CONTROL OF ELECTRIC FIELD AND VOLTAGE DISTRIBUTION OF A 765KVSYSTEM POLYMERIC INSULATOR USED IN INDIAN TRANSMISSION SYSTEMS.
M.Kanyakumari *, R.S.Shivakumara Aradhya, Central Power Research Institute, Bangalore, IndiaHemanth Jangawala, G.K. Xianghe Electricals ,Pvt. Ltd., India
Abstract: C omposite insulators are being subjected tohigher electrical stresses with the increased transmissionvoltage levels being adopted universally. In addition tohigh electrical stresses at the energized end, the electricfield distribution along the length of the insulator beingnon uniform may lead into problems associated withcorona causing deterioration of material of no ceramicinsulators. Therefore, the control of electric field strengthin the vicinity of such insulators is an important aspect to
be considered in the design. In this paper, the authorshave attempted to determine the adequacy of the grading
ring design proposed by the manufacturer M./S. G.K.Xianghe Electricals, Pvt. Ltd., India for use in 765kVsystem by computing the electric field distribution alongthe insulator length. In the next step, an attempt wasmade to suggest a suitable Corona / Grading ring designfor minimizing the electric field at the energized highvoltage end as well as at ground end in addition toreducing the non linearity in the electric field distributionalong its length using Finite Element Software. Theresults of the optimization study are presented here.
Key Words: Composite insulator, grading ring design,Electric field at triple junction, FEM simulation
INTRODUCTION: [1-3]:
The electric field distribution on transmission class polymer / non ceramic insulators (NCI), affects both thelong and short term performance. In order to design andapply composite insulators effectively, a fundamentalunderstanding of the Electric field distribution and itseffect on the insulator performance is needed. In general,the Electric field magnitudes are more close to theenergized and grounded ends of a composite insulator.Typically, the energized end is subjected to the highestfield magnitudes. In some cases the position of highestElectric field occurs adjacent to the end fittings, while inother cases it may occur within a short distance of theend fitting.
There are three main regions of interest whenconsidering the Electric field distribution of compositeinsulators namely,1) Polymer/air interface, polymer/ air / metal interface
called triple junction point2) Within the fiberglass rod and polymer rubber shed
material, as well as at the interfaces between thesematerials, and
3) on the metallic end fitting surface and in the airsurrounding it and attached corona rings.
The electric field magnitude in any of the these threeregions exceeding the critical values excessively results
into corona discharge activity and may affect the long orshort term performance of the insulator.The provision of corona rings to insulators reduces thecorona effect, reducing the maximum electric fieldgradient along insulator surface, and prevents excessivelevels of radio interference thus prolonging the life of theinsulator. To our knowledge, no standards governing thedesign parameters and placements of the corona rings onthe non ceramic insulators exist at present. Positioningthe corona ring at incorrect locations on the insulatormay lead to increase in maximum electric field instead of
reducing it. In these cases the grading ring simplyredistributes the voltage stress, resulting in coronadischarge elsewhere on the insulators. When the gradingring is designed properly, corona discharge occurs onlyunder contaminated and moist conditions. Therefore, it isvery critical to place the gradient ring on the insulator. Itis commonly accepted that the corona ring be placed nearthe energized terminal of the insulator. By having such acorona ring, it allows the electric field to be distributedmore uniformly along the insulator surface.
At present, there are no standards stipulating the limitson the electric field at the three critical points mentioned
earlier. As such different utilities follow different norms.As per the available literature, the electric field limit (atthe three critical points) for the design of grading rings toavoid the corona discharges on dry and clean polymerinsulators surface are 1), 0.5 – 0.7kV/mm, 2) 3kV/mm, 3)2.1kV/mm (rms) [1] .
This paper presents the results of 2D Finite Elementcalculations of the electrical field distribution along a765 kV system composite insulator for grading ringconfiguration as proposed by the manufacturer. Differentdesign parameters of the corona/ grading ring werevaried and the effect of these parameters on the
maximum values of the electrical field around the threecritical points was evaluated in detail. Finally, optimumvalues for these parameters were suggested whichlimited the electric field at the critical points to aroundthe acceptable value. The design parameters included thegrading ring diameter, grading ring pipe diameter and its
position. The results of this study are presented below.
Details of the insulator under study:The single suspension silicon rubber composite insulator
proposed by the manufacturer for use in 765kV system ishaving one grading ring at the line end and one at theground end. The grading ring is having a diameter of
380mm with a pipe diameter of 32mm at both the endfittings. The length of the insulator with about 170 no ofsheds is 6580 mm. The line end grading ring is
positioned at a height of 150mm from the end point of
2012 IEEE 10th International Conference on the Properties and Applications of Dielectric Materials July 24-28, 2012, Bangalore, India
the line end fitting. Similarly the ground end grading ringis positioned at a height of 135mm from the end point ofthe ground end fitting as shown in Fig. 1. The non-ceramic insulator with the proposed design has four maincomponents namely the fiberglass reinforced (FRP) rod,the polymeric sheath on the rod, the polymeric weather
sheds, and two metallic end fittings. With these givendimensions, the insulator was modeled using a 2-DElecnet software based on Finite Element Method(FEM ).
A L L I N F O R M A T I O N S C O N T A I N E D I N T H I S D O C U M E N T I S C O N F I D E N T I A L A N D S H O U L D N O T B E U S E D W I T H O U T P R I O R C O N S E N T O F M / S G
. K . X I A N
G H E E L E C T R I C A L S P V T . L T D
.
SASANPOWERLIMITED
Fig. 1: The 765kV system insulator under study.
Finite element Modeling details:In this study, electric field and potential distribution alonginsulator length are simulated by a two dimensional finiteelement model. In reality, this problem is not an axi-symmetric one due to the existence of power transmissionline, hardware and transmission tower effects. In order tosimplify the model and to use axi-symmetric property in thecylindrical coordinate system, transmission line, hardwareand ground effects are not taken into account in thesimulations. All simulations are performed with Elecnetsoftware based on 2-D finite element simulation program. Itis assumed that the relative permittivity of the silicon
rubber and fiberglass is 4.3, and 5.5 respectively. Theinsulator was modeled in free space with a potentialdifference of 462 kV Phase-ground RMS applied
between the end fittings. The steady state voltage of462kV was assigned as the specified boundary conditionon the bottom metal end fitting of the insulator and thegrading ring at that end. The metal end fitting at the topand the top grading ring are assigned a Zero voltage
boundary condition. Fig. 2 shows the Finite ElementModel (FEM) of the simulated insulator showing theGrid.
Fig 2. FEM Grid used for the study.
Results of the study:The critical points of high field strength are labeled asfollows for the purpose of reporting the results.Point H1- HV metal end fitting, H2- Triple junction atHV end, H3- around 1 st shed at HV end, Point G1-
Ground metal end fitting, G2- Triple junction at groundend, G3- around 1 st shed at ground end.
Electric field at critical points without grading ring:
The FEM simulation was carried out first withoutgrading ring, and then with the given grading ringdimension and position as per manufacturers design.The results are shown in Table 1. The values show thatthe electric field values at all the critical points withoutring are very high showing the effectiveness of the
grading rings even with the smallest pipe diameter of32mm. However the field values at all the critical pointsare above the limit values indicating the insufficient ringdimensions and position.
Table 1. Electric field at critical points with originalgrading ring and without ring
Electric field in kV/mm
H1 H2 H3 G1 G2 G3 R1 R2
Tubedia32mm
7.52 7.08 1.57 4.2 2.5 1.5 3.85 3.82
No
ring12.3 8.6 4.8 6.4 2.7 3.3 - -
Since the proposed grading ring configuration isinadequate as demonstrated in this step, The FEM caseswere executed for different cases by varying the design
parameters of grading ring and its effect on the voltagestress at the critical points was computed. From theseresults the optimized dimensions of grading ring wereevaluated. The results of the optimization study are
presented below.
Effect of grading ring tube diameter on electric fieldat critical points:
In order to evaluate the effect of grading ring tubediameter, the maximum electric field occurring at thecritical points mentioned above are computed for varioustube diameters of the grading rings (i.e. 32mm, 40mm,50mm, 60mm, 70mm. 80mm. 90mm and 100mm) bykeeping the grading ring diameter as constant at the predesigned value of 380mm . The results are presented inFig 3. The computed values clearly indicated that themaximum electric field at the metal end fittings isexceeding the air breakdown strength of 2.1kV/mmexcept the case with a pipe diameter of 100mm. Fromthis, it is evident that the end fittings will always be incorona under dry weather conditions itself. The electric
field at triple junction also exceeded the limit vale of0.7kV/mm both at the line end and at ground end. So atthe end of this step, it is decided that a minimum of100mm pipe diameter is required for proper voltagedistribution.
Fig 3. Variation of Field at critical points with HV ringtube diameter
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Effect of HV end grading ring position on the electricfield at critical points:The variation of electric field at critical points by varyingthe HV ring diameter is shown in Fig 4. The HV ringdiameter of 380mm, ground end ring position of6482mm is kept constant. The electric field values were
computed for different positions of the grading ring atHV end by keeping the position of the grading ring atground end and ring diameter unchanged. Thiscomputation was carried out for different pipe diametersconsidered in the above step. The computed valuesindicated that there was an overall reduction in theelectric field at the triple junction points and around thefirst shed at both the line and ground end by lowering the
position of the line end ring to 85mm from 150mm.Although there was an overall reduction in the electricfield, there seems to be an optimum position beyondwhich, the lowering of the ring position yielded a slightincrease in the field values. This optimum position of the
ring in most of the cases was observed to be the ring position corresponding to 126mm from the top of the lineend of the insulator. However the reduced values ofelectric field values at all the critical points including thegrading ring surface exceeded the air breakdown strengthof 2.1kV/mm and the values around the first shed at
both line and ground end exceed the limit value of0.7kV/mm. For eg. the electric field variation with ring
position is shown in Fig. 4 for tube diameter of 100mm ,ring diameter of 380mm. Hence 126mm is decided as theoptimum position of the HV end ring at this stage.
Fig 4. Variation of Field at critical points with HV ring position
Effect of ground end grading ring position on theelectric field at critical points:The electric field values were computed at variouscritical points by changing the position of the ground endgrading ring for a fixed position of the line end gradingring as determined from the above results (126mm) and
by taking the ring diameter as 380mm. The computationswere performed for various pipe diameters of 70mm,80mm, 90 mm and 100mm. It was observed from thecomputed values that the electric field distribution atmost of the critical points is reduced by moving groundend ring upwards. The electric field values at most of the
critical points were minimum for the ground end ring position of 6480mm and thereafter there was a slightincrease in the field values at few of the critical points.This indicated that there exists an optimum position for
the ground end ring. However the electric field values at both the metal end fittings and on the surface of both thegrading rings is exceeding the air breakdown strength of2.1kV/mm. The electric field values at the triple junctionat line and ground end exceeded 1kV/mm and around the1st and 2 nd sheds the values are greater than the limit
value of 0.7kV/mm in most of the cases. The variation offield at critical points with the ground end grading ring
position is shown in Fig 5 for grading ring tube diameterof 100mm, HV ring position of 126mm from HV top end
point and HV ring diameter of 380mm. The abovecomputations lead to the decision of considering theground end ring position of 6480mm as the suitablechoice.
Fig. 5 Variation of Field at critical points with Groundring position
Effect of HV end grading ring diameter on theelectric field at critical points:
After analyzing the above results, it was decided to vary
the ring diameter from 360mm to 380mm and 400mmand see the variation in the electric field distributionalong the insulator length. The electric field values arecomputed by considering the optimum pipe diameter of100mm and the ring positions of 126mm (for the ring atline end) and 6480mm (for the ring at ground end) asdetermined in the earlier steps. The computed values areshown in Fig.6. It is clear from the results of Fig. 6. thatthe computed electric field values at the metal endfittings is more than 2.1kV/mm with the ring diameter of360mm and 380mm. But these values reduced drasticallyto values lower than 2.1kV/mm by increasing the ringdiameter to 400mm. Also the electric field values at thetriple junction and around the 1 st and 2 nd sheds are wellwithin the limit value of 0.7kV/mm. However there is noconsiderable improvement in the field values byincreasing further the ring diameter to 420mm and theadditional cost to be incurred due to the increasingmaterial requirements will over weigh the improvementin the field values. Therefore the ring diameter of400mm was taken as the optimum diameter for the ring.
The optimized grading ring configuration designed at theend of optimization study is shown in Fig. 7. Fig. 8.shows the comparison of the electric filed distributionalong the length of the insulator with original gradingring, with optimized grading ring and without gradingrings. Fig 9. shows the equipotential contours andshaded electric field lines with optimized grading ringconfiguration.
8/10/2019 Control of Electric Field and Voltage Distribution of a 765kv
The optimized grading ring dimensions are independentof other associated hardware which may influence thecorona inception voltage.
Fig. 6. Variation of Field at critical points with ringdiameter
Fig. 7 Optimum grading ring dimensions
Fig. 8. Electric field distribution (kV/m) along theleakage distance of the insulator.
Fig.9. Equipotential lines and electric field shaded plots
CONCLUSIONS:- The grading ring pipe diameters of less than 100mm
considered for the study resulted into an electric fieldof more than limit magnitude of 2.1kV/mm at bothmetal end fittings. This resulted into suggesting aminimum pipe diameter of 100mm for both the rings.
- The position of both the grading rings at HV end and at
Ground end have a greater effect on the electric field atcritical points. The upward moment of HV ringtowards the HV end and the downward moment of theground ring towards the ground end metal part hasimproved the electric field values at the critical points .There was an optimum position for both the rings atwhich the field magnitudes were minimum and beyondthat position the moment of the rings towards the endmetal parts resulted into increase in the electric fieldvalues. Thus the ring position of 126mm and 135mmfrom the metal end fittings was suggested for the HVand ground end rings respectively as optimum values.
- The increasing grading ring diameter was found toreduce the electric field stresses at the critical pointsand an optimum diameter of 400mm was suggestedwhich minimizes the electric fields magnitudes at thecritical points to below the limit values.
Acknowledgements:The authors would like to thank the CPRI authorities forgiving permission to publish this paper. The helprendered by Mr. T.C. Nataraj from G.K. xiangeElectricals is acknowledged in coordinating the workcarried out.
REFERENCES
[1] IEEE Taskforce on Electric Fields and CompositeInsulators, Electric Fields On AC CompositeTransmission Line Insulator . IEEE Transactions onPower Delivery. Vol. 23, N ◦ 2, April 2008.
[2] U. Schümann*, F. Barcikowski, M. Schreiber, H. C.Kärner TU Braunschweig, Institute for High Voltageand Electrical Power Apparatus, Braunschweig ,J. M.Seifert Lapp Insulator GmbH, Wunsiedel,(Germany)“FEM Calculation and Measurement of theElectrical Field Distribution of HV CompositeInsulator Arrangements “,
[3]. B. Marungsri, W. Onchantuek, and A. Oonsivilai“Electric Field and Potential Distribution alongSurface of Silicone Rubber polymer InsulatorsUsing Finite Element Method “
