A Novel Nanomodified Cellulose Insulation Paper for Power Transformer

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 with scanning 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 the lowest relative permittivity of approximately 2.3 at 50Hz.Thebreakdown voltage of the paper-oil-paper composite insulation system increased from 50.3 kV to 56.9 kV. The tensile strength of the paper handsheet was also measured.


Introduction
Kraft paper is widely used as a form of cellulose insulation in oil-filled transformer equipment [1][2][3][4][5].For very long time, it has been made from wood fiber.However, Kraft paper is the preferred insulation for all oil-filled transformers for its low price and reasonably good performance.The relative permittivity of immersed oil Kraft is about 4.4 or more than twice that of oil (about 2.1 at 50 Hz).Thus, the oil gap shares higher electric field strength [6].The electric field strength of the oil gap would become lower, when the relative permittivity of Kraft paper was reduced.Uniform electric field distributions can be achieved in paper-oilpaper composite insulation systems.Therefore, the insulating distance in transformers can also be decreased, which means the miniaturization of transformer and the reducedamount of cellulose insulation paper.
In my previous work of our group, low relative permittivity polyimide (PI)-SiO 2 films and Kraft-SiO 2 paper handsheet were successfully prepared using SiO 2 hollow spheres with different weight percentages [7][8][9].The relative permittivity of all the composites has been decreased, and electric field strength has been improved at the same time.
In the present work, Kraft-montmorillonite insulation paper handsheets (K-MMT) were obtained successfully with different contents of MMT.The distribution of the MMT in the handsheet was observed by scanning electron microscopy (SEM).The effect of the content of the MMT on the relative permittivity of the oil-immersed handsheet was investigated by broadband dielectric spectroscopy.Breakdown tests of the paper-oil-paper composite insulation system with different relative permittivity papers were performed.The tensile strength of the paper handsheet was also measured.

Material and Methods
The pulp was coniferous wood pulp from Russia, purchased by Qingdao Xinhaifeng Co., Ltd.(Qingdao, China).A natural montmorillonite (Nanomer I.31PS, Nanocor) clay surface modified with octadecylamine and silane coupling agent was used as the reinforcement filler.The nanocomposite was prepared by physical blending.
The dielectric property was measured by a Novo Control Broadband Dielectric Spectrometer with the films dipped in oil over the frequency varying from 1 Hz to 10 MHz at room  The structures of the Kraft, K-MMT, and MMT were characterized by X-ray diffraction.Figure 2 presented the Xray diffraction spectrum of Kraft, K-MMT, and MMT.From the figure, both K-MMT and MMT have obviously peaks at about 4 ∘ , and Kraft did not have any sharp peak but a broad peak.This clearly proved the successful modification of the K-MMT composite.The contents of MMT nanoparticles in the handsheets were 0%, 3%, 6%, 9%, and 12%; they were thus designated as Kraft, K-3% MMT, K-6% MMT, K-9% MMT, and K-12% MMT, respectively.

The Tensile Strength of Kraft-Montmorillonite (K-MMT)
Paper Handsheet.The tensile strength of Kraft (immersed oil) modified of MMT nanosheet was measured following the ISO 1924-2:1994 method.Figure 3 contained the mechanical property of the series of modified K-MMT.From the figure, the tensile strength had a tiny decrease with the increase of the MMT content from 0 to 9%.However, the tensile strength exhibited a dramatic reduction when the MMT content exceeded the 9%.So K-12% MMT would not be discussed for its poor mechanical property.

The Relative Permittivity of Kraft-Montmorillonite (K-MMT) Paper
Handsheet.The relative permittivities of Kraft (immersed oil) modified of MMT nanosheet was tested at different frequencies ranging from 10 −2 Hz to 10 7 Hz at 25 ∘ C. Figure 4 possessed the relative permittivity spectrum of the K-MMT.It showed that the variation trends of the four samples were similar.The changes in the relative permittivities decreased moderately in the range from 1 Hz to 10 7 Hz and dramatically from 0.01 Hz to 1 Hz.

Breakdown Electrical Strength of a Paper-Oil-Paper Composite Insulation System.
A uniformly distributed electric field between two test electrodes was proven by simulation analysis.Figure 5 depicts the computational domain for the dielectric test electrode.Line ab is the symmetry axis of electrode.The developed model was built under two-dimensional axial symmetry configurations and implemented using a COMSOL Multiphysics package based on the finite element method.A sinusoidal voltage of 50 Hz with a peak value of 100 kV was applied to the dielectric test setup.
The electric field distribution between the two test electrodes was shown in Figure 6.The two test electrodes (Figure 5) had a range from 25 mm to 28.3 mm (Figure 6(a)) and −9.5 mm to 9.5 mm (Figure 6(b)).The electric field of paper or oil was uniform along the direction of line ab between the two test electrodes (Figure 6(a)).The curve (Figure 6(b)) shows the electric field distribution of oil along the vertical direction of line ab.The electric field distribution of paper along the vertical direction of line ab was in accordance with that in Figure 6(b).Thus, the electric field of paper or oil was also uniform in the vertical direction of line ab between the two test electrodes.Therefore, the electric field distribution between the two test electrodes was uniform.
The diagram of the electrodes for measuring the breakdown voltage of the handsheet was shown in Figure 7.The diameter and height of the high-voltage (HV) and ground electrodes were both 25 mm.A copper bar was used to connect the HV electrode with the HV AC current power.In this test, mineral oil was used for the dielectric in the stainless steel box.
The focus of the experiment was the effect of the relative permittivity on the breakdown voltage of the composite insulation system.Therefore, the thickness of the oil gap was only 3 mm.The oil gap was formed in the 3 cm diameter hole of the 3 mm thick paperboard.The external diameter of the paperboard was 6 cm.The thickness of the four kinds of experimental handsheet papers (Kraft, K-3% MMT, K-6% MMT, and K-9% MMT) were 0.15 mm in this experiment.Their relative permittivity at 50 Hz was 2.55, 2.51, 2.48, and 2.30, respectively.The handsheet paper was cut into 4 cm diameter circles.All samples were put into the vacuum chamber and were dried at 90 ∘ C for 48 h, and then the mineral oil at 40 ∘ C was infused into the glass bottles in the vacuum chamber to immerse samples for 24 h.The moisture content of oil impregnated paper was 0.4% through such processing.MMT was a kind of phyllosilicate with wide specific surface area.As a very good barrier, MMT nanosheet could certainly resist the current going through the insulating paper.Figure 8 presented the mechanism of MMT resisting the current.It seemed to increase the growth path for the electrical tree; thereby the breakdown electrical field strength has been improved.
The effect of the relative permittivity of the handsheet on the breakdown voltage of the composite insulation system is shown in Figure 9.The breakdown voltage of the paper-oilpaper composite insulation system increased from 50.3 kV to 56.9 kV with decreased relative permittivity of the paper from 2.55 to 2.30.

Conclusions
Kraft-montmorillonite insulation paper handsheets (K-MMT) are obtained successfully with different contents of MMT.The MMT was uniformly dispersed in the handsheet.The effect of the content of the MMT on the relative permittivity of the oil-immersed handsheet was investigated by broadband dielectric spectroscopy.In the paper-oil-paper composite insulation system, the electric field strength of the oil gap decreased with decreased relative permittivity of the paper.Simulation analysis indicated that the electric field distribution between the two test electrodes was uniform.
The breakdown voltage of the paper-oil-paper composite insulation system increased as well as the MMT content in the handsheet.The breakdown voltage of the composite insulation system increased from 50.3 to 56.9 kV when the relative permittivity of the paper decreased from 2.55 to 2.30.The experimental results were also consistent with the theoretically calculated data.The study has great significance for transformer miniaturization and reducing consumption of the cellulose insulation paper.

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

3. 1 .
Preparation of Kraft-Montmorillonite (K-MMT) PaperHandsheet.The MMT were dissolved in deionized water (1 : 100 wt%) and the slurry was homogenized by vigorous agitation with a magnetic stir bar for 10 min.MMT powder with different weight percentages were added to Kraft pulps.The mixtures were stirred for 3 min at 3000 r/min in a fiber disintegrating device and were used to prepare the handsheet.Each wet handsheet was pressed at 15 MPa for 5 min at 80 ∘ C and dried at 105 ∘ C for 7 min under a vacuum.Handsheet with a target basis weight of 120 g/m 2 was produced.MMT with a low content were uniformly dispersed with increased concentration (Figure1(a)).The MMT locally aggregated as almost individual particles in the matrix (Figure 1(b)).

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

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