Electrical Performance Study of 11 kV Coated Polrcelain, Coated Glass, and Polymer Outdoor High Voltage Insulators

The study in this paper investigates how contaminations and different types of pollutants affect the electrical performance of outdoor coated porcelain, coated glass, and composite insulators when subjected to an 11 kV AC voltage. The ceramic insulators (porcelain and glass) coating layer is assumed to be RTV silicon rubber material with a 0.5 mm thickness. The effect of these three pollution cases on the electrical performance were investigated using a commercial software called COMSOL Multiphysics based on the finite element method (FEM). The three pollution cases were uniform, non-uniform pollution


INTRODUCTION
The surface insulators pollution means increasing of moisture under weather conditions such as rain, fog, and dew. Thus, it is increasing of surface conductance and leakage current under an applied voltage, which may lead to the degradation of the surface electrical performance of polluted insulators [1]. Insulators are devices that are used on electricity supply networks, to support, separate, or contain conductors at high voltage [2]. After a long time of exposure to the air, especially in the industrial and coastal regions, high voltage insulators are often covered with a pollution layer. When the insulator profile becomes wet, high surface leakage currents start to flow causing dry bands. Partial arcs start to occur. If these arcs propagate along the surface of the insulator, a flashover phenomenon will occur leading to the degradation and failure of the insulator [3]. This poses a threat to the reliability and performance of these insulators. Therefore, investigating how contaminations and different types of pollutants affect the electrical performance of outdoor high voltage insulators is an important goal [4]. The most widely used type of porcelain and glass insulator, worldwide, is the cap and pin insulator [5]. The main problem with ceramic high voltage insulators is that water readily forms a continuous film on their surface. To reduce the incidence of insulator flashover, room temperature vulcanizing (RTV) silicone rubber is being widely used to coat insulators to enhance their electrical performance [6]. In this paper, the performance of coated porcelain, coated glass, and polymeric insulators under different pollution cases was investigated by using software called COMSOL Multiphysics based on the finite element method (FEM).

COMSOL Multiphysics and FEM
COMSOL Multiphysics is simulation software package for various engineering applications. The software facilitates conventional physics-based user interfaces and coupled systems of partial differential equations. COMSOL provides Integrateddevelo pment environment unified workflow for electrical, chemical, and mchanical applications. COMSOL Server is very good software for the management of COMSOL simulation applications in different sectors. There are many modules in COMSOL classified based on the applications areas of Electrical, Mechanical, Acoustic, Chemical. The Finite element method is used for solving partial differential equations in two or three variables [7]. The problem is solved by divides a large system into finite elements. This is developed by a particular space discretization in the three dimensions, which is achieved by the construction of a mesh of the object: the numerical domain for the solution, which has a finite number of points. The finite element method formulation of a boundary value problem finally results in a system of algebraic equations.

DESIGN AND MODELING
The insulators that were investigated in this paper are 81022 Ball-Socket porcelain and U100BL Ball and Socket glass insulators as shown in Fig.1, with dimensions and specifications demonstrated in Tables 1 and 2.

Polymer insulator design
The polymer insulator that was investigated is a standard 4-shed insulator as shown in Fig.2, with dimensions and specifications demonstrated in Table 3.

MODELS
The three insulators were modeled drawn on Autocad 2007 software, and modeled and imported to Comsol Multiphysics software as DXF files. The high voltage electrode was energized with 11 kV AC voltage with a frequency of 50 HZ and the low voltage electrode was grounded. A 0.5 mm RTV coating layer was modeled on the surface of porcelain and glass insulators. The three models are shown in Fig.3.

Uniformly polluted insulators
The surface water is considered to be the dominant substance when the surface is completely wet and saturated with moisture, thus the pollution layer was assigned with a relative permittivity of 80 and a conductivity of 6×10 −6 S/m [8]. The pollution layer was distributed uniformly with a pollution layer of 0.5 mm. Fig.4 shows the three uniformly polluted insulators.

Non-uniformly polluted insulators
The pollution layer was non-uniformly distributed as patches with thicknesses varying from 0.5 to 1 mm near HV and LV electrodes and along the surface of the three insulators as shown in Fig.5.

Water droplets near HV and LV electrodes
Water droplets were simulated near HV and LV electrodes in hemispherical shapes with a diameter of 1 mm as shown in Fig.6.

Material properties
Each region of the model was specified with appropriate material properties. The properties of material used in the simulations are summarized in Table 4.

Meshing
As part of the FEM procedure the entire domain is divided into non-overlapping triangular mesh elements. The finite element method (FEM) is one of the most widely used methods for solving partial differential and integral equations of met in many engineering problems and mathematical models. This method states that the area or the domain under investigation is subdivided into small non-separated, non-overlapping regions called finite elements where the differential equations will be implemented. This sub-division procedure is called meshing [9]. Mesh discretisation of the insulators domain problem can be seen in Fig.7. The potential and field distribution is computed by solving the differential equation in the software given by equation (1)  Where: : external current density (A/ 2 ).
The surface dissipated power in pollution layer is computed per unit surface area using the following equation: Where: Et: the tangential electrical field in (V/m). tp: the thickness of the pollution layer along the creepage path of the insulator surface. σ: the conductivity of the pollution layer in([S/m).

Uniform and non-uniform pollution cases
The same results of the potential distribution were obtained in the uniform and non-uniform pollution cases, as shown in Fig.8. For coated glass and porcelain insulators, the potentials followed very similar trends. A steep rise up to 12 kV at a surface distance of 50 mm for the porcelain and 12.3 kV for the glass at a surface distance of 80 mm approximately was noticed. The voltage is in a steady state until it reaches 15.6 kV near the HV electrode. For the silicon rubber insulator, the voltage seemed to be smooth and more uniform starting from 0 kV near the LV electrode and ending with 15.6 kV near the HV electrode.

Water droplets pollution case
When water droplets were added on the surface of the three insulators' models, the voltage distribution was significantly changed as shown in Fig.10. For porcelain and glass insulators, the curve initially increased in a sharp orientation to about 8 kV at a surface distance of 20 mm, and then it fluctuated to a surface distance of 150 mm till it reached 15.6 kV near the HV electrode. For the silicon rubber insulator, the shape of the curve had become non-uniform, a transition in the form of plateaus from 0 kV at LV electrode to 15.6 kV near the HV electrode was observed. The presence of water droplets on the three insulators' surfaces produced more undulation seen on the voltage profiles in Fig.9. The magnitude of the resulting voltage variations could be sufficient for the formation of dry bands.

Uniform pollution case
The electrical field distribution in the uniform pollution case is illustrated in Fig 10. For coated porcelain insulator, the highest electrical field strength magnitudes were observed at the metal end fittings with values of 3 kV/cm at a surface distance of 20 mm, which is very close to the LV electrode and 8.2 kV/cm at a surface distance of 340 mm, which is very close to the HV electrode. These peaks confirmed the concentrated equipotential lines near the metal end fittings. Away from the metal end fittings, the gradient of the electrical field was reduced to lower values in a way that seems to be uniform. For the coated glass insulator, the highest electrical field values were observed near the HV electrode with a value of 9.5 kV/cm and near the LV electrode with a value of 2.5 kV/cm. Between the metal end fittings, the electrical field is constant until it reaches a surface distance of 280 mm, then it fluctuates reaching the maximum value near the HV electrode. For the silicon rubber insulator, the maximum electrical field was noticed at the tip of the first and last shed compared with other sheds with a value of 1.2 kV/cm. A similar trend of the electrical field distribution was noticed in the shank areas between the metal end fittings with peaks of 0.8 kV/cm. This is correlated with the uniform equipotential lines distribution.

Non-uniform case
The electrical field distribution along the three insulators surfaces in the non-uniform condition is shown in Fig.11. For the coated porcelain insulator, it can be observed that the electrical field near the HV electrode has increased to 8.5 kV/cm and remained constant near the LV electrode. Also, the distribution has become more non-uniform and the fluctuations increased near the HV electrode. For the coated glass insulator, the electrical field intensity has increased near the metal end fittings and reached a maximum value near the HV electrode of about 12 kV/cm. Also, the electrical field at the LV electrode increased to a value of about 4 kV/cm. More undulations can be seen near the HV electrode. For the silicon rubber insulator, there is no change in the magnitude of the electrical field near the metal end fittings. Therefore, silicone rubber has been widely used to coat insulators [6]. Away from the metal fittings, the electrical field distribution became more non-uniform in shank areas. These areas exhibited a similar trend with an electrical field value of 1 kV/cm. The obtained results are similar with that reported in literature for electrical insulators [6].

Water droplets case
The three insulators models in the presence of water droplets. For the coated porcelain insulator, the maximum electrical field occurred near the LV electrode with a value of 4 kV/cm. The electrical field near the HV electrode has increased with values ranging from 2 kV/cm to 8.5 kV/cm. The electrical field has become more non-uniform. For coated glass insulator, the electrical field near the metal end fittings has reached maximum values ranging from 5 kV/cm to 12.1 kV/cm near the HV electrode and from 4 kV/cm to 1 kV/cm near LV electrode. Such high field magnitudes, if increased to higher magnitudes, it will lead to surface heating. Initiating partial arcs that consume the silicon fluid and result in loss of hydrophobicity in high field regions. For the silicon rubber insulator, the electrical field has significantly increased near ‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬ : ‫العلوم‬ ‫التطبيقية‬ Journal of Alasmarya University: Applied Sciences the metal end fittings with a value of 4.6 kV/cm approximately. The tips of the upper and lower sheds exhibited non-uniform distribution of the electrical field with values ranging from 3 kV/cm to 3.5 kV/cm near LV electrode and 2 kV/cm to 2.7 kV/cm near HV electrode.

Power dissipation in the uniform pollution layer of three models
The dissipated power was calculated along the creepage path of the three insulators in the uniform pollution case as illustrated in Fig.13. The maximum dissipated power occurred in the regions which have been subjected to the highest electrical field intensity. For coated porcelain insulator, the maximum dissipated power was 185 W/m 2 near the HV electrode. For coated glass insulator, the maximum dissipated power occurred near the HV electrode with a value of 260 W/m 2 . For silicon rubber insulator, the maximum dissipated power occurred at the upper shed near the HV electrode with a value of 3.6 W/m 2 . Peaks of dissipated power can also be seen in the shank areas with a peak of 1.8 W/m 2 . Therefore, these investigations can be summarized as the silicon rubber insulator provided better performance for power disspation in the uniform pollution layer as compared with the other studied two insulator models (coated porcelain and coated glass insulators). These finding is in good agreement the previously reported investigations [6].

CONCLUSION
This paper aims to understand and compare the electrical performance of nowadays available outdoor insulators under different pollution conditions. Electrical potential distributions, electrical field distribution, and dissipated power were computed and investigated along the creepage path of coated porcelain, coated glass, and silicon rubber insulators. From the obtained results, for the uniform pollution case, the coated glass insulator exhibited the highest electrical field amongst the three models with a value of 9.5 kV/cm. For the non-uniform pollution condition, electrical field distribution became more non-uniform for the three models. For the water droplets pollution case, more fluctuations appeared in the metal end fittings regions where water droplets were located. The dissipated power was at its maximum value for the coated glass insulator with a value of 260 W/m 2 . In addition, it was found that coated porcelain experienced a power loss with a value of 185 W/m 2 . Therefore, coated porcelain offered better performance compared with the coated glass insulator. Furthernore, the lower value of 3.6 W/m 2 of dissipated power was found for the silicon rubber insulator. Also, high field regions were identified to be near the metal end fittings. In summary, the silicon rubber insulator offered better insulation performance compared with coated porcelain and coated glass. This study provides useful information that may help for design suitable and appropriate insulators to give better performance under harsh environmental conditions.