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ELECTRIC FIELD COMPUTATION OF 400KV AC PORCELAIN STRING INSULATOR

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ELECTRIC FIELD COMPUTATION OF 400KV AC PORCELAIN STRING INSULATOR Powered By Docstoc
					 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), ©
                             TECHNOLOGY (IJEET)
 IAEME
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 3, Issue 2, July – September (2012), pp. 174-181
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  ELECTRIC FIELD COMPUTATION OF 400KV AC PORCELAIN
                  STRING INSULATOR
                         Vinayaka V Rao and Pradipkumar Dixit
                    Department of Electrical & Electronics Engineering,
              M S Ramaiah Institute of Technology, M S R Nagar, MSRIT Post,
                           Bangalore-560054, Karnataka, India.
                    +919739290784, E-mail: vintesh.1976@gmail.com


ABSTRACT

Satisfactory operation of outdoor porcelain insulators under polluted environment is
immediately related to the electric field distribution along the insulator string .This paper
presents results of the calculation of the electric field (E field) distribution for porcelain
string or suspension insulators subjected to 400kV, 50Hz, AC, using PSPICE .Field
distributions are calculated for three types of strings under dry, uniform pollution, non-
uniform pollution distribution. The results obtained shows that anti-fog and /or
combination of anti-fog and standard disc insulator strings perform better under polluted
& dry conditions.

Keywords: Arc, Electric Field, Flashover, Outdoor insulator, Porcelain and
String/suspension.

   1. INTRODUCTION

Pollution flashover, observed on insulators used in high voltage transmission, is one of
the most important problems for power transmission. Pollution flashover is a very
complex problem due to several reasons such as modeling difficulties of insulator
complex shape, different pollution density at different regions, non-homogenous
pollution distribution on the surface of insulator and unknown effect of humidity on the
pollution. The performance of insulators under polluted environment is one of the
guiding factors in the insulation coordination of high voltage transmission lines. On the
other hand, the flashover of polluted insulators can cause transmission outage of long
duration over a large area. Flashover of polluted insulators is still a serious threat to the
safe operation of a power transmission system. It is generally considered that pollution
flashover is becoming even more important in the design of high voltage lines.
    Outdoor insulators are being subjected to various operating conditions and
environments. The surface of the insulators is covered by airborne pollutants due to


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6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME

natural or industrial or even mixed pollution. Contamination on the surface of the
insulators enhances the chances of flashover. Under dry conditions the contaminated
surfaces do not conduct, and thus contamination is of little importance in dry periods. As
the surface becomes moist because of rain, fog or dew, the pollution layer becomes
conductive because of the presence of ionic solids. The leakage current flows through the
conducting surface film, generating heat which tends to increase the film temperature
most rapidly at those points where the current density is high, i.e., at narrow sections of
the insulator, such as the area around the pin. Eventually, the temperature in these areas
approaches boiling point, and rapid evaporation of the moisture occurs producing dry
bands.
    The process of dry band formation has been analysed by William et el at [1].They
showed that if the evaporation rate exceeds the wetting rate then the electric field E is
sufficiently high causing dryband formation.
    The basic phenomena of flashover is formation of dry bands due to the flow of
leakage currents, initiations of arc/s across dry bands and propagation of the arc on the
wet pollution layer, finally triggering a complete flashover. Several models [2,3,4,5]
have been developed to predict flashover voltage of polluted insulators and most of them
depend directly or indirectly on arc voltage and or voltage across the pollution layers. It
has been observed form the literature that, one of the major factor governing the electrical
performance of the outdoor insulators is the electric filed distribution along the surface of
the insulator string[6]. The resulting electric filed concentration can lead to corona &
accelerate several failure modes[7].
    In the present paper 400 kV AC porcelain string insulator in PSPICE has been
modelled & analysis of electric filed distribution under dry, uniform pollution and non-
uniform pollution have been discussed for three different cases
    i)      standard disc
    ii)     Anti Fog
    iii)    Combination of standard and anti fog insulators.

   2. MODELING OF PORCELAIN STRING INSULATOR

    In the present work three different cases are considered for the study such as i)
standard disc insulator ii)Anti-fog insulators iii)Combinations of standard & anti-fog
insulators. The dimensional details of the insulators are given in Table 1.

        Table 1: Dimensional details of insulators used
                                         Leakage
          Number of                                                              Height
                            Type of      length /             Shed
Case insulators in                                                        of insulator/
                          insulator/s      disc      diameter (cm)
           the string                                                       disc (cm)
                                           (cm)
                                 Stan           3
                   26                                         25.4                 14.6
                         dard disc          0.3
                                  Anti          4
                   17                                         30.0                 14.6
   I                       -fog disc        7.0
                                 Com            3
                                                              25.4/
                   20    bination of I      0.3                                    14.6
  II                                                      30.0
                              & II        &47.0
     An electrical equivalent circuit shown in Fig.1 for string insulator reported in [8] has
been adopted for the field analysis under dry conditions. The values for the different


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capacitances are obtained from the field for a 400kV AC transmission system .The
description and values for different capacitances used in present study are given below
       Ci-pin to line capacitance = 5pF
       Coi-section capacitance = 50 pF
       Cig-pin to ground capacitance = 4 pF




           Fig1: Equivalent circuit for voltage /filed distribution along insulator string

    The mechanism of long strong flashover consists of different phases instantly, the
contaminated insulator surface is completely dry and so the voltage/field distribution on
the string can be regarded to be the same as that on a dry and clean insulator string. The
equivalent circuit can be represented by network of capacitances only as shown in Fig.1.
As the wetting progresses ,the surface impedance is a combined & resistance, The
equivalent circuit of contaminates insulator string is as shown in Fig.2.




                          Fig. 2 Electrical Equivalent of Polluted String


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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME

    In order to calculate the pollution resistance of each unit ,a dynamic model that takes
into account the instantaneous change in the arc parameters employed[9, 10].
    Equation of the computation of the pollution resistance (Roi) for a given conductivity
(σ) of pollution layer are given below.

                      =     ∙                                                  (1)

                     =                                                         (2)

       Where, dL is the movement of the arc in time dt.
       r is the radius of the insulator at creepage distance Larc
       Larc is the arc length.
       L is the creepage length of the insulator string.
       FF is the form factor.
       σ is the conductivity of the pollution layer.

3. ESTIMATION OF ELECTRIC FIELD
    In the present work a 400kV porcelain string insulator is considered and E field has
been estimated for three different strings
    i)      standard disc
    ii)     Anti Fog
    iii)    Combination of standard and anti fog insulators.
    Each string has been analysed under dry, uniform pollution and non-uniform
pollution. The following section explains the estimation of E field for the three conditions
mentioned above.

3.1 Electric field under dry conditions
    The electric field have been calculated after performing the analysis in PSPICE for
string model shown in Fig.1. Number of insulators used in string is 26, 17 and 20
respectively for strings of standard disc, anti-fog and combination of standard and anti-
fog insulators.
    The electric field has been calculated along the string length from the energized line
and up to the ground end. Figure.3 shows the variation of E field as a function of length
of the string. The maximum of about 7.5kV/cm near the pin of the first insulator or near
high voltage line end. This field is less than the field required to initiate the arc under dry
conditions.




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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME


                                               8
                                                                        S ta n d a rd d is c s
                                                                        A n tifo g in s u la to rs
                                                                        C o m b in e d s ta n d a rd d is c a n d A n tifo g
                                               6
                                                                    D ry c o n d itio n




                     Electric Field in kV/cm
                                               4




                                               2




                                               0
                                                   0   100   200       300      400       500      600        700     800      900
                                                                   L e n g th a lo n g th e s trin g in c m


                                                       Fig.3 Electric Field under Dry condition

3.2 Electric field under uniform pollution condition

    The surface of porcelain insulator is assumed with a uniform pollution of 10µS and
corresponding pollution resistance of all the 26 insulators are calculated using (1) and
simulation is carried out in PSPICE with string model shown in Fig.2.The same
procedure is repeated for anti-fog & combined insulator stings.

    Figure 4 shows the electric field along the length of the strings for 231 kV (line to
ground) system voltages. The electric field required to initiate the arc/scintillation along
the polluted porcelain insulator has been reported in the range of 2.1 to 2.8kV/cm[11] and
1.45 to 2 kV /cm[12].
    From Fig.4, it can be observed that field across the insulator disc 1 and 2 are higher
than the minimum field required for the arc initiation in case of string with standard discs.
In the case of string with anti-fog discs insulators and combined anti-fog & standard disc
insulator, only across the first insulator from the line end exceeds the minimum field for
arc initiation.




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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME


                                                  8
                                                                           S ta nd a rd d iscs
                                                                           A n tifog in sula to rs
                                                                           C o m bin ed stan da rd d isc a nd A ntifog
                                                  6
                                                                      U niform P ollution (10uS )




                   Electric Field in kV/cm
                                                  4




                                                  2




                                                  0
                                                      0   100   200      300       400       500       600       700      800      900
                                                                      Le ng th a lo ng the strin g in cm


                            Fig.4. Electric Field under uniform pollution
3.3 Electric field under non uniform pollution condition
    In practice the accumulation of the pollution on the surface of the insulator is not
uniform because of the reason i)non uniform distribution of the voltage across the string
and ii) partially washing of some pollutants on the surface of the insulator surfaces. To
mimic this present work also considers non uniform pollution of string from high
level(5µS) to heavy level (30 µS). Heavy pollution has been consider on the line end
insulators and decreased pollution near the ground end. Again using (1) ,the pollution
resistance were calculated for all the three cases mentioned earlier and simulation has
been performed to estimate the electric field.
    Figure 5, shows the plot of electric field as a function of length along the string for
standard disc string, string of anti-fog and string of combination of anti-fog & standard
disc insulators. It can be observed from the Fig.5 that across the first insulator from the
line end ,the field is about 3kV/cm in case of standard disc string & it is about 2.3kV/cm
in case of anti-fog insulators.
                                                  8
                                                                           S ta n d a r d d is c s
                                                                           A n tifo g in s u la to rs
                                                                           C o m b in e d s ta n d a r d d is c a n d A n tifo g
                                                  6
                                                                       N o n -u n ifo rm P o llu tio n
                        Electric Field in kV/cm




                                                  4




                                                  2




                                                  0
                                                      0   100   200       300      400       500      600        700      800      900
                                                                      L e n g th a lo n g th e s trin g in c m


                                                      Fig.5 Electric Field under non-uniform pollution


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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME

    As mentioned earlier, the field across the first unit of the insulator string exceeds the
threshold field of the arc initiations leading to flashover of the first disc which in turn
redistributes the voltage causing high field across the second and/or third insulator. This
process continues until the arc reaches the critical length and finally bridging the whole
insulator leading to a complete flashover.
    From the above discussion, it is observed that, the strings of anti-fog insulator and
combined insulator perform better that gives lesser field compared with strings of
standard disc insulators. To get more insight, string efficiency has been calculated for all
the three cases under dry conditions and tabulated in Table 2.

       Table2: Calculated string efficiency under dry condition
                Insulator type in the string  Percentage efficiency
                     Standard discs                  27.8
                     Anti-fog discs                  42.2
                Combination of standard &
                                                     46.0
                      anti-fog discs

Table 2 shows that combination of anti-fog and standard disc insulator string yields better
string efficiency.

4. CONCLUSIONS
    Paper describes the estimation of electric field of porcelain insulator string used in
400Kv transmission systems. For this purpose three insulator string has been modelled as
RC network and used in PSPICE to find the node potential/potential difference across the
each insulator disc. The values of the node potential are further used to calculate the
electric field along the length of the string.
    Anti-fog insulator and combination of anti-fog and standard disc insulator string not
only performs better in polluted environment, it also yields better string efficiency under
dry conditions. Using anti-fog disc insulators and /or combinations of anti-fog and
standard disc insulators, results in increased creepage length and reduced leakage current.
    Electric field analysis plays a critical role in the design, selection and application of
ceramic insulators to ensure better performance under polluted environment.

REFERENCES

[1] D. L. Williams, A. R. Rowlands, A. Haddad and                         R. T. Waters,
    “Characterisation of dry bands on Insulators under clean fog conditions”, in Proc. Of
    10th International Symposium on High Voltage Engineering, vol.3, 1997, pp.45-48.
[2] P. Claveri. & Y Procheron., “How to choose insulators for polluted areas”, IEEE
    Trans. PAS, vol.PAS-98, No.3, 1973, pp.1121-1131.
[3] L.L Alston. & A Zoledziowski, “Growth of discharges on polluted insulation”, Proc.
    IEE, vol.110, No.7, July 1963, pp. 1260-1266.
[4] F.A.M. Rizk & D.H Nguyen., “AC source-insulator interaction in HV pollution tests”,
    IEEE, PAS, PAS-103, No.4, April 1984, pp. 723-730.
[5] IEC 507, “Artificial pollution tests on high voltage insulators to be used on ac
    systems”, No.1, 1991.
[6] T. Doshi, R.S.Gorur, “Electric Field Computation of Composite Line Insulators up to
    1200kV AC”, IEEE Trans. DEI, vol.18, No.3, June 2011, pp.861-867.



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[7] J.S.T. Looms, Insulators for High Voltages, Peter Peregrinus Ltd., London United
    Kingdom, 1988.
[8] Pradipkumar Dixit, H.G.Gopal, “Getting Leakage Current Wave-shapes along the
    equivalent circuit of Polluted Porcelain Insulator using PSPICE software”, Proc. of
    International Conference on Energy, Information, Technology and Power sector
    PEITSICON-2005 (organized by IEE(UK)), at Kolkata. Paper No. 89, January 28-29,
    2005. pp. 73-75.
[9] R. Sundararajan & R.S.Gorur, “Dynamic Arc Modeling of Pollution Flashover of
    Insulators under DC Voltage”, IEEE Trans. EI, vol.28, No.2, April 1993, pp.209-218.
[10] IEC 507, “Artificial pollution tests on high voltage insulators to be used on ac
    systems”, No.1, 1991.
[11] H G Gopal, “An Electrothermal Model for pollution induced flashover of
    Insulators”, Ph D dissertation, Department of High Voltage Engineering, Indian
    Institute of Science, Bangalore, India, May 1998.
[12] Pradipkumar Dixit & Dr. H.G.Gopal, “Experimental Determination of Inception
    & Breakdown Gradients of Contaminated Ceramic Insulators”, 12th International
    Symposium on High Voltage Engineering, (ISH-2001), IISc., Bangalore, Paper No.5-
    45,Vol-3, August 2001, pp.763-765.


Acknowledgment
  Authors are grateful to the Principal & Management of M.S. Ramaiah Institute of
Technology, Bangalore for their constant support and encouragement, in carrying out this
work.

BIOGRAPHICAL SKETCH OF AUTHORS

Vinayaka V Rao received the BE degree in Electrical and Electronics Engineering from
Mangalore University and M.Tech degree in Illumination Technology, from VTU,
Belgaum, India in 1998 and 2000. He currently working as Assistant Professor in the
department of Electrical and Electronics Engineering, M S Ramaiah Institute of
Technology, Bangalore. His research interests include outdoor insulation and Energy
Management.

Pradipkumar Dixit was born in Bijapur, Karnataka, India in 1964. He received the BE
degree in Electrical and Electronics Engineering from Mysore University in 1989 and
M.Tech. degree in Power and Energy Systems from NITK, Surathkal, Mangalore
University, India in 1995 and the Ph.D. degree from Visvesvaraya Technological
University, Belgaum, India in 2010 for his thesis titled “Studies on Pollution Performance
of Ceramic Insulators”. Currently, he is working as Associate Professor in the department
of Electrical and Electronics Engineering, M S Ramaiah Institute of Technology,
Bangalore. His research interests include outdoor insulation, insulation engineering and
artificial intelligence to electrical engineering.




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