This paper evaluates the effect of different corona ring designs on electric field intensity near the live end of polymer insulators using 3D modeling software. Five corona ring designs with diameters ranging from 200mm to 425mm were modeled and tested. Both simulation and practical high voltage testing were performed. Simulation results found electric field intensities exceeded 3kV/mm at inception voltages that were generally within 10% of practical test values. Larger diameter rings had higher inception voltages. The research demonstrates how ring geometry influences electric field distribution and provides guidance for insulator design.

1. International Conference on High Voltage Engineering and Technology
(ICHVET-2015)-January 29-30, 2015, Hyderabad
Introduction & Effects of Corona ring Design by Electric Field Intensity using 3D-COULOMB Page 1
INTRODUCTION & EFFECTS OF CORONA RING DESIGN BY
ELECTRIC FIELD INTENSITY USING 3D-COULOMB
Kiran Kumar Reddy Bana, Sampengala Venkatesh*
kiran.bana@goldstone.net, venkateshsampengala@goldstone.net
M/s. Goldstone Infratech Limited, Hyderabad
ABSTRACT:
This paper presents the evaluation of electric field on corona & grading rings along with the
Polymer Insulator. This paper shows the difference between the corona occurred on Grading rings
theoretically by simulation studies and practically by testing using HVAC source. The components of
EHV/UHV lines and substations can produce significant corona. To limit the consequent Radio
Interference and Audible Noise on these systems, suitable corona control rings are employed. The
shapes of these rings could vary from circular to rectangular with smooth bends. Many manufacturers
seem to adopt trial and error method for arriving at the final design. As such, neither the present
testing standard nor the final design adopted consider the practical scenario like corona produced by
deposition of dirt, bird droppings, etc. The present work aims to make a first step in addressing this
practically important problem. This requires an accurate evaluation of the electric field and a reliable
method for the evaluation of corona inception. Based on a thorough survey of pertinent literature, the
critical avalanche criteria as applicable to large electrodes, has been adopted. Taking the rain drop on
the surface as the biggest protrusion, conducting protrusions modeled as semi-ellipsoid is considered
as representative for deposition of dust or the boundary of bird droppings etc. Through examples of
400 kV and 765 kV class toroidal corona rings, the proposed method is demonstrated. This work is
believed to be useful to corona ring manufacturers for EHV/UHV system.
1 INTRODUCTION
Polymer insulators are extensively used in high
voltage transmission systems. A simplified
diagram of a high voltage insulator is shown in
figure 1. In recent times, a number of
experimental studies have focused on
evaluating the degradation of polymer material
due to corona occurring with different grading
rings. A number of different grading rings on
polymer insulator are used in the research
studies at M/s. Goldstone Infratech Limited. All
of these Grading rings fitted to polymer
insulator, to reduce the electric field along the
corona ring surface. The rings are fitted at live
end where the electric field generated has the
greatest value.
Figure 1- Composite insulator with corona ring
Figure 2 shows the impact the ring has in reducing the electric field at the live end. It can be seen that
the field can be significantly reduced close to the fitting when a ring is used. The irregular shape of the
graph is due to the discontinuity of the electric field at boundary interfaces due to different dielectric
End Fitting
Weather sheds
sheath
Fibre glass rod
Corona control ring

2. International Conference on High Voltage Engineering and Technology
(ICHVET-2015)-January 29-30, 2015, Hyderabad
Introduction & Effects of Corona ring Design by Electric Field Intensity using 3D-COULOMB Page 2
properties of the materials, principally at the air-polymer interface. Peaks are due to the sheds
protruding from the sheath. These sheds have a higher permittivity than air.
Figure 2 – a) Effect of Electric field on live end with ring Figure 2- b) Effect of Electric field on live end without ring
The size, shape and positioning along the insulator (height) of the corona rings varies from one
manufacturer to another. Generally, the corona ring is level with the first or second shed. There is no
fixed standard to specify the size of the grading ring required for a particular voltage level, and the
usual practice is to use the rings recommended by the manufacturers. It is of particular interest to this
research how grading ring geometry affects the ring’s performance. Detail results from this
investigation are reported in this paper.
2. MODELLING METHOD
The electrostatic simulations carried out in this research were done with the help of the software
package Coulomb 3D [3]. This package uses the Boundary Element Method (BEM) to perform the
electrostatic calculations. To aid a faster solution, the geometrical models were simplified, using
findings from research carried out by Zhao & Comber [1] and Weiguo and Sebo [2]. Insulator models
consisted of the fibre glass rod, end-fittings; sheath and sheds, however only the first 8-10 sheds
closest to the end-fittings were incorporated in the model. This is because the higher fields in these
regions are influenced somewhat by the sheds, whereas the sheds in the regions with low fields do not
have a significant impact on the accuracy of the results. The permittivity of the materials used was 5.3
for the fibre glass, 3.7 for the silicone rubber and 1 for the end-fittings and air.
The insulator assembly with the grading ring and the end fittings arrangement is as shown in fig.3. The
various different grading rings were connected individually to the insulator sample.
Figure 3 – Polymer insulator geometry at the live end

3. International Conference on High Voltage Engineering and Technology
(ICHVET-2015)-January 29-30, 2015, Hyderabad
Introduction & Effects of Corona ring Design by Electric Field Intensity using 3D-COULOMB Page 3
2. The effect of different corona rings at the live end to measure Electric Field
The design of the Insulator sample which is researched practically is similar with the figure shown below
Figure 4 - Reference Insulator sample
A. Insulator sample with 200mm outer Diameter corona
ring
The corona ring with 200mm diameter was connected to the insulator
sample of 1M length. The one end of the sample is connected to the
live voltage and the other is connected to the ground.
Practically by using High voltage AC power source, the voltage raised
gradually from initial voltage to the peak till the corona appears. At
the value of 208kV, the visibility of corona has occurred on the
corona ring and is noted as a peak voltage of the particular corona
(grading) ring.
Type-1: Grading Ring with 200mm Dia
Figure 5 - Electric field on 200mm Dia corona ring

4. International Conference on High Voltage Engineering and Technology
(ICHVET-2015)-January 29-30, 2015, Hyderabad
Introduction & Effects of Corona ring Design by Electric Field Intensity using 3D-COULOMB Page 4
*As per the researchers, we observed that the voltage greater than 3.0kV/mm leads to appear visibility of corona
on corona ring.
B. Insulator sample with 300mm outer Diameter corona
ring
Simultaneously, as per the procedure done for 200mm diameter
corona ring.
The voltage gradually raised till the corona appears. At the value of
223kV,
the visibility of corona has occurred on the ring and is noted as peak
voltage.
As theoretically, the Electric field exceeded 3.0kV/mm by applying
the voltage of 223kV in 3D-Coulomb. The reference electric field
occurred at the value of 2.996kV/mm by applying the voltage of
211kV. So, this gives us the results that there is a small variation in
the theoretical values when compared to the practical.
Type-2: Grading Ring with 300mm Dia
Figure 6 - Electric field on 300mm Dia corona ring
C. Insulator sample with 370mm outer Diameter corona ring
The visibility of corona appeared at the voltage of 204kV and is noted as inception voltage.
Theoretically, by applying the voltage of 230kV, the electric field observed is around 2.979kV/mm.

5. International Conference on High Voltage Engineering and Technology
(ICHVET-2015)-January 29-30, 2015, Hyderabad
Introduction & Effects of Corona ring Design by Electric Field Intensity using 3D-COULOMB Page 5
Figure 7 - Electric field on 370mm Dia corona ring Type 3: Grading ring with 370mm Dia
This results that there is variation in theoretical values by
comparing with practical values.
D. Insulator sample with 400mm Dia Grading ring
The corona appeared on the grading at the voltage of
270kV and is noted as inception voltage.
Theoretically, the same voltage is applied on the
grading ring and the electric field occurred is
3.060kV/mm. shown in fig.8
Type 4: Grading ring with 400mm Dia
Figure 8 - Electric field on 400mm Dia corona ring
E. Insulator sample with 425mm Dia Grading ring
The corona found at the voltage of 370.2kV on425mm Dia grading ring practically by HV source. At
340.3kV, the Electric field observed is 2.988kV/mm by using 3D-Coulomb.

6. International Conference on High Voltage Engineering and Technology
(ICHVET-2015)-January 29-30, 2015, Hyderabad
Introduction & Effects of Corona ring Design by Electric Field Intensity using 3D-COULOMB Page 6
Figure 9 - Electric field on 425mm Dia corona ring Type 5: Grading ring with 425mm Dia
Table 1: Tabulation of Electric Fields at Different locations:
The Table below illustrates the Electric field at various locations on different types of grading rings
along with insulator.
S. No.
Type of
Grading ring
Stress on FRP
(kV/mm)
Stress on Sheath
(kV/mm)
Stress on Ring
(kV/mm)
1 Type-1 1.80 0.61 3.01
2 Type-2 2.99 1.05 2.99
3 Type-3 0.97 1.23 2.97
4 Type-4 0.92 1.03 3.06
5 Type-5 0.93 1.17 2.98
4. CONCLUSIONS
The effect of corona ring geometry on the electric field around the live end of high voltage insulator
has been studied using 3-D simulation software. This paper gives the information that there is a small
variation in the corona values comparing between the practical by HV source in line and theoretically
by 3D-coulomb. By this, we can calculate with tolerance of +10% to the result observed in the 3D-
coulomb. Always the corona inception voltage observed in 3D-coulomb simulation is less than the
corona inception voltage observed in line voltage. The value will be approx. 10% more than the
simulation done in 3D-coulomb. This research shows how grading ring geometry affect its Electric
Field Intensity in simulation studies when compared with HVAC source.
Acknowledgement:
The authors are grateful to the management of M/s. Goldstone Infratech Limited to publish this paper.
We are thankful to Mr. N. K. Rawal, CEO and Mr. N. Sudhakar Reddy, A.V.P (Operations), M/s.
Goldstone, Hyderabad for their continual support and encouragement. I also thank to Mr. Y.
Sundaraju, A.M (Designs), M/s. Goldstone for supporting us during the research.

7. International Conference on High Voltage Engineering and Technology
(ICHVET-2015)-January 29-30, 2015, Hyderabad
Introduction & Effects of Corona ring Design by Electric Field Intensity using 3D-COULOMB Page 7
REFERENCES
[1] T. Zhao, M.G. Comber, “Calculation of electric field and potential distribution along non-ceramic insulators
considering the effects of conductors and transmission towers”, IEEE Transactions on Power Delivery, vol. 15 ,
no. 1, pp. 313 – 318, 2000.
[2] Q. Weiguo, S.A Sebo, “Electric field and potential distributions along dry and clean non-ceramic insulators”,
Electrical Insulation Conference and Electrical Manufacturing & Coil Winding Conference, Proceedings, 16-18,
pp. 437-440, 2001.
[3] Coulomb 3D website – http://www.integratedsoft.com/products/coulomb/
Authors Biography:
Mr. B. Kiran Kumar Reddy received his Masters Degree M.E (CAD/CAM) in the year
2004 from Osmania University, Hyderabad and working as a General Manager in M/s.
Goldstone Infratech Limited, Hyderabad since from 9 years.
Mr. Sampengala Venkatesh pursuing his Masters on Power Systems at JNTU Hyderabad
and working as H.V Test Engineer in M/s. Goldstone Infratech Limited since from 2 years
and have 1year experience in CPRI, Hyderabad.