, pp.1-7 http://dx.doi.org/10.14257/astl.2017.147.01 Control of Electric Field Stress in Gas Insulating Busduct using Nano-Nitride Fillers Hannah Monica Anoop 1, G. V. Nagesh Kumar 1, K Appala Naidu 1, D. Deepak Chowdary 2 and B Venkateswara Rao 3 1 Vignan s Institute of Information Technology, Visakhapatnam, India 2 Dr. L. Bullaya College of Engineering for Women, Visakhapatnam, India 3 V R Siddhartha Engineering College (Autonomous), Vijayawada, India Abstract. Gas Insulated Substation (GIS) gains an ever increasing importance due to the ever growing demand for electricity and energy density in metropolitan cities. The insulation integrity of the spacer ensures the reliability of GIS. So, it is of at most importance that the electric field distribution along the spacer s surface is simulated and an optimization of the same is done in order to prevent the flashovers especially at the triple junction (TJ) formed by SF6 gas, the spacer and the electrode. The distribution of electric field depends considerably on the geometric shape of the spacer. Epoxy resin exhibits excellent electrical, thermal and mechanical properties. This can be further enhanced to a great extent by reinforcing epoxy with nano filler mixture as a dielectric coating. The condition assessment can be done using Finite Element Method, one of the proven methods for calculating the electric field density at various points under consideration and the condition enhancement is done by changing the filler concentration or by changing the thickness of the dielectric coating. In this paper, the distribution of along the spacer s surface is plotted, the relative permittivity, breakdown voltage, thermal conductivity and maximum electric field are calculated for a conical spacer are determined and analyzed for different filler concentrations and different thickness of the dielectric coating. Keywords: Electric Field; Gas Insulated Systems; Nano Nitrides; Polymeric Insulators 1 Introduction Gas Insulated Systems (GIS) which are highly reliable, compact and pollution-free have a potential to lead to a breakthrough in the Indian power scenario whose major challenges are rapid urbanization, increasing energy density and scarcity of land. The reliability of Gas Insulated Substations is of at most importance as any small failure can elevate to a major problem in the grid it is connected to resulting in blackouts in the power system. A survey of the failures in Indian GIS has shown that almost 30% of the failures are due to selection of wrong materials, improper material substitutions and material failures [1]. The condition assessment and enhancement of same is a thrust area of the research and has drawn the attention of many researchers. ISSN: 2287-1233 ASTL Copyright 2017 SERSC
As the compactness of GIS is increased, the electric field stress developed in the Gas Insulated Busduct (GIB) comprising of the solid insulator called spacer, gaseous insulator and the conductor, increases. A HV system experiences extreme conditions which include high electric fields, high temperatures, mechanical stress and intense radiations. From [2], the electric field stress developed at the ends of the solid insulator increases at certain conditions leading to partial discharges weakening the dielectric strength and early degradation. Under severe conditions, a complete flashover occurs. It should have the capability to withstand not only regular voltages but also over voltages caused by lightning, switching etc. Upgradation to ultra-high voltage lines and extra-high voltage lines necessitates insulating materials which can withstand high voltages, polarity reversal and space charge accumulation. Generally, pure SF 6 or mixture of SF 6 and N 2 is used at high pressure are used as the gaseous insulator. The solid insulators called spacers which contain the conductors are also used to support and separate various sections in GIB. Spacers produce a complex dielectric field and intensify the electric field on the spacer s surface. The dielectric strength of SF 6 is sensitive to maximum electric field. The dielectric strength along the surface of the spacer is generally lower than that of gaseous medium [3]. So the spacers should be designed such that, more or less a uniform electric field distribution occurs along the spacer s surface which will be more reliable and flashover free. High operating temperatures and accumulation of heat causes heating up of the equipment. It results in looseness between the devices and consequent reduction of its lifespan. So the enhancement of the thermal properties of the insulating materials is very important. Other factors like particle dispersion, electric erosion, electrical treeing and interface properties greatly affect the breakdown voltage. The future of the power systems lies in the progress of the insulating materials with superior thermal, mechanical and electrical properties. The development of polymers which conduct heat through vibration of atoms, groups and chains, led to the synthetic materials like varnish, resins, impregnated insulating fiber and composites [4] which had better insulating properties to be used in even extreme conditions. In [5], the performance of the spacers in various shapes like cone, smooth disc and corrugated disc has been reviewed. The intensification of the local electric field which is a major problem has been considered in [6] in a conetype spacer fitted between the flanges in GIB. Various techniques have been implemented to obtain improved insulating properties and uniform electric field but with the limitation of a complex geometry of the spacer rendering it almost impossible to manufacture. The breakdown of dielectric occurs at submicron or nanoscale weak points like interface between dielectric and electrode or other interface regions within the dielectric. In 1994, Lewis introduced the concept of nano-dielectrics [7]. Investigations have proved that epoxy with nano-composites exhibit superior electrical and mechanical properties when compared to pure epoxy resin and epoxy resin with micro-fillers at low concentrations [8]. It was proved that the permittivity depends greatly on the type and size of filler [9], combination of matrix and filler and the smoothness of the samples [10]. In [11], it was shown that the epoxy nanocomposites accumulate lesser charge compared to that of the clean epoxy resins. From [12], it is shown that the charge dynamics are faster in epoxy nano-composites 2 Copyright 2017 SERSC
and it is observed most evidently in case of negative charges. They exhibit a high resistance towards partial discharges and electrical treeing and low dissipation factor. Research has shown orderly arrangement of spherulite structures which prevents the development of electric erosion helping the polymers to resist corona and partial discharge [13-14]. The thermal conductivity of the dielectrics can be improved by adding nanoparticles. By changing the amount, type and surface modification method the thermal properties can be enhanced. Various fillers such as Al 2O 3, BN, AIN and BNNT have been modified and added to different matrices, which include polyamide, epoxy and silicone rubber. The zone of interaction between the polymer matrix and nanoparticles is considered as an independent area. When nanoparticles are in isolated dispersion, the carriers are restrained in the interaction area. This results in the reduction of the density of charge carriers as well as the mobility of the charge carriers. The thickness of the interaction zone increases with an increase in the filler concentration which greatly increases the mobility and density of the carriers. The interaction strength between the polymer matrix and the nanoparticles greatly affects the thickness of the interaction zone. The incorporation of nanofillers into the polymer matrix results in a structural change of polymer caused by the polymer-nanofiller interaction. Using inorganics nanofillers like aluminum-nitride and boron-nitride in polymeric matrices reduces the cost, improves the fire resistance, mechanical characteristics like tensile strength and permittivity. In this paper, the distribution of electric field along the spacer s surface coated with dielectric coatings of nanonitrides with different concentrations is calculated using Finite Element Method (FEM). The overall insulation integrity of GIB is determined. 2 Calculation of Relative Permitivity The electric field in a given volume will be weakened when a material whose dielectric constant is high is placed in it. Polyethylene can be placed between the inner conductor and the outer enclosure in a coaxial cable. Epoxy/epoxy based nanocomposites are preferred insulating materials for electrical applications for bushings, GIS spacers etc. In epoxy nano composite, nanocomposites play a vital part in the enhancement of the properties of epoxy because the permittivities of fillers are high. Due to the higher individual permittivities of the fillers and on combining with epoxy resin overall permittivity of the composite increases when compared to net epoxy and epoxy micro composite. The filler loading can considered up to certain extent based on the advantage of the interaction zone. It filler concentration is increased to a high value which leads to over lapping of the interaction zone between polymer matrix and filler due to which conductivity increases. The overlapping of the nanoparticles in epoxy nanocomposite depends upon the rate of dispersion of nanoparticles in the epoxy resin. The permittivity of a two phase dielectric satisfies the Lichtenecker-Rother mixing rule which can be extended and written as shown in equation (1) Copyright 2017 SERSC 3
c Log xlog ylog zlog (1) c 1 2 3 where is resultant composite permittivity, 1,, and epoxy and x and y, are the concentrations of filler and polymer. 2 3 are the permittivities of the filler 3 Breakdown Voltage For transmission and distribution of electric power three-phase common enclosure GIB is used GIS. Inner surface of the bus duct is dielectric coated with epoxy nanocomposites. To determine the breakdown voltage in terms of coating thickness and permittivity can be written as -1 v = v 1+ tε r b d where t is the thickness in µm, V is the voltage applied, d is the gap and ɛ r is the relative permittivity. The use of without surface treatment of nanofillers in epoxy nanocomposite there is no changes in breakdown voltage. The breakdown voltage is calculated at γ is 3 10-3. (2) 4 Thermal Conductivites The nanofillers have the individual thermal conductivity values are high. The epoxy resin thermal conductivity is 0.168w/m.k. The thermal properties of epoxy resin, nanocomposites are added to the matrix. The thermal conductivity is predicted from the Agari and Uno model; Logk =.c.logk + 1-.Log c.k c 1 f 1 m where, c 1 and c 2 are the adjustable constants, k f is the thermal conductivity of the filler, k m is the thermal conductivity of polymer matrix, k c is the resultant thermal conductivity, Φ is the volume fraction of the filler additives. w f w 1 w m (3) (4) where w is weight fraction, ρ f is the density of the filler, ρ m is the density of polymer matrix. The weight percentage of nanofillers increase then thermal conductivity of epoxy increases well. 4 Copyright 2017 SERSC
5 Results and Discussions The filler concentrations of aluminium nitride are varied and variation of various parameters like resultant permittivity, break down voltage and maximum electric field are calculated and presented in Table 1. As the filler concentrations of aluminium nitride are increased there is a gradual increase from 3.61 (for filler concentration of 0.2) to 4.52 (for filler concentration of 10). The break down voltages are calculated for various filler concentrations of aluminium nitride. It is observed that with the decrease in the thickness, there is an increase in the breakdown voltage. However, there is a minor change in the maximum Electric field from 1.14 to 1.16. As the filler concentrations of Boron nitride are increased there is a gradual increase from 3.60 (for filler concentration of 0.2) to 4.14 (for filler concentration of 10). The Break down voltages is calculated for various filler concentrations of Boron nitride. It is observed that with the decrease in the thickness, there is an increase in the breakdown voltage. However, there is a minor change in the maximum Electric field from 1.14 to 1.1548. Table 1. Variation of Various Parameters with Aluminium Nitride Filler Concentration Filler Concentratio n Resultant Permitivity Breakdown Voltage at 40µm Breakdown Voltage at 130 µm Maximum Electric Field 0.2 3.61 1059.60 1057.25 1.14 0.4 3.63 1059.60 1057.25 1.1415 0.6 3.64 1059.59 1057.22 1.142 0.8 3.66 1059.59 1057.20 1.1423 2 3.76 1059.56 1057.10 1.143 4 3.94 1059.51 1056.93 1.145 6 4.13 1059.45 1056.76 1.152 8 4.32 1059.40 1056.58 1.156 10 4.52 1059.34 1056.39 1.164 The variation of relative permittivity with filler concentration of aluminium nitride and boron nitride are plotted in Fig 1. It is observed that there is a linear increase in permittivity with increase in filler concentrations. Permittivity with aluminium nitride filler concentration is more than that of boron nitride filler concentration. Copyright 2017 SERSC 5
Fig. 1. Plots between permittivity and filler concentration 6 Conclusion The flashovers in critical areas which lead to complete breakdown of the insulators can be prevented during the design by having a precise knowledge of the distribution of the electric filed. The electrostatic field developed is greatly influenced by the geometric shape of the electrode. The electric field distribution along the electrode surface and the dielectric surface has to be carefully considered during the design and optimization of the high voltage equipment. The model has been developed for a single phase enclosure with an objective to obtain a quasi-stationary electric field distribution. Nano composites enhanced the electrical and thermal strengths of insulating materials in a gas insulated bus duct. Nitrides like aluminum nitride and boron nitride enhanced the break down voltage and electric field. References 1. V. Aaradhi and K. Gaidhani,: Special problems in gas insulated substations (GIS) and their effects on indian power system, 2012 IEEE International Conference on Power System Technology (POWERCON), Auckland, pp. 1-5, (2012). 2. Marungsri,W. Onchantuek, A. Oonsivilai and T. Kulworawanichpong,: Analysis of Electric Field and Potential Distributions along Surface of Silicone Rubber Insulators under Various Contamination Conditions Using Finite Element Method, pp. 156-166,International Journal of Electrical and Computer Engineering,(2010). 3. T.Hasegawa. K. Yamaji, M. Hatano, H. Aoyagi, Y. Taniguchi and A.Kobayashi,: DC Dielectric Characteristics And Conception Of Insulation Design for DC GIS,vol. 2. no.4, pp. 1776-1782, IEEE Transactions On Power Delivery, (1996). 4. Lei, Q.: Recent progress of engineering dielectrics, Science Press, Beijing, 1st edn. (1999). 5. J.C.Cronin, E.R.Perry,: Optimization of Insulators for Gas Insulated Systems,vol.92, no.2,pp.558-564, IEEE Transactions on Power Apparatus and Systems, (1973). 6. H. Tsuboi,T. Misaki,:Optimization of Electrode and Insulator Contours by Using Newton Method, vol. 106A, pp. 307_314, IEEE Trans. (1986). 6 Copyright 2017 SERSC
7. Lewis, T.J.,:Nanometric Dielectrics,vol.1, pp.812 825, IEEE Trans. Dielectr. Electr. Insul. (1994). 8. Singha, S.; Thomas, M.J,: Dielectric properties of epoxy nanocomposites,vol.15, pp.12 23, IEEE Trans. Dielectr. Electr. Insul.(2008). 9. Kadhim, M.J.; Abdullah, A.K.; Al-Ajaj, I.A.; Khalil, A.S,: Dielectric properties of epoxy/al2o3 nanocomposites,vol.3, pp.468 477, Int. J. Appl. Innov. Eng. Manag. (2014). 10. Lau, K.Y., Vaughan, A.S., Chen, G.: Nanodielectrics: pportunities and challenges, vol.31, issue.4, pp. 45 54, IEEE Electr. Insul. Mag., (2015). 11. Castellon, J.; Nguyen, H.N.; Agnel, S.; Toureille, A.Frechette, M.; Savoie, S.; Krivda, A.; Schmidt, L.E,: Electrical properties analysis of micro and nano composite epoxy resin materials,vol.18, pp.651 658, IEEE Trans. Dielectr. Electr. Insul. (2011). 12. Fabiani, D.; Montanari, G.C.; Dardano, A.; Guastavino, G.; Testa, L.; Sangermano, M.,: Space charge dynamics in nanostructured epoxy resin. In Proceedings of the Conference on Electrical Insulation and Dielectric Phenomena, CEIDP, Quebec, QC, Canada, 26 29 October 2008; pp. 710 713, (2008). 13. Maity, P., Basu, S., Parameswaran, V..: Degradation of polymer dielectrics with nanometric metal-oxide fillers due to surface discharges, vol.15, no.1, pp. 52 62, IEEE Trans.Dielectr. Electr. Insul., (2008). 14. Kozako, M., Okazaki, Y., Hikita, M,: Preparation and evaluation of epoxy composite insulating materials toward high thermal conductivity, 2010 10th IEEE Int. Conf. on Solid Dielectrics, pp. 1 4, (2010). Copyright 2017 SERSC 7