A Study of Critical Positive Impulse Flashover of 24 kv Contaminated Insulators

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1 The 5 th PSU-UNS International Conference on Engineering and 467 Technology (ICET-2011), Phuket, May 2-3, 2011 Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand Study of Critical Positive Impulse Flashover of 24 kv Contaminated Insulators N. Sirimasakul 1*, S. Chotigo 2,. Pungsiri 3 Department of Electrical Engineering, King Mongkut's University of Technology Thonburi 126/1 Prachauthit Road, angmod, Thungkhru angkok Thailand. si.narongsak@gmail.com bstract: This paper studies and compares the dielectric characteristic of various insulator types, line-post class 57-2, pin-post class 56/57-2, pin class 56-2 and suspension class 52-2 used in the distribution systems 24 kv of Metropolitan Electricity uthority (ME) of Thailand. In angkok and nearby area, most insulators are contaminated from sand or soil dust which is winded up from traffic or industries. Most of contaminated ingredient, CaSO 2 and NaCl, are soluble. This contamination causes the insulator to flash over and then the distribution system will be affected. In this present work, five levels of contamination following in IEC , mg/cm 2, 0.05 mg/cm 2, 0.1 mg/cm 2, 0.2 mg/cm 2 and 0.4 mg/cm 2 are used to study the flashover characteristics of insulators. The contamination which consists of kaolin, water and salt in a certain amount is sprayed through the surface of insulators and then waits until the surface is dry. The results show that each type of insulator has different flashover voltages depending on its size and structure. t the highest contamination level, the reductions of flashover voltage from that of the clean insulators are 7.3%, 6.6%, 7.9% and 3.9% for line-post, pin-post, pin and suspension types respectively. The percentage of flash over reduction rate will show the insulator characteristic for resisting the contamination of each type of insulator. surface conductance of insulator will be high, and then the flashover occurs easier. One of a method to reduce the contamination level is cleaning insulator regularly or designing the insulator to be suitable with condition where the insulator will be used. The aim for these types of research is designing and selecting suitable insulators for polluted area. 2. TEST OJECTS ND PROCEDURES 2.1 Insulators In this present work, four types of 24 kv insulators are used: four of line-post class 57-2(called Type I), four of pin-post class 56/57-2(called Type II), four of pin class 56-2(called Type III) and three bunches of suspension class 52-2 (called Type IV) as shown in Fig. 1. Each type of insulator has different manufacturer so the name of each insulator manufacturer is called as, and C. Key words: C insulator, flashover voltage, pollution 1. INTRODUCTION t present, the electrical energy needed in Thailand is higher. The distribution system in Thailand is overhead line therefore insulators are used for electrical and mechanical purposes. However, sometimes surface flashover can occur at insulators due to its contamination which causes fault in the system as seen from [1, 2]. When those problems occur, the distribution system will get damage. This leads to economics disaster, especially in angkok and others provinces around angkok area which will be more serious than other provinces area. The main cause for insulator flashover in distribution system is the contamination accumulation on insulator. When the contamination accumulation is high, the Fig.1. Test insulators 1) Line-Post Type Insulator class 57-2 (Type I) 2) Pin-Post Type Insulator class 56/57-2 (Type II) 3) Pin Type Insulator class 56-2 (Type III) 4) Suspension Type Insulator class 52-2 (Type IV) The properties of each insulator were measured as shown in table I-IV.

2 468 TLE I PROPERTIES OF INSULTOR TYPE I 57-2 Name 1 2 C (a) TLE II PROPERTIES OF INSULTOR TYPE II 56/57-2 C Name 1 2 C TLE III PROPERTIES OF INSULTOR TYPE III C Name 1 2 C TLE IV PROPERTIES OF INSULTOR TYPE IV Name 1 2 Configuration height Experimental Circuit The experimental circuit shown in Fig. 2 consists of 400 kv impulse generator, test object, voltage divider, and oscilloscope Tektronix TDS340. (b) (a) Diagram (b) ctual Circuit 1) Impulse Generator 400 kv, 2) Voltage Divider, 3) Test Object 2.3 Test Procedure Fig. 2. Test circuit of insulators The clean insulator is tested as a reference, then the contamination of insulators is done following IEC [8]. The solvent sprayed on the surface of insulator consists of Kaolin 40 g, water 1 liter and salt which depends on the required conductance. The conductance can be referred as contamination of insulator surface as shown in Table V in this present work which is referred from Table 3 in IEC The solvent is sprayed to the insulator surface using spray gun which connected to pump. The solvent is sprayed to the insulator surface repeatedly, both in vertical and in horizontal. litre of solvent can be roughly sprayed to 5 insulators. fter spraying, the insulator has been left for at least 24 hours until it dries as seen in Fig. 3. Then, the contaminated insulator is tested for surface flashover voltage under power frequency voltage. fter finishing surface flashover voltage test, the insulator is cleaned and then has been left for at least 24 hours before increasing the contamination level. This process is done repeatedly at the contamination level of insulator surface of 0.025, 0.05, 0.1, 0.2 and 0.4 mg/cm 2. The installation of insulators for testing and voltage application follows the method in IEEE [10] and NSI C [9].

3 469 TLE V PPROXIMTE CORRESPONDENCE ETWEEN THE REFERENCE DEGREES OF POLLUTION ON THE INSULTORS ND VOLUME CONDUCTIVITY OF THE SLURRY T TEMPERTURE OF 20 O C Salt Deposit Density SDD (mg/cm 2 ) Volume conductivity of the slurry σ 20 (S/m) Fig. 5. Critical positive impulse flashover voltage of insulator type II Fig. 3. Contaminated insulators 3. TEST RESULTS Test results are shown in two cases. Firstly, the surface flashover voltage is shown as a function of contamination level. Secondly, the average electric field per dry and wet arc s is shown as a function of contamination level. The surface flashover voltage shown in this present work is corrected by the correction factor in NSI C Surface Flashover Voltage of Insulators The critical positive impulse flashover voltage of insulators, Type I IV, for different manufacturers as a function of contamination level, 0.025, 0.05, 0.1, 0.2 and 0.4 mg/cm 2, is shown in Figs Fig. 6. Critical positive impulse flashover voltage of insulator type III Fig. 7. Critical positive impulse flashover voltage of insulator type IV 3.2 The Comparison of Surface Flashover Voltage of Insulators Type I-IV From test results, the maximum critical impulse flashover voltages of each insulator type are plotted as a function of the contamination level as shown in Fig.8. Fig. 4. Critical positive impulse flashover voltage of insulator type I Fig. 8. Comparison of the maximum critical positive impulse flashover voltage obtained from each type

4 Comparison of the veraged Electric Field Strength In this section, results of the averaged electric field strength per dry arc and leakage s will be shown in Figs The averaged electric field strength of insulators per dry arc and leakage s is obtained from eqs. (1) and (2) respectively. E h = U 50% /h (1) E L = U 50% /L (2) E h is averaged electric field strength per dry arc. E L is averaged electric field strength per leakage. U 50% is critical positive impulse flashover voltage. h is dry arc. L is leakage. Fig. 11. The averaged electric field strength per dry arc as a function of contamination level of insulator type III 1) The veraged Electric Field Strength per Dry rc Distance: The arc in eq. (1) is defined as the arc measured when the insulator is dry. The data of this arc for each insulator is shown in Tables I- IV. The averaged electric field strength per dry arc of insulators as a function of contamination level is shown in Figs Fig. 12. The averaged electric field strength per dry arc as a function of contamination level of insulator type IV 2) The veraged Electric Field Strength per Distance: The leakage in eq. (2) is defined as the minimum measured between electrodes. Some part of this leakage will prevent the insulator surface to be wet during the raining so the insulator will be able to with stand the higher voltage. The value of this leakage for each type of insulators is shown in Tables I-IV. The averaged electric field strength per leakage of insulators as a function of contamination level is shown in Figs Fig. 9. The averaged electric field strength per dry arc as a function of contamination level of insulator type I Fig. 13. The averaged electric field strength per leakage as a function of contamination level of insulator type I Fig. 10. The averaged electric field strength per dry arc as a function of contamination level of insulator type II

5 471 3) From results, it shows that the suspension type insulator has the highest surface flashover voltage. This can be explained by their longest arc and leakage s. 4) The type III insulator has the lowest averaged electric field strength per dry arc and the type I insulator has the lowest averaged electric field strength per leakage arc. Fig. 14. The averaged electric field strength per leakage as a function of contamination level of insulator type II Fig. 15. The averaged electric field strength per leakage as a function of contamination level of insulator type III 5. REFERENCES [1] J. M. Seifert, W. Petrusch and H. Janssen, Comparison of the Pollution Performance of Long Rod and Disc Type HVDC Insulators, IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 1, pp , February [2] J.P.Holtzhausen and W.L.Vosloo, The Pollution Flashover of ac Energized Post Type Insulators, IEEE Trans. Elect. Insulation, Vol.8, No. 2, pp , pril [3] Xingliang Jiang, Shaohua Wang, Zhijin Zhang, Shujiao Xie, and Yan Wang, Study on C Flashover Performance and Discharge Process of Polluted and Iced IEC Standard Suspension Insulator String, IEEE Transactions on Power Delivery, Vol. 22, No. 1, pp , January [4] Xingliang Jiang, Jihe Yuan, Lichun Shu, Zhijin Zhang, Jianlin Hu, and Feng Mao, Comparison of DC Pollution Flashover Performances of Various Types of Porcelain, Glass, and Composite Insulators, IEEE Transactions on Power Delivery, Vol. 23, No. 2, pp , pril [5] Fuzeng Zhang, Xin Wang, Liming Wang and Zhicheng Guan, Experimental Investigation on Flashover Performance of Glass Insulator for DC Transmission Lines at High ltitudes, IEEE Conference, pp , February [6] R. Matsuoka, H. Shinokubo, K. Kondo, Y. Mizuno, K. Naito, T. Fujimura and T. Terada ssessment of asic Contamination Withstand Voltage Characteristics of Polymer Insulators, IEEE Transaction, Vol. 11 No. 4, pp , October [7] oudissa, R., Djafri, S., Haddad,., elaicha R., and earsch, R., Effect of Insulator Shape on Surface Discharges and Flashover under Polluted Conditions, IEEE Transaction, Vol. 12 No. 3, pp June [8] rtificial Pollution Test on High Voltage Insulators to be used on.c. Systems, IEC 507, [9] merican National Standard for test method for electrical power insulator, NSI C29.1, [10] IEEE Standard Techniques for High-Voltage Testing, IEEE Std.- 4, Fig. 16. The averaged electric field strength per leakage as a function of contamination level of insulator type IV 4. CONCLUSIONS For the critical positive impulse flashover voltage testing of these four types of insulators, it can be concluded as follows: 1) The critical positive impulse flashover voltage of insulators decreased as the contamination level increases. 2) t the contamination level of 0.4 mg/cm 2, the reduction of the critical positive impulse flashover voltage from that of the clean insulators is 7.3%, 6.6%, 7.9% and 3.9% for line-post type, pin-post type, pin type and suspension type insulators respectively considering from the maximum critical positive impulse flashover voltage of each type. The percentage of flash over reduction rate will show the insulator characteristic for resisting the contamination of each type of insulator.