If you can't read please download the document
Upload
lyduong
View
228
Download
2
Embed Size (px)
Citation preview
Control of Electric Field Stress in Gas Insulating
Busduct using Nano-Nitride Fillers
Hannah Monica Anoop1, G. V. Nagesh Kumar1, K Appala Naidu1,
D. Deepak Chowdary2 and B Venkateswara Rao3
1Vignans Institute of Information Technology, Visakhapatnam, India 2Dr. L. Bullaya College of Engineering for Women, Visakhapatnam, India
3V 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 spacers 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 spacers 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.
Advanced Science and Technology Letters Vol.147 (SMART DSC-2017), pp.1-7
http://dx.doi.org/10.14257/astl.2017.147.01
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 SF6 or mixture of SF6 and N2 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 spacers
surface. The dielectric strength of SF6 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 spacers 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 cone-
type 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
Advanced Science and Technology Letters Vol.147 (SMART DSC-2017)
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 Al2O3, 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 spacers 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)
Advanced Science and Technology Letters Vol.147 (SMART DSC-2017)
Copyright 2017 SERSC 3
1 2 3cLog xLog yLog zLog (1)
where c is resultant composite permittivity, 1 , 2 , 3 are the permittivities of the filler
and epoxy and x and y, are the concentrations of filler and polymer.
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
-1trv = v 1+
b d
(2)
where t is the thickness in m, V is the voltage applied, d is the gap and 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 is 310-3.
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;
(3)
where, c1 and c2 are the adjustable constants, kf is the thermal conductivity of the
filler, km is the thermal conductivity of polymer matrix, kc is the resultant thermal
conductivity, is the volume fraction of the filler additives.
(4)
where w is weight fraction, f is the density of the filler,
m is the density of polymer matrix. The weight percentage of nanofillers increase then thermal
conductivity of epoxy increases well.
c 1 f 1 mLogk = .c .Logk + 1- .Log c .k
1
w
fw w
m
Advanced Science and Technology Letters Vol.147 (SMART DSC-2017)
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
40m
Breakdown
Voltage at
130 m
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.
Advanced Science and Technology Letters Vol.147 (SMART DSC-2017)
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).
Advanced Science and Technology Letters Vol.147 (SMART DSC-2017)
6 Copyright 2017 SERSC
7. Lewis, T.J.,:Nanometric Dielectrics,vol.1, pp.812825, IEEE Trans. Dielectr. Electr. Insul. (1994).
8. Singha, S.; Thomas, M.J,: Dielectric properties of epoxy nanocomposites,vol.15, pp.1223, 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.468477, 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. 4554, 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.651658, 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, 2629
October 2008; pp. 710713, (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. 5262, 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. 14, (2010).
Advanced Science and Technology Letters Vol.147 (SMART DSC-2017)
Copyright 2017 SERSC 7