EXPERIMENTAL INVESTIGATION OF EFFECT OF UV RADIATION ON FLASHOVER VOLTAGE OF POLYMERIC INSULATORS WITH AND WITHOUT CONTAMINATION

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1 EXPERIMENTAL INVESTIGATION OF EFFECT OF UV RADIATION ON FLASHOVER VOLTAGE OF POLYMERIC INSULATORS WITH AND WITHOUT CONTAMINATION A. Majzoobi *, I. A. Joneidi, S. Mohajer, H.Mohseni and A. A. Shayegani High Voltage Lab, School of Electrical & Computer Eng., University of Tehran, Iran * Abstract: In recent years Polymeric insulators have been found wide spread usage in all voltage levels in IRAN, especially in southern areas, due to environmental condition and high contamination in these areas. The main property that led to this increasing tendency toward this kind of insulators is enhancement of contamination flashover voltage through hydrophobicity property of polymeric insulators. Environmental factors such as Ultra violent (UV) radiation, affect the hydrophobicity property of polymeric insulators and reduction of this property can reduce flashover voltage of insulators. In this paper two tests have been done on polymeric insulators. In the first test flash over voltage of insulators has been measured in different humidity condition. In the second test, five kinds of insulators have been exposed to UV radiation for 160 days. Then the flashover voltage of aged insulators has been measured in different levels of contamination. The results showed that in the low humidity condition, humidity does not have any meaningful effect on flashover voltage, but in the high humidity, reduction in flashover voltage has been observed. Results of the other test showed that hydrophobicity of aged insulators start to decrease during the time and increasing in conductivity of contamination, lead to reduction of flashover voltage. 1 INTRODUCTION Polymeric insulators have been used in outdoor service for about sixty years. bisphenol epoxy resins have been used commercially for indoor applications in the mid 1940's and in the 1950's, cycloaliphatic epoxy resins have been used in the United Kingdom in outdoor service due to their better performance.[1] insulators for transmission lines have been developed as early as 1964 in Germany, and by other manufacturers in England, France, Italy, and the U.S [2]. In recent years, a lot of researches have been done in performance evaluation of polymeric insulators especially about advantages and disadvantages of these insulators compared to ceramic ones [1,3,5]. The most important advantages are [1]: 1- Reduction in weight up to 90% which can benefit towers design and transportation and installation of insulators economically. 2- Higher mechanical strength. 3- Reduction of serious damage from vandalism such as gunshots, etc. 4- Much better performance in the presence of heavy pollution. 5- Better withstand voltage in polluted condition. 6- Reduction of the maintenance costs such as insulator washing which is often required for ceramic insulators in contaminated environment. The main disadvantages of polymeric insulators are [1, 9]: 1- They are subject to chemical changes on the surface due to weathering and from dry band arcing. 2- Suffer from erosion and tracking which may lead to failure of the insulator. 3- Life expectancy is difficult to evaluate. 4- Detection of faulty insulators is difficult. Despite these disadvantages, advantages of polymeric insulators persuade electric power companies to use them in power transmission and distribution system. So in recent years Polymeric insulators have been found wide spread usage in all voltage levels including distribution networks, overhead transmission lines and high voltage equipments in IRAN. Especially polymeric insulators are increasingly being used in southern areas of IRAN. Environmental condition and high contamination in these areas are the most important reason for installation of this kind of insulator instead of ceramic ones. The main property that led to this increasing tendency toward this kind of insulators is enhancement of contamination flashover voltage through hydrophobicity property of polymeric insulators. Another reason for this growing tendency is an up to 90% reduced weight of them compared to ceramic ones. With increasing usage of polymeric insulators, concerns about the performance of these insulators after aging of polymeric materials have been raised. Hydrophobicity is one of the major advantages of polymeric insulators because it can increase the flashover voltage of polymeric insulators in the wet and contaminated condition,

2 Table 1: Specification of samples NO. of Sheds 7 10 Creepage distance(mm) compared to ceramic ones. Environmental condition such as UV radiation affect the hydrophobicity property of polymeric insulators and reduction of this property can reduce flashover voltage of insulators. Owing to importance of effect of UV radiation on performance of polymeric insulators, experimental studies in various conditions have been done in recent years [4, 6, 7]. A(mm) A'(mm) A"(mm) B(mm) B'(mm) C(mm) D(mm) D'(mm) Sample Sample EXPERIMENT SETUP AND PROCEDURE 2.1 Specification of samples All of the samples which used in this experiment were commercial domestic products. Description of samples geometry has been given in Table1. These samples have different design features, but all of them have the same voltage level of 24kV. Figure 1: Specification of samples 2.2 Flashover voltage in fog chamber In this test as shown in "figure 2", cylindrical plastic chamber prepared in High Voltage Lab. Diameter of this chamber is about 1m and its height is 2m. Insulators were placed in the middle of chamber in vertical position. An electrical vapour maker has been used for preparation of fog and humidity in the chamber. Tests have been done in different temperature and humidity. So temperature and humidity were checked and recorded via thermometer and humidity meter continuously. Voltage is applied to upper electrode of insulator and the lower electrode is connected to earth. For measuring the withstand voltage of insulators, voltage must be increased gradually till flashover occur. This voltage will be recorded as flash over voltage of insulator in clean fog condition. Figure 2: Sample in fog chamber in Laboratory. 2.3 Ultra Violet (UV) radiation UV radiation is classified by its wavelength into three types A, B and C [8]: The wavelength of UV-A is between 315 and 400nm, UV-B is between 290 and 315nm and UV- C is less than 290nm. As UV radiation with shorter wavelength is more harmful for insulator, UV-C must be the most problematic but UV-C is filtered by atmosphere, so UV-B is the most problematic radiation. Silicone rubbers have high resistance against UV damage because the photons do not contain enough energy to break the siloxane (Si-O) bonds. UV resistance can be increased with addition of carbon, but carbon reduces insulation property of insulators. So a balance should be between increasing the resistance against UV radiation and reducing the resistance to tracking.[8]

3 In this test 5 samples which their specification mentioned in table1, were exposed to UV radiation. The hydrophobicity of surface usually can be determined by measurement of the contact angle between surface of insulators and water droplets. A metal structure 1m x 1m x 1m was made and 8 UV lamps and insulators were placed in this structure as shown in "figure 3". Figure 3: Samples in expose to UV radiation in Laboratory. UV lamps are UV-C with 40W power. Insulators were exposed to UV radiation for 160 days. Flashover voltage of insulators has been measured two times, after 80 days and 160 days UV aging, in 3 levels of contamination. In order to have various contamination levels, salt s with different electrical conductivity were sprayed over insulators. Electrical conductivity of these s is as follows: Running water: 0.4 ms/cm Salt No.1: 8.5 ms/cm Salt No.2: 18.5 ms/cm Figure 4: Hydrophobicity of new samples. 3 RESULTS AND DISCUSSION 3.1 Visual observation In comparison with new insulators, the aged insulators with UV and even after flashover voltage tests in contaminated condition, there is no visible difference between them. 3.2 Hydrophobicity The surface hydrophobicity property of polymeric insulators is one of the most important parameters which affects the electrical insulation property of insulators directly. On a hydrophobic surface, water appears in droplet form, whereas a hydrophilic surface is easily wetted by water. Reduction in hydrophobicity property leads to decrease of the flashover voltage. Silicone rubber regains its hydrophobicity because of low molecular weight (LMW) polymer chains diffusing through the bulk of the material to the surface forming a thin layer covering over the pollution layer [8]. Figure 5:Hydrophobicity of samples after UV aging

4 In order to analyze changes of hydrophobicity after aging of insulators STRI method has been applied [10]. The Swedish Transmission Research Institute (STRI), has been categorized hydrophobicity of surface to 7 levels, HC1 to HC7. HC7 refers to surface with the least hydrophobicity and water covered surface. With decrease of index of HC from HC7 to HC1, hydrophobicity increase. By comparison between photos of samples of insulators as shown in "figure 4 &5 " and reference pictures which STRI has been published [13], it can be concluded that new insulators belong to HC1 or HC2, whereas hydrophobicity of the aged samples is between HC4 and HC5. So these pictures and conclusions show that UV radiation affects hydrophobicity of insulators obviously. 3.3 Flashover voltage withstand voltage of 3 insulators have been measured in clean fog chamber and in various temperature and humidity condition. The results of tests have been put in table 2-4. In order to increase accuracy of results, tests have been done 3 times in each condition. Table 2: Flashover voltage of sample1 in clean fog chamber. Humidity Temp. 1 st test 2 nd test 3 rd test Table 5: Flashover voltage of new insulators (kv). Dry & Clean condition Type of contamination Running water No No Sample Sample Table 6: Flashover voltage of UV aged insulators, after 80 days(kv). Dry & Clean condition 90 Type of contamination Running water No No Sample Sample Table 7: Flashover voltage of UV aged insulators, after 160 days(kv). Dry & Clean condition 88 Type of contamination Running water No No Sample Sample Table 3: Flashover voltage of sample2 in clean fog chamber. Humidity Temp. 1 st test 2 nd test 3 rd test Table 4: Flashover voltage of sample3 in clean fog chamber. Humidity Temp. 1 st test 2 nd test 3 rd test In the other tests, UV aged samples have been tested in different condition, after 80 days and 160 days of UV aging. In order to make results close to what is happening in practice, insulators which were contaminated for tests after 80 days, were not cleaned and were exposed to UV radiation aging with contamination. The results of these situations have been shown in table 5-7. Figure 6: Arcing of samples in clean fog chamber

5 4 CONCLUSION In the low humidity condition, according to results of tables 2-4, humidity does not have any meaningful effect on flashover voltage, But in the high humidity condition which humidity creates the little drops of water on the surface of insulators, the flashover voltage has been reduced. This reduction in flash over voltage is because of conductive layer of water in the insulator surface. In the test of effect of UV radiation, the following results can be concluded: (a) 1- There is not any difference between flashover voltage of aged insulators and new samples in dry and clean condition. So UV radiation doesn't have any effect on insulators in non-contaminated condition. 2- Contamination with higher electrical conductivity leads to much lower Flashover voltage of insulators. 3- Flashover voltage of insulators has been reduced extremely after 80 days UV aging. (b) 4- Flashover voltage of insulators doesn't have any difference after 160 days rather than 80 days aging. Even it has been increased a little in some cases. Because of contamination which has been remained on the surface of insulators UV radiation doesn't reach to surface of insulators, therefore contamination prevents insulators from further aging and polymeric insulators recover their hydrophobicity property themselves. So unlike previous generation insulators that should be cleaned periodically, presence of contamination on the surface of polymeric insulators have even positive effects. Also it should be mentioned that the rate of aging of insulators reduce as time goes by. 5 REFERENCES Figure 7: Arcing of UV aged insulators. (a):dry and clean condition ; (b): Contaminated with No.1 ; (c): (b): Contaminated with No.2 (c) [1] R. Hackam: "Outdoor HV Composite Polymeric Insulators", IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 6 No. 5, October 1999 [2] E Hall, "History and Bibliography of Polymeric Insulators for outdoor applications", IEEE Transactions on Power Delivery, Vol. 8, No. 1, January 1993 [3] S.M. Gubanski, A. Dernfalk, J. Andersson, H. Hillborg: "Diagnostic Methods for Outdoor Polymeric Insulators", IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 5; October 2007

6 [4] M. Ehsani, H. Borsi, E. Gockenbach, J. Morshedian, G. R. Bakhshandeh, A. A. Shayegani: "Effect of Aging on Dielectric Behavior of Outdoor Polymeric Insulators", Iniernaiiorzal Conference on Solid Dielecrrics, Toulouse, France, July 5-9, 2004 [5] E.G. da Costa, T.V. Ferreira, Max G. G. Neri, I.B. Queiroz, A.D. Germano: "Characterization of polymeric Insulators using Thermal and UV Imaging under Laboratory Conditions", IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 4; August 2009 [6] M. AMIN, M. AHMED: "Effect of UV Radiation on HTV-Silicon Rubber Insulators with Moisture", IEEE, 2007 [7] M. Amin, M. Akbar, R. Matsuoka: " Effect of UV radiation, temperature and salt fog on polymeric insulators", IEEE, 2006 [8] C.A. Spellman, H.M. Young, A. Haddad, A.R. Rowlands and R.T. Waters: " Survey of polymeric insulator aging factors", High Voltage Engineering Symposium, Conference Publication No. 467, IEE, 1999 [9] B. Marungsri, W. Onchantuek, A. Oonsivilai and T. Kulworawanichpong: "Analysis of Electric Field and Potential Distributions along Surface of Silicone Rubber Insulators under Various Contamination Conditions Using Finite Element Method", International Journal of Electrical and Electronics Engineering 2: [10] M. Amin, M. Akbar: "Effect Of Uv Radiations On Heavily Polluted Polymeric Insulators", IEEE ICET nd International Conference on Emerging Technologies Peshawar, Pakistan November 2006 [11] Hydrophobicity Classification Guide, STRI Guide 92/1.