Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 2, 2012 © 2012 by the authors – Licensee IJASER- Under Creative Commons License 3.0 Research article www.ijaser.com editorial@ijaser.com ISSN 2277 – 9442 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition Mohammad Bagher Asadpoor, Mohammad Mirzaie Babol University of Technology, Department of Electrical & Computer Engineering, P. O. Box 484, Babol, Iran doi:10.6088/ijaser.0020101017 Abstract: Satisfactory operation of suspension insulator strings is intimately related to the voltage distribution of the string. At operational voltage, the voltage distribution along insulator string is affected by stray capacitances, which causes a non-uniform voltage distribution. In addition, the performance of insulator changes by the accumulation of environmental pollution over its surface that will deteriorate by absorption of moisture sharply. In order to explain this dilemma clearly, a full equivalent circuit, which takes into account the insulator material properties and the stray capacitances, is derived from the Finite Element Method Based Software, which is implemented in the EMTP-ATP package to calculate the voltage distribution, power loss and the leakage current, flowing through the string with and without pollution under power frequency voltage. Finally, the simulation results of voltage distribution have been compared with experimental results, which were obtained in the High Voltage Laboratory. In experimental procedure, an attempt to form the pollution on the surface of the insulator has been carried out. For this investigation the dry pollutant on the insulator surface has been approached according to IEC60507, which has led to the resistance of pollutants. Then, the voltage distribution of clean and polluted insulator strings has been measured by sphere gap method. Keywords: Insulator, Pollution, FEM, EMTP-ATP, Sphere Gap. Nomenclature C EMTP-ATP FEM IEC Megger Pi Rp Ud Uti ρ ε Capacitance Electromagnetic transient modeling software for power systems- Alternative transients program Finite element method International Electrotechnical Commission Mega ohm meter Percentage of voltage across the i-insulator unit Pollution resistance Breakdown voltage of sphere gap Applied voltage to the insulator string for i-insulator Electric charge density Dielectric constant 1. Introduction Insulator’s capability to insulate the power lines as well as their function in bearing the weight of the line conductor, makes it to be of significant importance in the overhead transmission lines. The knowledge of the voltage distribution and electric field within and around high voltage insulators is of 165 *Corresponding author (e-mail: mirzaie@nit.ac.ir) Received on, Feb, 2012; Accepted on Feb. 2012; Published on Feb 24, 2012 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition paramount importance for the engineer involved in the design of power lines insulation. For instance, voltage distribution is helpful for the detection of punctured insulators in the string. Corona, radio/TV interference and premature aging of insulation are the results of high level electric fields. Ideally, the potential distribution along the insulator string should be uniform. The capacitance of each insulator can be determined approximately by the geometry and permittivity of each consisting element, while neglecting the impact of stray fields. However, for a more accurate computation of the potential distribution, a finite-element package is required to compute the insulator string capacitances (including stray capacitances). Contamination level and the design of the fittings, conductors and tower are eminent facts that influence the voltage distribution over an insulator string. The line insulators are often covered with contaminations, especially in industrial and coastal regions due to the long time exposure in the air. In a condition of high humidity, the salt in the contamination will be dissolved by the moisture because of rain, fog or dew. Therefore, the conductivity of the surface pollution increases, leading to a possible flashover accident. To compute the electric fields and potentials along a polluted insulator, numerous methods have been used such as boundary element method (QueW, 2002; Rasolonjanahary, 1992), finite difference method (Morales et al, 2001) and finite element method (Asenjo, 1997; Ashouri et al, 2010; Faisal, 2011). Potential distribution along a suspension insulator string have been determined by FEM (Kontargyri et al, 2005) and theoretical model which considered the contamination effect in (Dhalaan 2003 a; Dhalaan 2003 b). In experimental part, reference (Pattanadech, 2004) have been measured the voltage distribution along three insulators string by sphere gaps method. Also, this method is used as a reliable method in (Kontargyri et al, 2005) to calculate the voltage distribution on glass insulator string. However, no remarkable endeavor regarding the polluted condition and its impacts upon the voltage distribution in test procedure has been made so far. In the present paper, a full electric circuit of insulator string, considering the insulation geometry, permittivity and stray capacitances, which were derived from a FEM-based software, have been presented. The circuit was simulated in ATP-EMTP in order to calculate the voltage distribution and leakage current following throw the string under various pollution levels. To verify the simulation results, insulators are subjected to the pollution of different severity, according to IEC 60507 and the simulation results of voltage distribution for clean and polluted insulators, have been compared with experimental results using sphere gap method which have an acceptable accordance. 2. Simulation process in FEM-based software 2.1. FEM procedure In electrostatic field problem for isotropic, linear and equalizing dielectric, potential v can be determined according to the Poisson equation : ∇2v = − ρ ε (1) The finite element method is one of numerical process based on the variation approach and has been widely used in electric and magnetic field analyses which have illustrated widely in (He et al, 2009). In this investigation electrostatic problem should be solved to compute the voltage distribution of insulator string. The boundary problem of the electrostatic field, by supposing that the domain under consideration does not contain any space and surface charges, is turned to evaluate the functional equation: Mohammad Bagher Asadpoor, Mohammad Mirzaie Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 166 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition 1 F(v) = 2 2 2 ∂v 2 ∂v ∂v ε x + ε y + ε z dxdydz D ∂x ∂z ∂y ∫ (2) In case of isotropic material, dielectric constant distribution is ( ε x = ε y = ε z = ε ). Whole domains of the solution divide into many triangles, which equation (2) must be applied to them. The calculation of the electric potential at every knot in the total network composed of many triangle elements was carried out by minimizing the functional F(v) , that is, ∂F(vi ) =0 ∂vi ; i = 1,2,3,....n (3) Where n is the total number of knots in the solution region. 2.2. Insulator string simulation In this study, disk shaped porcelain insulator was selected to simulate potential distributions. The basic design of this insulator is as follows; porcelainc with the relative dielectric constant of 6, attached with two alumina with 14 dielectric constant, used as fixing material to the cap and pin. Surrounding of the insulator string is air having relative dielectric constant 1.0. Also, the numbers of cap and pin type insulators, in the string are 13 units which are used for 230 kV overhead transmission lines. The geometrical characteristics of each insulator unit are depicted in figure 1. Figure 1: Profile of a cap and pin insulator Mohammad Bagher Asadpoor, Mohammad Mirzaie Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 167 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition 2.3 Calculation of the equivalence circuit parameters for insulator string Experimental investigating the behavior of an insulator, we can see that, when it is clean and has no pollution on its surface, there is practically no leakage and the insulator behaves as a capacitor. As pollution is deposited on the insulator surface, without humidity, it behaves as a capacitance in parallel with a high resistance. When, the humidity level of the pollution on the insulator surface rises because of, for instance, hoar-frost, mist, drizzle, and frost, and then an electrolytic solution is formed. This results in reduction of insulator pollution resistance, leading to the development of leakage current. The equivalent circuit of a polluted insulator is shown in Figure 2, where C and Rp are the capacitance and pollution resistance of the insulator, respectively. Figure 2: Simplified equivalent circuit of a polluted insulator For the simulation of insulator string in different pollution condition, equivalent circuit parameters shall be determined. In order to calculate the self capacitance of each insulator unit and stray capacitances between caps and ground, caps and energized conductor and also each insulator cap and other caps, the insulator string structure is drawn in a FEM-based software. The complete part of a transmission line; including insulator string, tower and conductor and its equivalent circuit are shown in Figure 3. a. b. Figure 3 a. Complete structure of insulator string in FEM-based software b. Equivalent electric circuit Mohammad Bagher Asadpoor, Mohammad Mirzaie Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 168 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition First, the voltage and field stress distributions on the insulator string were computed. Then computed electric field values are used to obtain the electrical energy (We) stored in various parts of the model. These energies values combine with the computed potentials (V) to allow determination of the various capacitances of the equivalent circuit. It should be mentioned that, pollution level for all of the insulators in the string are the same ( Rp = Rp = ... = Rp ). In addition, the pollution resistances are 1 2 measured by Megger test that will be represented in the next section. 2.4 Measuring the insulator resistance in different pollution levels In order to measure the insulator resistance, the Megger test has been conducted. Relative humidity during the test was 45-50% (ambient humidity) which provides a dry pollution on insulator surface. 2.5 Insulators contamination In order to artificially contaminate the surface of insulators, solid layer method has been chosen (IEC 60507, 1991). This method can produce an approximately uniform contamination layer and can also be performed rapidly, so that the results will be satisfying. To prepare the contamination solution, according to the IEC 60507, some kaolin and salt was poured in one liter distilled water and have sprayed on insulators surface. The amount of salt has a direct effect on the electrical conductance of contamination slurry, so three different types of solutions was prepared to contaminate the insulators in different stages of pollution based on Table 1. In the next stage, insulators were suspended vertically to dry out. Figure 4 shows a contaminated insulator. Table 1: Electrical conductance and ESDD in 20 oC Ins. No. Kaolin /Salt (gr/lit) 1 40/10 Measured electrical Conductance in 20oC (S/m) 0.0168 2 40/10 3 Salinity (Sa) ESDD Pollution level 0.089 0.028 Light 0.0155 0.082 0.026 Light 40/10 0.0144 0.076 0.024 Light 1 40/40 0.0885 0.494 0.156 Moderate 2 40/40 0.0845 0.471 0.149 Moderate 3 40/40 0.0989 0.554 0.175 Moderate 1 40/70 0.198 1.133 0.358 Heavy 2 40/70 0.181 1.031 0.326 Heavy 3 40/70 0.212 1.215 0.384 Heavy 2.6 Data measurement In order to measure the conductance of pollution layer for each insulator the value of the leakage current was recorded in thirty minutes by Megger. For every pollution level three insulators have been Mohammad Bagher Asadpoor, Mohammad Mirzaie Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 169 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition tested, that the average resistance of them are shown in Figure 5. As it mentioned, the higher pollution degree exist, the lower pollution resistance occurred. For the simulation, the average value of resistance in thirtieth minute has been considered. Figure 4: Contaminated insulator. 7 x 10 4 800 4000 2000 4 3 2 3500 3000 2500 In su la to r R esista n ce (M o h m ) 5 In su la to r R esista n ce (M o h m ) 750 In su la to r R esista n ce (M o h m ) In su la to r R esista n ce (M o h m ) 6 1750 1500 1250 1 700 650 600 550 500 2000 1000 0 0 10 20 Time (min) 30 a. 0 10 20 Time (min) b. 30 0 10 20 Time (min) 30 450 0 c. 10 20 Time (min) 30 d. Figure 5 Average resistance of Polluted Insulators a. Without pollution b. Moderate pollution c. Light pollution d. Heavy pollution 3. Simulation results Since getting the leakage current throw insulator string and lower running time, ATP-EMTP has been used. The lumped equivalence circuit in Figure 3.b is running in ATP-EMTP and results have been obtained. Mohammad Bagher Asadpoor, Mohammad Mirzaie Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 170 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition 3.1 Performance of insulator string under clean condition Figure 6 demonstrates the voltage distribution of insulator string under clean condition. The first insulator unit in this simulation is placed near the tower and the thirteenth is placed near the energized conductor. Due to the stray capacitances existing between the discs, conductor and ground, the distribution of the voltage along the string is not uniform, and the discs around the conductor being more highly stressed. So the rate of puncture of these insulators is more reported in transmission lines. Also, the leakage current in this condition is capacitive and its value is very low. The amplitude of the leakage current is 0.5923 mA. Voltage distribution (%) 20 18 16 14 12 10 8 6 4 2 0 2 4 6 8 10 Insulator number 12 14 Figure 6: Voltage distribution along insulator string under clean condition 3.2 Performance of insulator string under different pollution condition To investigate the performance of insulator string under pollution condition, as it mentioned, a resistance with higher value is paralleled with each unit. In Figure 7, voltage distribution of insulator string in different pollution levels has been represented. As it can be depicted, any significant changes haven’t occurred on voltage distribution, since the value of pollution resistance under dry condition isn’t considerable in comparison with the impedance of each unit. The same conclusion can be getting for leakage current, as is represented in Table. 2. In addition string loss for different pollution levels are shown in this table. Table 2: Maximum leakage current passing through insulator string with different pollution levels and string loss Pollution level Without pollution Light pollution Maximum leakage current (mA) 0.5923 0.5923 String loss (Watt) 0.031 0.463 Moderate pollution 0.5924 0.925 Heavy pollution 0.5934 2.467 Mohammad Bagher Asadpoor, Mohammad Mirzaie Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 171 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition Voltage distribution (%) 20 Without pollution Light pollution Moderate pollution Heavy pollution 15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Insulator number Figure 7: Voltage distribution along insulator string with different pollution levels 4. Experimental procedure To measure the voltage distribution of an insulator string, sphere gap method has been used in this paper. The required facilities for the test are shown in Figure 8. Self Transformer 220 v 0-220 v HV Transformer V1 Electrostatic voltmeter 0-100 kv Figure 8: Experimental set-up used to measure of the voltage distribution The voltage is measured by using an electrostatic voltmeter, measuring the high voltage in the secondary part of the transformer (IEC 60060-1, 1989). Distance between two spheres is fixed during the voltage measurement. The sphere gap is installed parallel with each insulator. The capacitance of the sphere gap is very small compared to the capacitance of the insulator, so it can be neglected. The applied voltage to the insulator string is up turned until the spark over of the sphere gap at the critical voltage (Ud) occurred. Ten tests for each of the insulators has been carried out to obtain the accurate results and the average of them have been considered as Ut. Then the percentage of voltage across the i-insulator unit Pi , is calculated by: Mohammad Bagher Asadpoor, Mohammad Mirzaie Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 172 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition Pi = Ud ×100% U ti (4) By moving the sphere gap along the insulator string, rate Pi for every insulator will achieve until thirteenth. Ut for each insulator varies, since the existence of stray capacitances between the cap and pin of the insulators, tower and conductor. Totally, the critical voltage of the sphere gap is calculated by the equation: 13 13 i =1 i =1 ∑ Pi = Ud ∑ 1 =1 U ti (5) As a result, by calculating Ud, the voltage percentage of each insulator will obtain. The experimental and simulation results for clean and dry insulator string are shown in Figure 9. Voltage distribution (%) 20 Simulation Results Exprimental Results 18 16 14 12 10 8 6 4 2 0 2 4 6 8 10 Insulator number 12 14 Figure 9: Comparison between simulation and the experimental results for clean and dry insulator string Voltage distribution (%) 20 Simulation Results Exprimental Results 18 16 14 12 10 8 6 4 2 0 2 4 6 8 10 Insulator number 12 14 Figure 10 Comparison between simulation and the experimental results for heavy pollution level Mohammad Bagher Asadpoor, Mohammad Mirzaie Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 173 Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution condition It shows that there should be more consideration to the insulators near the conductor in comparison with other insulators in the string. As it can be seen, there is an acceptable concord between the results. In this comparison the root mean square and maximum value of errors for all units are 2.7 and 5. Also all of the insulators in the string are contaminated according to the IEC60507 and suspended in the chamber. The same experimental procedure for voltage measurement is conducted in the ambient humidity and heavy pollution level and results are shown in figure 10. The experimental results admit the simulation ones that show, dry pollution doesn’t affect the voltage distribution of the string. 5. Conclusion In this work, a full electric circuit of insulator string, which takes into account the insulator material properties and the stray capacitances, was derived. The capacitances values are calculated using a finite-element package. The circuit was implemented in the EMTP-ATP package in order to simulate the voltage distribution and the leakage currents flowing through the insulator string under various scenarios of pollution. Also, indoor laboratory experiments for the investigation of pollution severity on potential distribution along a 230 kV I-string insulator have been carried out in low humidity that made a dry pollution on insulator surface. A very satisfactory agreement has been ascertained when comparing experimental results with results from simulations. The results show the voltage distribution and leakage current on pollution condition didn’t change remarkably in comparison with the clean condition. As can be concluded, when pollution level increased to heavy, the electric potential over the unit nearest to the line conductor decreased from 19.98 % to 19.9 %, which had a reduction of about 0.4%. Although power loss on insulator string in clean and pollution condition is very low, it changes from 0.031 W in clean condition to 2.467 W in heavy pollution condition. 6 References 1. Asenjo. E, Morales N, Valdenegro., 1997. Solution of low frequency complex fields in polluted insulators by means of the finite element method, IEEE Transactions on Dielectrics and Electrical Insulation. doi:10.1109/94.590856. 2. Ashouri. M, Mirzaie. M, Gholami. A, 2010. Calculation of Voltage Distribution along Porcelain Suspension Insulators Based on Finite Element Method, Electric Power Component System. doi: 10.1080/15325000903489694. 3. Dhalaan, S.M.A.; Elhirbawy, M.A., 2003. Investigation on the characteristics of a string of insulator due to the effect of dirt, Transmission and Distribution Conference and Exposition, IEEE PES; September., 3, 915 – 920. doi: 10.1109/TDC.2003.1335059. 4. Dhalaan, S.M.A. Elhirbawy., M.A. 2003. Simulation of voltage distribution calculation methods over a string of suspension insulators, Transmission and Distribution Conference and Exposition, IEEE, PES; 3, 909 – 914. doi: 10.1109/TDC.2003.1335058. 5. Faisal, Sh.M, 2011. 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