C5 | Insulator (Electricity) | Ultraviolet

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insulator paper
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  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 *Email: <A.Majzoobi@ece.ut.ac.ir> 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, XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011  Table 1:  Specification of samples 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]. 2 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 UVradiation and reducing the resistance to tracking.[8] Parameter NO. of Sheds Creepage distance(mm)  A(mm) A'(mm) A (mm) B(mm) B'(mm) C(mm) D(mm) D'(mm) Sample1 Sample2 Sample3 7 10 6 670 600 600 190 275 240 100 110 110 110 120 130 55 75 90 27.5 25 45 22 23 22 40 25 35 30 15 28 Sample4 6 680 320 100 110 50 50 22 40 40 Sample5 6 640 300 100 110 90 45 22 40 25 XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011  In this test 5 samples which their specification mentioned in table1, were exposed to UV radiation.  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 solutions with different electrical conductivity were sprayed over insulators. Electrical conductivity of these solutions is as follows: Running water: 0.4 ms/cm Salt solution No.1: 8.5 ms/cm Salt solution No.2: 18.5 ms/cm 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]. The hydrophobicity of surface usually can be determined by measurement of the contact angle between surface of insulators and water droplets. Figure 4: Hydrophobicity of new samples. Figure 5: Hydrophobicity of samples after UV aging XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011  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. Sample1 Humidity Temp. 1 s  test 2 n  test 3 r   test 25 54 69 25 25 27 94.6 95 92 96.3 94.5 90 100 97 95 95 29 84 81 87 Table 3:  Flashover voltage of sample2 in clean fog chamber. Sample2 Humidity Temp. 1 s  test 2 n  test 3 r   test 37 55 70 25 26.5 28 110 108.5 106 110 110 105.3 112 109 104 96 29 93 94 91 Table 4:  Flashover voltage of sample3 in clean fog chamber. Sample3 Humidity Temp. 1 s  test 2 n  test 3 r   test 25 54 70 26 25 28 108.8 107 104.5 108 107.5 107 107.5 106 103 97 30 92.5 95 94 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. Table 5:  Flashover voltage of new insulators (kV). Parameter Type of contamination Dry & Clean condition Running water solution No.1 solution No.2 Sample1 Sample2 Sample3 96 104 102 91 100 99 68 49 70 43 22 29 Sample4 114 110 90 65 Sample5 112 109 77 40 Table 6:  Flashover voltage of UV aged insulators, after 80 days(kV) . Parameter Type of contamination Dry & Clean condition Running water solution No.1 solution No.2 Sample1 Sample2 Sample3 90 100 102 84 95 97 58 30 56 35 13 17 Sample4 115 108 70 45 Sample5 108 105 55 20 Table 7:  Flashover voltage of UV aged insulators, after 160 days(kV) . Parameter Type of contamination Dry & Clean condition Running water solution No.1 solution No.2 Sample1 Sample2 Sample3 88 100 98 78 96 94 55 35 54 35 16 19 Sample4 110 108 76 48 Sample5 110 108 62 24 Figure 6:  Arcing of samples in clean fog chamber XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011
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