Comparison of effects of Various Contaminations on SiR insulators

Transcription

1 Volume 120 No , ISSN: (on-line version) url: Comparison of effects of Various Contaminations on SiR insulators Satheesh Gundlapalli 1,B. Basavaraja 2, Pradeep M. Nirgude 3 1,2 Department of E.E., 3 UHV Research Laboratory 1 S R Engineering College Warangal, India. 2 University B.D.T. College of Engineering Karnataka, India. 3 Central Power Research Institute Telangana, India. 1 satheeshtrrec@gmail.com October 12, 2018 Abstract The essential part of High Voltage (HV) power transmission system is an insulator. Its unsatisfactory performance will result in considerable loss of capital. The main aim of this paper is to compare the effect of pollution on flash over voltage (FOV) of 11 KV straight and alternate shed silicone rubber (SiR) insulators under different scenarios. The different scenario conditions are clean condition and various contamination conditions. These scenarios are seen in the field with in-service insulators located near industries. In the first case, two types of insulators were tested under clean condition and in the second case the insulator samples were tested considering only cement dust along their surfaces. In the remaining cases the insulators were tested with plywood dust, cement dust water drops, water drops 1 609

2 only and plywood dust water drops. Above tests revealed that in polluted environments alternate shed insulators have higher flash over voltages compare to straight shed insulators. Hence in polluted environments alternate shed insulators are to be used compared to straight shed insulators. So, we can predict the scheduled maintenance or replacement of the SiR insulators under different environments. Key Words:: Breakdown voltage; Contamination; Insulator; Dust; Water 1 Introduction Electrical insulators support the conductors and they must withstand normal voltages, various operating conditions and environments. Overhead line insulators are being subjected to various operating conditions and environments. So insulator surfaces were covered by pollutants due to natural or industrial or even combination of both. Contamination on the surface of the overhead line insulators enhances the chances of flashover [1]. The contamination is little important in dry conditions because the contaminated surfaces do not conduct under dry periods. Because of the presence of ionic solids the pollution layer becomes conductive as the insulator surface becomes moist because of rain, dew or fog [2]-[3]. The most important problem in power transmission is flashover observed on insulators used in HV power transmission [4]. On the other hand, the flashover of polluted insulators can cause transmission line outage. The flashover of polluted insulators was the motivation for the installation of a test station in order to perform tests on polluted insulators [5]. The insulator begins to fail when the pollutants that exist in the air settle on the surface of the insulator and combine with the humidity of the rain, fog or dew. The mixture of pollutants and humidity form a layer that become conductor and allows passage of currents that will facilitate the conditions of short circuit [6]-[7]. This is due to decrease of the resistance of insulator surface. Unless there is an adequate maintenance or natural cleaning, the electrical activity will be affected by a flashover on the insulator

3 2 SIR INSULATOR Insulators are used in electrical power transmission to contain, suspend and separate the conductors. Composite insulators did not come out until 1970s, and Germany is the first country developing and using this kind of insulators. Compared to conventional ceramic insulators silicon rubber (SiR) insulators offer more advantages in their application. Hence it is very advantageous to use SiR Insulators. So, to analyze their characteristics, SiR insulators are tested with different types of dusts. Structure of SiR insulator is shown in Fig. 1. The basic design of a SiR insulator is as follows; fiber reinforced plastic core (FRP), attached with two metal fittings is used as the load bearing structure. The presence of moisture and dirt in combination with stress results in the occurrence of discharges causing deterioration of the material such as erosion and tracking. In order to protect the FRP core from various stresses, such as acid, ultraviolet, ozone etc., and to provide a leakage distance within a limited length of insulator under wet and contaminated conditions, weather sheds are installed outside the FRP core. SiR is mainly used for polymeric insulators as housing material. Fig. 1. Structure of SiR insulator 3 EXPERIMENTAL SET UP AND TEST PROCEDURE The tested straight and alternate shed insulators have an insulating part made up of silicone rubber material. The tests are conducted on above two types of 11kV silicone rubber insulators. The measurement of the breakdown voltage set up consisted of cascaded transformer, HV bus bar and insulators to be tested and control panel with required metering. The specifications of the cascaded transformer are: 3 611

4 Number of stages : 2 KVA rating : 50 KVA Voltage generated per unit : 250 KV Total voltage generated : 500KV Cooling method : Oil cooling A. Circuit Diagram The insulators are tested in the high voltage laboratory and in the testing process the flashover voltages corresponding to the applied voltage are noted down until the breakdown of the insulators occurs. The circuit diagram for the Flashover Voltage (FOV) measurement is as shown in Fig. 2. Fig. 2. Circuit diagram for testing insulators in the HV laboratory B. Test Procedure The procedure for measuring the FOV of SiR insulators in the HV Laboratory is as follows: The one end of test specimen i.e. 11 kv straight shed insulator is connected to high voltage side and other end to the ground as shown in Fig. 2. Source voltage of power frequency is applied across the insulator. The source voltage is increased at uniform rate in steps of 5 or 10 kv until breakdown occurs. Observe the breakdown voltage accurately as it suddenly goes off to zero after breakdown. Tabulate the readings of voltages at breakdown under clean and various contaminated conditions. Repeat the same process for alternate shed insulator. The main precaution is that the grounding must be properly given and also the readings should be noted down with care. C. Experimental Set up 4 612

5 After washing the insulator samples with water and kept dry for 24 hours, they are placed in the experimental setup with a ground clearance of approximately two meter. One end of the insulator was connected to high voltage side and the other end to ground as shown in Fig. 3. The occurrence of flashover of 11 kv straight and alternate shed insulators is as shown in Fig. 4. They are energized to measure the flashover voltages under clean and contaminated conditions. Fig. 3. (a) straight shed insulator (b) alternate shed insulator 5 613

6 Fig. 4. Flashover of (a) straight shed insulator (b) alternate shed insulator 4 TEST RESULTS A. Testing of Alternate shed type insulator under clean and various polluted conditions The Alternate shed type insulator is tested without and with various contaminations on its surface to study the variation of the breakdown voltage. Following are the test results (flashover voltages) of insulator without and with various contaminations on its surface. TABLE I. FLASHOVER VOLTAGE UNDER VARIOUS CONTAMINATION CONDITIONS FOR 11KV ALTERNATE 6 614

7 SHED INSULATOR The plot drawn between flashover voltage and various dust conditions using Table I is shown in Fig. 5 Fig. 5. Flashover voltages of 11kV alternate shed insulator under clean and various contaminated conditions From Fig. 5 it is observed that the flashover voltage of Alternate shed insulator is reduced by 2.35 % under cement dust compared to clean condition, 5.88 % under plywood dust compared to clean condition, % under cement dust and water drops compared to clean condition, % under water drops compared to clean condition and % under plywood dust and water drops compared to clean condition. Therefore flashover voltage is less for polluted insulator compared to unpolluted insulator. B. Testing of Straight shed type insulator under clean and various polluted conditions The straight shed type insulator is tested without and with various contaminations on its surface to study the variation of the breakdown voltage. Following are the test results (flashover voltages) of insulator without and with various contaminations on its surface. TABLE II. FLASHOVER VOLTAGE UNDER VARIOUS 7 615

8 CONTAMINATION CONDITIONS FOR 11KV STRAIGHT SHED INSULATOR Fig. 6. Flashover voltages of 11kV straight shed insulator under clean and various contaminated conditions From Fig. 6 it is observed that the flashover voltage of Straight shed insulator is reduced by 2.63 % under cement dust compared to clean condition, 6.58 % under plywood dust compared to clean condition, % under cement dust and water drops compared to clean condition, % under water drops compared to clean condition and % under plywood dust and water drops condition compared to clean condition. Therefore flashover voltage is less for polluted insulator compared to unpolluted insulator. C. Analysis of results A comparison is made between the flashover voltages of the two types of insulator samples under different contamination conditions. The plot drawn between flashover voltage and various dust conditions is shown in Fig

9 Fig. 7. Comparison of flashover voltages of 11kV alternate and straight shed insulators under clean and various contaminated conditions From the plot in Fig. 7 the flashover voltages of alternate shed insulator are higher than that of straight shed type insulators under different polluted conditions. 1. The flashover voltage of alternate shed insulator is % higher than that of straight shed insulator under clean condition. 2. The flashover voltage of alternate shed insulator is % higher than that of straight shed insulator under cement dust condition. 3. The flashover voltage of alternate shed insulator is % higher than that of straight shed insulator under plywood dust condition. 4. The flashover voltage of alternate shed insulator is % higher than that of straight shed insulator under cement dust and water drops condition. 5. The flashover voltage of alternate shed insulator is % higher than that of straight shed insulator under water drops condition. 6. The flashover voltage of alternate shed insulator is % higher than that of straight shed insulator under plywood dust and water drops condition. 5 Conclusion The flashover voltage measurement is done on 11 kv alternate shed and straight shed insulators without and with contaminations. The contamination conditions considered are: (1) Clean condition, (2) Cement dust condition, (3) Plywood dust condition, (4) Cement dust and water drops condition, (5) Water drops condition, and (6) Plywood dust and water drops condition. The contamination conditions considered are similar to pollutions that exist on overhead line insulators in industrial areas. The flashover voltages in case 9 617

10 of alternate shed insulators with and without pollution are higher than straight shed insulators. Also flash over voltage is higher for polluted insulator compared to unpolluted insulator. It is observed that the flashover voltage in case of plywood dust water contaminated condition is lower than that of the other conditions in both types of insulators. Similarly, the flashover voltage is less for conditions containing water drops than the other conditions i.e. clean, cement dust and plywood dusts. This is because of the fact water and water along with dusts has high conductivity value than the other conditions. From the above facts it is observed that the insulators are to be designed based on environment present around the insulator. Also in polluted environments alternate shed insulators are to be used compared to straight shed insulators. From the results obtained we can predict the schedule maintenance or replacement of the insulators under different contamination conditions. References [1] M. T. Gencoglu and M. Cebeci, The pollution flashover on high voltage insulators, Electric power systems research, vol. 78, no. 11, pp , November [2] V. Vinayaka Rao and D. Pradip kumar, Electric field computation of 400kv ac porcelain string insulator, International Journal of Electrical Engineering Technology, vol. 3, no. 2, pp , July-September [3] V. Jayaprakash Narayanan, M. Sivakumar, K. Karpagavani and S. Chandrasekar, Prediction of Flashover and Pollution Severity of High Voltage Transmission Line Insulators Using Wavelet Transform and Fuzzy C-Means Approach, Journal of Electrical Engineering and Technology, vol. 9, no. 5, pp , September [4] S. A. Suflis, I. F. Gonos, F. V. Topalis and I. A. Stathopulos, Study of the dielectric behaviour of non-uniformly polluted insulators, XIII th international symposium on High voltage engineering, School of Electrical and Computer Engineering, Electric Power Department, High Voltage Laboratory

11 National Technical University of Athens, Greece, Netherlands, Smit (ed.), Rotterdam, pp. 1-4, [5] M. A. M Piah and A. Darus, Modelling leakage current and electric field behavior of wet contaminated insulators, IEEE Transactions on power delivery, vol. 19, no. 1, pp , [6] P. Jirapong and W. Thipprasert, Electrical Performances of Line Post Insulators in 22kV Distribution System, International Journal of Electronics and Electrical Engineering, vol. 4, no. 2, pp , April [7] S. Venkataraman, R.S. Gorur and A.P. Mishra, Impact of weathering on flashover performance of nonceramic insulators, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 15, no. 4, August

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