-
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.
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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
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4 Journal of Nanomaterials
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
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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).
References
[1] T. A. Prevost and T. V. Oommen, “Cellulose insulation in
oil-filled power transformers: part I—history and development,”IEEE
Electrical Insulation Magazine, vol. 22, no. 1, pp. 28–35,2006.
[2] Z. Deng,M. Chen, S. Zhou, B. You, and L.Wu, “A novel
methodfor the fabrication of monodisperse hollow silica
spheres,”Langmuir, vol. 22, no. 14, pp. 6403–6407, 2006.
[3] A.M. Emsley, X. Xiao, R. J. Heywood, andM. Ali,
“Degradationof cellulosic insulation in power transformers. Part 2:
formationof furan products in insulating oil,” IEE Proceedings:
Science,Measurement and Technology, vol. 147, no. 3, pp. 110–114,
2000.
[4] L. E. Lundgaard, W. Hansen, D. Linhjell, and T. J.
Painter,“Aging of oil-impregnated paper in power transformers,”
IEEETransactions on Power Delivery, vol. 19, no. 1, pp. 230–239,
2004.
[5] T. V. Oommen and T. A. Prevost, “Cellulose insulation
inoil-filled power transformers: part II—maintaining
insulationintegrity and life,” IEEE Electrical Insulation Magazine,
vol. 22,no. 2, pp. 5–14, 2006.
[6] Y. Kamata, E. Ohe, K. Endoh et al., “Development of
low-permittivity pressboard and its evaluation for insulation of
oil-immersed EHV power transformers,” IEEE Transactions
onElectrical Insulation, vol. 26, no. 4, pp. 819–825, 1991.
[7] Y. Yuan, Y. Wang, and B. Xie, “Preparation and propertiesof
polyimide/SiO
2hollow spheres composite films with good
dielectric property and amazing XRD spectra,” Materials Sci-ence
Forum, vol. 663–665, pp. 584–587, 2011.
[8] Y. Yuan, B.-P. Lin, and Y.-M. Sun, “Novel
low-dielectric-constant copolyimide thin films composed with
SiO
2hollow
spheres,” Journal of Applied Polymer Science, vol. 120, no. 2,
pp.1133–1137, 2011.
[9] T. Liu, R. J. Liao, F. Z. Zhang, and L. J. Yang,
“Preparation andperformance of a low relative permittivity
insulation paper,”in Proceedings of the International Conference on
High VoltageEngineering and Application, pp. 211–214, 2012.
[10] M. A. Wahab, I. Kim, and C.-S. Ha, “Microstructure and
prop-erties of polyimide/poly(vinylsilsesquioxane) hybrid
compositefilms,” Polymer, vol. 44, no. 16, pp. 4705–4713, 2003.
[11] Y. B. Kim and K.-S. Yoon, “A physical method of
fabricatinghollow polymer spheres directly from oil/water emulsions
ofsolutions of polymers,” Macromolecular Rapid Communica-tions,
vol. 25, no. 18, pp. 1643–1649, 2004.
[12] J. Lin and X. Wang, “Preparation, microstructure, and
prop-erties of novel low-𝜅 brominated epoxy/mesoporous
silicacomposites,” European Polymer Journal, vol. 44, no. 5, pp.
1414–1427, 2008.
[13] C.-L. Chung and S.-H. Hsiao, “Novel organosoluble
fluori-nated polyimides derived from
1,6-bis(4-amino-2-trifluoro-methylphenoxy)naphthalene and aromatic
dianhydrides,” Poly-mer, vol. 49, no. 10, pp. 2476–2485, 2008.
-
6 Journal of Nanomaterials
[14] Y.-J. Lee, J.-M. Huang, S.-W. Kuo, and F.-C. Chang,
“Low-dielectric, nanoporous polyimide films prepared from PEO-POSS
nanoparticles,” Polymer, vol. 46, no. 23, pp. 10056–10065,2005.
[15] S.-H. Hsiao, C.-P. Yang, and C.-L. Chung, “Synthesis
andcharacterization of novel fluorinated polyimides based
on2,7-bis(4-amino-2-trifluoromethylphenoxy)naphthalene,” Jour-nal
of Polymer Science A: Polymer Chemistry, vol. 41, no. 13,
pp.2001–2018, 2003.
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