘C was infused into the glass bottles in thevacuum chamber to immerse samples for 24 h. The moisture

HVAC powersupply

25mm25

mm

HVelectrode

Groundelectrode

Mineral oil

R: 3mmMineral oildielectric

Stainlesssteel box

Figure 7: Diagram of electrodes.

content of oil impregnated paper was 0.4% through suchprocessing. MMT was a kind of phyllosilicate with widespecific surface area. As a very good barrier, MMT nanosheetcould certainly resist the current going through the insulatingpaper. Figure 8 presented the mechanism of MMT resistingthe current. It seemed to increase the growth path for theelectrical tree; thereby the breakdown electrical field strengthhas been improved.

The effect of the relative permittivity of the handsheet onthe breakdown voltage of the composite insulation system isshown in Figure 9. The breakdown voltage of the paper-oil-paper composite insulation system increased from 50.3 kV to56.9 kVwith decreased relative permittivity of the paper from2.55 to 2.30.

4. Conclusions

Kraft-montmorillonite insulation paper handsheets (K-MMT) are obtained successfully with different contents of

Journal of Nanomaterials 5

I

MMT

Figure 8: Schematic of MMT blocking the current.

58

56

54

52

50

Brea

kdow

n vo

ltage

(kV

/mm

)

0 2 4 6 8 10

Percent (%)

Figure 9: Effect of MMT content on the breakdown voltage of oilimmersed K-MMT handsheet.

MMT. The MMT was uniformly dispersed in the handsheet.The effect of the content of the MMT on the relativepermittivity of the oil-immersed handsheet was investigatedby broadband dielectric spectroscopy. In the paper-oil-papercomposite insulation system, the electric field strength of theoil gap decreased with decreased relative permittivity of thepaper. Simulation analysis indicated that the electric fielddistribution between the two test electrodes was uniform.The breakdown voltage of the paper-oil-paper compositeinsulation system increased as well as the MMT contentin the handsheet. The breakdown voltage of the compositeinsulation system increased from 50.3 to 56.9 kV when therelative permittivity of the paper decreased from 2.55 to2.30. The experimental results were also consistent with thetheoretically calculated data. The study has great significancefor transformer miniaturization and reducing consumptionof the cellulose insulation paper.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

The work is supported by the Fundamental Research Fundsfor the Central Universities (Project no. CDJZR12130046),the Natural Science Foundation Project of CQCSTC (Projectno. cstcjjA50007), and the Special Financial Grant from theChongqing Postdoctoral Science Foundation (Project no.XM20120037).

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research article a novel nanomodified cellulose insulation ...research article a novel nanomodified...

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