Simulation of electric field distribution in nanodielectrics based on XLPE

Simulation of electric field distribution in nanodielectrics based on XLPE

Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 3 (2016) 2381–2386 www.materialstoday.com/proceedings Recent A...

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Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 3 (2016) 2381–2386

www.materialstoday.com/proceedings

Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)

Simulation of electric field distribution in nanodielectrics based on XLPE Arjun Jayakrishnana, D. Kavithab, A. Arthi a, Niveditha Nagarajana, Meera Balachandrana,* b

a Department of Chemical Engineering and Materials Science Department of Electrical and Electronics Engineering, Amrita VishwaVidyapeetham, Coimbatore-641112, India

Abstract Recently, there has been a growing interest in the use of nano-sized fillers in order to enhance the properties of a material. The mechanical, barrier and electrical properties of polymers can be enhanced by use of nanofillers in appropriate amounts. The tremendous improvement in polymer properties on incorporation of nanoparticles arises from large interfacial area and large interfacial interaction between the nanofiller and the polymer matrix. There is several reported literature on the influence of nanofillers on dielectric properties of polymeric insulators, mainly epoxy resins. Literature on theoretical analysis for determining the influence of fillers and the dependence of its size, shape and composition on the electrical properties of the composite are limited. Cross linked polyethylene (XLPE) is widely used as insulation in underground high tension cables. The life of cables can be extended by delaying the breakdown of insulation. The breakdown of insulation depends on the distribution of electrical stress in insulation. The electric field and stress distribution in polymer insulation can be altered by adding nanoparticles, depending on permittivity of the nanofiller and its weight percentage in the polymer. This paper attempts to simulate the electric field distribution in XLPE and the effect of different kinds of nanofillers, namely nanoclay, nanosilica and nanoclacium carbonate on the same. Simulations were also performed in COMSOL Multiphysics software to evaluate the effect of nanofiller content on electrical field distribution in XLPE. It was found that the composite with the highest difference in the electric field values between the polymer and the nanofiller offers the best resistance against electric field propagation and that the dielectric properties of the nanocomposites become better with increased amount of nanofillers. However, at a very high weight percentage, the properties of the composite deteriorate due to the fact that for higher weight percentage the inter-particle distance reduces allowing for more agglomeration. The nano-filler with the highest difference is found to be nano-silica. © 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords: Nano-silica; Nano-calcium carbonate; Nano-clay; Tanaka equation; Dielectric; Mechanical

* Corresponding author. E-mail address:[email protected] 2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].

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1. Introduction Over the recent years, a lot of work has been done in the field of nano-materials and composites. The applications of nano-composites have gained ground in the aerospace, automotive, construction and electrical industries, to name a few. Reinforcing a polymer with nano-sized fillers has known to yield materials with a superior performance[1,2,3]. The enhancement in properties of nano-composites arises from the large specific surface area of the nano-fillers. One such material that is of particular interest is the nano-composite of cross linked polyethylene (XLPE). XLPE has been used as an insulation material in high tension underground cables and submarine cables [4,5,6]. These cables are designed to withstand tremendous pressures and are very useful in transferring electricity through long distances. But with continued exposure to moisture and electrical stress, a damaging phenomenon called treeing takes place inside the material [4,5,6]. This is caused due to partial discharges, and it progresses in a path resembling the branches of a tree, and this reduces the life of the cable. Reinforcing the XLPE with certain nano- fillers may reduce the damage done by electrical treeing and increase the life of a high tension cable [7,8]. According to literature, there are reports of enhanced dielectric properties and reduced treeing on addition of certain nano-fillers [7,8]. In this paper, we compare the electrical and mechanical properties of different XLPE nanocomposites using three types of nano-fillers, nano-clay, nano-silica and nano-calcium carbonate, with varying compositions, using the COMSOL simulation software. 2. Materials and Methods The materials that have been investigated are • Cross Linked Polyethylene (XLPE) • Nano-clay • Nano-silica • Nano-calcium carbonate The relative permittivity of XLPE is 2.4 and its Young’s modulus is 1E+09 Pa. The density of XLPE is 930 kg/m3. The following table (Table 1) shows the relative permittivity of the nanodielectrics which been used in the simulation of their electric field distribution. Table 1: Relative Permittivity of Nano-Fillers

Nano-filler

Relative Permittivity

Nano-clay

3.2

Nano-silica

3.9

Nano-calcium carbonate

8.5

The simulations were carried out in COMSOL Multiphysics 4.3. A 2-dimensional diagram of the nano-composite is drawn by keeping the cross-sectional area constant across the different nano-fillers and weight percentages. The distance between two nano-fillers was calculated using the Tanaka equation [9]. Both electrostatics and solid mechanics physics were chosen at the same time. The boundary conditions that the system was subject to were: 1) ground at one side, 2) electric potential at the opposite side, 3) fixed constraint on one side and 4) boundary load of 150 MPa [10] on the opposite side. The simulations were carried out to get the electric field and mechanical stress distribution inside the system at the given conditions. 3. Results and Discussion The surface graphics obtained after the\simulations have been depicted in Figure 1.The progress of electric treeing may be obstructed by the presence of nano-fillers.In order to calculate the distance between the centres of two nano-filler particles, we used the Tanaka equation9

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1 ⎡ ⎤ 3 ⎢ ⎧⎪ π S n 100 ⎡ wt % ⎛ S m ⎞ ⎤ ⎫⎪ ⎥ − − D = ⎢⎨ 1 1 ⎢ ⎜ ⎟⎥ ⎬ d 6 S m wt % ⎣ 100 ⎝ S n ⎠ ⎦ ⎭⎪ ⎥ ⎪ ⎩ ⎢⎣ ⎥⎦ The maintained fixed size for all the simulation and varied the weight percent of nanofillers. The electric field stress for blank xlpe is 2.24E+07 V/m.Electrical field distribution obtained from the COMSOL simulations gives an approximate idea about the electric treeing process. The electric field distribution depends on the permittivity and weight percentage of the nano-fillers. To compare different nano-fillers of different weight percentages, we compare the difference in the electric stress experienced in bulk XLPE material and right above a nano-filler. Table 2 gives the difference in electric field between bulk XLPE and material right above the nano-filler of different nano-fillers and weight percentages.

Table 2: Electric field stress at the interface of the nano-filler Nano-filler

Nano-clay(V/m)

Nano Calcium carbonate (V/m)

Nano-silica(V/m)

2.5 wt%

2.33E+07

2.91E+07

2.67E+07

5 wt%

2.37E+07

2.99E+07

2.71E+07

7.5 wt%

2.40E+07

3.03E+07

3.15E+07

10 wt%

2.41E+07

3.04E+07

3.35E+07

Fig. 1. Electrostatics simulations from COMSOL of XLPE with 5 wt% (a) nanoclay ; (b) nano silica; (c) nano calcium carbonate.

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The dielectric loss of the nanocomposite should be le lesser than that of pure XLPE. We see from Figure 2 that when the range of frequency is high (100-200 kHz), th the nano-clay and nano-calcium carbonate nanocomposites have a lower dielectric loss than that of pure XLPE, where ereas that of nano-silica is much higher. This implies that the dielectric strength of nano-clay and nano-calcium carbon onate will be greater than that of nano-silica. Between nanoclay and nano-calcium carbonate, nano-clay is a filler tha that forms layers in the XLPE, thus they have dimensions in the range of nanometers in atleast one direction. This imp mplies that the path taken by the treeing will be longer. Thus, in terms of dielectric loss, nano-clay is the most suitablee nnano-filler.

Fig 2. Comparison of dielectric loss with respect to frequency for a blank ank sample, nano-clay, nano-calcium carbonate and nano-silica

The effective permittivity of the nanocomposite shou ould also be lesser than that of pure XLPE. We find from Figure 3 that the effective relative permittivity of nano-cal calcium carbonate is higher than that of the blank sample and that of nano-clay and nano-silica is lesser than that of the he blank sample. Thus, in terms of relative permittivity nanoclay and nano-silica are suitable nano-fillers. We also kee keep in mind that with an increase in weight percentage of a nano-filler, the relative permittivity of the nanocompo posite increases. Thus, we find that an optimum weight percentage is 5 wt%. The composite with the highest difference in the elec lectric field values offers the best resistance against electric treeing. The properties of the nano composite become be better with more added fillers. But it must be noted that at a very high weight percentage, the properties of the compos posite deteriorates. This may be accounted for by the fact that for higher wt% the inter-particle distance reduces allow owing for more agglomeration. Thus, the 5 wt% nano-filler concentration is the optimum concentration for nano-fille llers. From this preliminary testing, it may be concluded that the 5 wt% nano-silica filler offers the best resistance again ainst electric treeing.

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Fig 3. Comparison of relative permittivity with respect to frequency for the a blank sample, nano-clay, nano-calcium carbonate and nano-silica Table 3: Difference in electric field stress at the interface of the nano-filler

Nano-filler

Nano-clay

Nano Calcium carbonate (V/m)

(V/m)

Nano-silica (V/m)

2.5 wt%

0.09E+07

0.67E+07

0.43E+07

5 wt%

0.13E+07

0.75E+07

0.47E+07

7.5 wt%

0.16E+07

0.79E+07

0.91E+07

10 wt%

0.17E+07

0.80E+07

1.11E+07

4. Equations In order to calculate the distance between the centres of two nano-filler particles, we use the Tanaka equation9: 1 ⎡ ⎤ 3 ⎢ ⎧⎪ π Sn 100 ⎡ wt % ⎛ Sm ⎞ ⎤ ⎫⎪ ⎥ 1 1 D = ⎢⎨ − − ⎢ ⎜ ⎟⎥ ⎬ d 6 Sm wt % ⎣ 100 ⎝ Sn ⎠ ⎦ ⎭⎪ ⎥ ⎢⎣ ⎩⎪ ⎥⎦

(1)

where, D = Interparticle distance (nm) = Specific gravity of XLPE ( = Specific gravity of nanofiller(

) )

d = diameter of nanofiller (nm) wt% = nanofiller weight percent 5. Conclusion In order to extend the lifetime of XLPE insulation and protect it from damage caused by electric treeing, XLPE nano-composites are used in place of XLPE. The protection against treeing is directly proportional to the difference

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in the electric field values. Therefore, the higher the difference in the electric field values, the better the resistance against electric treeing. The nano-filler with the highest difference is found to be nano-silica. Also, an optimal concentration of nano-fillers is found to be 5 wt%. Therefore, it may be concluded that the 5wt% nano-silica filler offers the best resistance against electrical treeing. References [1]Pavlidou S., Papaspyrides C.D., Prog. Polym. Sci. 33, 1119 (2008) [2]Ray S.S., Okamoto M., Prog. Polym. Sci. 28, 1539 (2003) [3]Hussain F., Hojjati M., Okamoto M., Gorga R.E., Journal of Composite Materials, 40, 1511 (2006) [4]Mauseth, F. ; Amundsen, M. ; Lind, A. ; Faremo, H., Electrical Insulation and Dielectric Phenomena (CEIDP), 2012 Annual Report Conference, Montreal, QC Montreal, 692 (2012) [5]Fabiani, D. ; DIE, Univ. of Bologna, Bologna, Italy ; Cavallini, A. ; Montanari, G.C. ; Saccani, Electrical Insulation and Dielectric Phenomena (CEIDP), 2012 Annual Report Conference, Montreal, QC Montreal, 315 (2012) [6]Danikas M.G., Tanaka T., Electrical Insulation Magazine, IEEE, 25, 19 (2009) [7]Ding H.Z., Varlow B.R., Electrical Insulation and Dielectric Phenomena (CEIDP),2004 Annual Report Conference, 332 (2004) [8]Lau, Yiew K., Piah, M. A. M., Malaysian Polymer Journal, 6, 58 (2011) [9]T. Tanaka, “Dielectric Nanocomposites with Insulating Properties”,IEEE Trans. Dielectr. Electr.Insul., Vol. 12, No. 5, pp. 914-928, 2005. [10]Jackson L.A., Submarine communication cable including optical fibres within an electrically conductive tube. U.S. Patent 4278835, December 14, 1978.