Electric Field Modelling of Non-Ceramic High Voltage Insulators

Electric Field Modelling of Non-Ceramic High Voltage Insulators K. Eleperuma Powerlink Queensland 33 Harold Street Virginia Qld 4014 keleperuma@powerlink.com.au T. K. Saha University of Queensland Queensland 4072 Australia saha@itee.uq.edu.au T. Gillespie Powerlink Queensland 33 Harold Street Virginia Qld 4014 tgillespie@powerlink.com.au ABSTRACT This paper applies a three-dimensional (3D) electric field analysis program to calculate the field distribution at the live-end of 110kV non-ceramic insulators used in the Queensland transmission system. A number of 110kV insulator configurations have been modelled, including a post, a longrod and a horizontal vee assembly, to determine the electric field on the sheath at the live end. mechanical stresses. Figure 1 illustrates a 110kV double circuit pole with horizontal vee insulator assemblies composed of post and longrod insulators. It has been observed that birds will only chew the dead end of insulators. Based on modelling and the proximity of bird chewing damage to the live end, the maximum electric field that birds will tolerate has been estimated. Of particular interest to this research was the effect of corona ring geometry on the electric field at the live-end of high voltage non-ceramic insulators. At 110kV, manufacturers do not recommend the fitting of corona rings to reduce the electric field. However, for 330kV non-ceramic insulators, manufacturers recommend fitting corona rings at the live end to reduce the electric field so that sheath damage due to corona discharge is minimised. The 110kV non-ceramic insulator electric field values from modelling are compared with those of 330kV longrods from earlier work. This paper comments on whether corona rings should be applied to 110kV non-ceramic insulators based on the maximum levels of corona on the sheath at the live end of longrods. A number of different corona ring geometries were compared. The effect of changing ring cross-sectional shape, diameter, and placement along the insulator are investigated. 1. INTRODUCTION [1-4] Quality high voltage insulators are required to achieve high electrical performance of HV networks. They also play a major role in heavily contaminated environments. Their primary function is to act as the electrical isolators that separate the HV power lines and the grounded structures. They also act as the mechanical supports for the cables by keeping them connected with the structures. Subsequently, HV insulators must have the ability to withstand extremities of both electrical and Figure 1: 110kV Pole Structure with post and longrod insulators The available insulator types are ceramic insulators (i.e. porcelain), glass insulators, and polymer (i.e. nonceramic) insulators. Regardless of the insulator brand and its design, the level of the electric field present on the surface of a composite insulator varies considerably from the energized terminal to the grounded terminal [3]. To reduce the maximum electric field on the sheath, corona rings (also known as gradient rings) are installed on the energised end fittings of HV insulators. Typically, for lower system voltages such rings are built into the insulator structure. However, for higher voltage levels, separate corona rings are installed into the insulator

structures. External rings which are designed for installation in only one orientation and location to avoid miss application are preferred. Such corona rings are often manufactured out of aluminium to attain greater levels of weight reduction and higher levels of corrosion resistance. Illustrated in Figure 2 is a cross section of a typical composite high voltage insulator. End fitting Fibre Glass Rod Wether-sheds Sheath incorrect locations on the insulator may lead to amplifications in the maximum electric field (as opposed to reductions of it) [5]. Therefore, it is crucial to research the most appropriate position to place the gradient ring on the insulator. It is commonly accepted that the corona ring be placed near the energised terminal of the insulator. As indicated in Figure 4, this is often adjacent to the first shed on the energised terminal of the insulator. By having such a corona ring, it allows the electric field to be distributed more uniformly along the insulator surface. Vancia [1] had earlier discovered that it is essential to model positioning of the corona ring on the insulator for each insulator in the market as the design parameters vary from one manufacturer to another. Consequently, a number of corona rings were modelled (Figure 5). It should be noted that these rings were modelled isolated in free-space while being energised at 63.5kV (i.e. 110/3 kv). Corona Ring Figure 2: Components of a high voltage insulator [1] As stated by Phillips, Childs and Schneider [2], the maximum recommended electric field to ensure minimal corona is 0.42. The inclusion of corona rings to insulators aid the system by reducing the corona effect, thus prolonging the life of the insulator, reducing the maximum electric field gradient along insulator surface, and preventing excessive levels of radio interference. Figure 3 shows the impact a corona ring has in reducing the electric field on the sheath at the live end of a 110kV composite longrod insulator. As indicated on the horizontal axis, only 10% of the insulator is shown. It can also be seen that a reduction in maximum electric field of up to 0.5 was achieved with the introduction of a corona ring. Ring Radius Ring Inner Diameter Ring Height Ring Outer Diam. Figure 4: Ring positioning on a typical 330kV longrod insulator (HV terminal shown). Note: the insulator was modeled using coulomb 3D, isolated in free-space with one terminal grounded (0V) and the other energized at 190.5kV (i.e. 330/3 kv). 2. 110KV LONGROD AND POST INSULATORS [1-6] Figure 3: Before/after effects of the energized terminal of a 110kV composite post insulator with/without a corona ring. Note: the insulator was modeled using coulomb 3D, isolated in free-space with one terminal grounded (0V) and the other energized at 63.5kV (i.e. 110/3 kv). Presently there are no standards governing the design parameters and placements of the corona rings on high voltage structures. Hence, positioning the corona ring at In conducting the simulations for this research, it was recognised that the implementation of grading rings on the 110kV transmission network would drastically reduce the effect corona poses on the HV insulators. Initially only a portion of the structure was modelled (Figure 6). Later, it was noticed that modelling insulators in free-space gave higher electric fields than modelling the horizontal vee assembly and structure

(Figures 7 and 8). This is due to the presence of the earthed metal structure; i.e. having a large grounded device in the vicinity of the insulators would also aid in reducing the maximum electric field [1]. As shown in Table 1, both the longrod and the post insulators would benefit from the introduction of corona rings. Figure 7: 3D electric field contour plot of a typical 110kV post insulator (energized terminal). Note: the insulator was modeled isolated in free-space. The energized terminal was applied with 63.5kV (i.e. 110/3 kv) while the grounded terminal with 0V. Ring A B C D E Max e- field 0.25 0.38 0.45 0.49 0.31 Figure 5: Various corona rings as modeled in free-space using Coulomb 3D. Note: all rings were modeled isolated from one another and energized at 63.5kV (i.e. 110/3 kv). Figure 6: Partial 110kV structure as modeled in Coulomb 3D. Note: the above was modeled isolated in free-space with the structure energized at 0V and the interconnecting rod energized at 63.5kV (i.e. 110/3 kv). Figure 8: 3D electric field contour plot of a typical 110kV longrod insulator (energized terminal). Note: the insulator was modeled isolated in free-space. The energized terminal was applied with 63.5kV (i.e. 110/3 kv) while the grounded terminal with 0V. Insulator Type Without Corona Ring With Corona Ring Longrod 0.342 0.097 Post 0.152 0.102 Table 1: Maximum surface electric field on insulator before/after installing a corona ring.

It was later noted that the 110kV post and the longrod insulators required two different types of corona rings to successfully reduce their maximum surface electric field. Furthermore, of the various corona rings tested (Figure 5), the corona ring with the circular cross sectional area yields the lowest level of maximum surface electric field. The primary reason for this being the simplicity with the least number of sharp protruding sections. However, this design required the most material and therefore is the highest cost. Later it was shown that the 110kV longrod insulator must be coupled with a circular shaped corona ring with a diameter of 280mm to achieve minimum electric field on the sheath. Furthermore, the ring must be placed at a height of 465mm from the energised termination point of the insulator. This can be approximated to be adjacent with the insulator s first shed. Placing corona rings adjacent to the first shed (towards the energised terminal) of an insulator is understood as common practice in the field of high voltage insulators [4]. As indicated in Figure 9, the end result of this process revealed that the 280mm ring placed at a height of 465mm from the energised termination point of the insulator imposed the minimum value for the maximum surface electric field (0.097). This is highlighted in red on the magenta coloured plot on Figure 9. Figure 9: Effects of various diameter corona rings on the 110kV polymer longrod insulator. Note: the insulator and corona ring were modelled isolated in free-space with the grounded terminal of the insulator energized at 0V and the energized terminal and the corona ring energized at 63.5kV (i.e. 110/3 kv). A similar experiment conducted on the 110kV post insulator proved that a corona ring of diameter 300mm placed at a height of 465mm from the energised termination point of the insulator imposed the minimum value for the maximum surface electric field 0.102. This is highlighted in red on the yellow coloured plot on Figure 10. 3. COMPARISON WITH THE 330KV INSULATORS Electric fields of 330kV and 110kV longrod insulators with corona rings were compared. Tables 2 and 3 illustrate the comparison of these two insulators. Both insulators were modelled using Coulomb 3D in freespace with no attachments. Furthermore, a 250mm diameter circular corona ring (same ring as ring A on Figure 5) was implemented on both insulators for the ease of comparison of the two insulators. Figure 10: Effects of various diameter corona rings on the 110kV polymer post insulator. Note: the insulator and corona ring were modeled isolated in free-space with the grounded terminal of the insulator energized at 0V and the energized terminal and the corona ring energized at 63.5kV (i.e. 110/3 kv). The following phenomena were noticed on the 330kV and 110kV insulators; Maximum surface electric field of the 110kV network increases with the corona ring diameter, while that of the 330kV network decreases with increased ring diameter; i.e. the most suitable ring for the 330kV insulator is the 340mm ring, while that for the 110kV network is the 260mm ring (highlighted in Tables 2 and 3). The maximum surface electric field varies by ±0.02 with different ring diameter for both 110kV and 330kV longrod insulators. The 330kV longrod insulator s maximum sheath electric field varies from 0.41-0.45 while that of the 110kV varies from 0.10-0.12. Ring Diameter 260mm 280mm 300mm 320mm 340mm Max surface e-field 0.45 0.43 0.42 0.42 0.41 Table 2: Maximum surface electric fields for different diameter corona rings on the 330kV longrod insulator. Note: the insulator and corona ring were modelled isolated in free-space with the grounded terminal of the insulator energized at 0V and the energized terminal and the corona ring energized at 190.5kV (i.e. 330/3 kv). 4. BIRD DAMAGE ON HV INSULATORS [5, 6] One of the problems is chewing damage caused by parrots on the 110kV HV post insulators. In general, damage to the sheds does not present a serious problem as the remaining creepage distance is typically sufficient to provide adequate insulation to the surrounding objects. However, a higher level of risk is presented when the insulator is erected in a highly polluted area

where any reduction in the insulator s creepage length could become critical. Such areas include close to power stations and heavily industrialised locations. If the sheath is damaged exposing the core as in Figure 12, the insulator must be replaced due to the risk of flashover under the sheath along the core rod. Ring Diameter 260mm 280mm 300mm 320mm 340mm Max surface e-field 0.10 0.10 0.11 0.12 0.12 Table 3: Maximum surface electric fields for different diameter corona rings on the 110kV longrod insulator. Note: the insulator and corona ring were modeled isolated in free-space with the grounded terminal of the insulator energized at 0V and the energized terminal and the corona ring energized at 63.5kV (i.e. 110/3 kv). noticed that the electric field varies from ~0 to 0.152 throughout the insulator. Furthermore, the highest electric field at the limit of the chewed area is approximately 0.085. As it can be seen in Figure 13, this is approximately 0.35m from the grounded terminal of the insulator (this translates to approximately 70% of the post insulator being chewed). Such a phenomenon clearly indicates that the parrots are able to withstand electric fields of up to 0.085. Hence, it would be desired if an attachment could be designed that can be used in conjunction with the insulator to maintain its maximum electric field levels at or above 0.085. Insulator degradation occurs from parrots chewing sheds Figure 11: Max surface electric field for longrod insulators on the 330kV and 110kV networks. Note: the insulators and corona ring were modeled isolated in free-space with the grounded terminals of the insulators energized at 0V and the energized terminals and the corona rings energized at 63.5kV (i.e. 110/3 kv) and 190.5kV (i.e. 330/3 kv) appropriately. Parrots sometimes position themselves on the grounded terminals of HV post insulators and chew their path along the sheds of the insulator towards the energised terminal. Once the electric field approaches levels that are irritable to the birds, they flee the insulator leaving only a small portion of its sheds operational (Figure 12). Figure 13: Coulomb 3D model of the 110kV NGK post insulator. Note: the insulator was modeled isolated in free-space. The energized terminal was applied with at 63.5kV (i.e. 110/3 kv) while the grounded terminal with 0V. The area indicated with the red arrow indicates the region for which the max surface electric field is less than 0.085. However, this poses supplementary concerns as the primary task of the study is to increase the lifespan of HV insulators by reducing the maximum electric field present on the insulator surface. Consequently, increasing the electric field to levels greater than 0.085 throughout the insulator (to avoid bird damage) would greatly aid in accelerating the corona degradation process. Another option would be to design a separate structure that can be attached to the insulators without affecting (increasing) its electric field. Such a structure (shield) could then be used as an overall protection device to guard the insulators from the parrots. Two such alternatives were suggested; Figure 12: An 110kV post insulator with bird damage on sheds In analysing the 110kV post insulator (modelled in Coulomb 3D isolated with no attachments), it was Firstly, the insulator sheds could be coated with a material that is unappreciated by birds. A critical problem arising from this would be its environmental impact; i.e. any side affects the coating substance may pose on the birds. Simultaneously, such a coating may

also adversely affect (damage) the silicone sheds of the insulator. Hence, it can be seen that there are a number of options to consider when designing such a substance. The use of aluminium sulphate is currently being researched by Powerlink Queensland. The second suggestion is the installation of audible or ultrasonic bird scaring devices on HV structures. This option will not inflict any environmental damage or damage to the insulators. However, it has a relatively high capital and maintenance cost. Furthermore, the audible sounds generated by such devices may not be tolerable in populated areas. Consequently, these devices can only be installed at limited number of locations. 5. CONCLUSIONS Note: All of the simulations conducted in this study were implemented using Coulomb 3D. All individual insulators were modelled isolated from external sources and attachments in free-space. All insulators and pole structures were energised as per below; All grounded structures (grounded terminal of insulators, poles, etc.) were set to 0V. All corona rings and energised terminals of insulators were energised with the appropriate voltage. This study analysed the insulators mounted on the 110kV pole structures on the Darling Downs, Queensland. Simulations conducted using Coulomb 3D determined the maximum electric field on the sheath of the 110kV polymer insulators. The second phase of the study compared the electric fields between the insulators on the 330kV and 110kV networks. The results of the study revealed that; The most efficient corona ring for the 110kV long rod insulator is the 280mm circular cross sectional ring positioned at a height of 465mm above the energized tip of the insulator. Similarly, a 300mm circular corona ring was proposed for the 110kV post insulator positioned at a height of 465mm above the energized tip of the insulator. It was also noted that the bird chewing is restricted to an electric field of less than 0.085 on the 110kV post insulators. A number of possible solutions were provided to overcome the bird chewing problem, including the coating of insulator sheds with a material that is unappreciated by birds, and installing audible or ultrasonic bird scaring devices. The 330kV longrod insulator s maximum sheath electric field was found to be 0.41-0.45 compared to that of the 110kV which was 0.10-0.12. This further indicated that all HV insulators must be individually modelled for optimum corona ring placement (see Figure 11). Future research that may be conducted to improve the efficiency of HV insulators include: Corona ring design As this study only examined a small number of ring designs, more emphasis could be placed on designing various rings to further reduce the surface electric field on the HV insulators. Simulation of complex components Coulomb lacks CAD tools required to create the complicated shapes. Such components could be constructed using SolidWorks/AutoCAD and imported into Coulomb 3D to increase the accuracy of the simulation process. Eliminating the bird chewing problem this study suggested a number of possible techniques for eliminating bird chewing on HV insulators. These techniques could be further examined prior to implementation. 6. ACKNOWLEDGEMENTS This project could not have been completed without the support provided by Powerlink Queensland, in providing the resources required. Their help is much appreciated. REFERENCES [1] B. Vancia, Modelling of electric fields on composite insulator. University of Queensland & Powerlink Student Thesis. [2] A.J. Phillips, D. J. Childs and H. M. Schneider, Aging of Non-Ceramic Insulators Due to Corona from Water Drops, IEEE Transaction on Power Delivery, vol. 14, no. 3, pp. 1081-1089, 1999. [3] Q. Weiguo, S.A Sebo, Electric field and potential distributions along dry and clean nonceramic insulators, Electrical Insulation Conference and Electrical Manufacturing & Coil Winding Conference, Proceedings, 16-18, pp. 437-440, 2001. [4] T. Zhao, M.G. Comber, Calculation of electric field and potential distribution along nonceramic insulators considering the effects of conductors and transmission towers, IEEE Transactions on Power Delivery, vol. 15, no. 1, pp. 313 318, 2000. [5] W. Sima, F. P. Espino-Cartes, A. Cherney, S. H. Jayaram, Optimisation of corona ring design for long rod insulators using FEM based computational analysis, Conference Record of the 2004 IEEE. International Symposium on Electrical Insulation.