PERFORMANCE ANALYSIS OF OVERHEAD PORCELAIN INSULATOR UNDER DIFFERENT CONTAMINATIONS

Transcription

1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 10, October 2017, pp , Article ID: IJMET_08_10_028 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed PERFORMANCE ANALYSIS OF OVERHEAD PORCELAIN INSULATOR UNDER DIFFERENT CONTAMINATIONS Rajamohan Jayabal and K. Vijayarekha School of Electrical and Electronics Engineering, SASTRA University, Thanjavur, India ABSTRACT Overhead insulators play an important role in electrical transmission network. When insulators is subjected to operation in open environment their insulation strength decreases. Mixture of pollutants in addition with the humidity forms a layer on the surface which increases conductivity that leads to the flow of leakage current leading to flashover. Artificial test is conducted in the laboratory as per the standard to analysis the performance of insulator. In this analysis an 11 kv porcelain insulator is subjected to power frequency test under different pollution condition. It is clear from the experiments done on the insulators that flashover occurs at lesser voltages when the pollution level is high which in turn increases conductivity. Key words: Ceramic insulator, Contamination, Leakage Current, Flashover. Cite this Article: Rajamohan Jayabal and K. Vijayarekha, Performance Analysis of Overhead Porcelain Insulator under Different Contaminations, International Journal of Mechanical Engineering and Technology 8(10), 2017, pp INTRODUCTION Electrical insulators play vital role in electrical transmission systems [1]. It prevents flow of current from the conductor to ground from its supporting points. When the insulators are subjected to various atmospheric conditions their performance degrades over time. The pollution in the atmosphere gets deposited as a layer on the surface which increase conductivity leads to flow of leakage current thereby leading to partial arcs. Natural and artificial pollution test are two types of pollution test carried out in the laboratory. Since natural pollution test is time consuming and difficult, artificial pollution test based on ESDD (equivalent salt deposit density) values is carried out. In artificial pollution test there are two methods such as salt fog and solid layer method. In solid layer method the insulator is pre-contaminated with sodium chloride and kaolin. Pollution severity is expressed in terms of layer conductivity in micro Siemens or salt deposit density in mg/cm^2. Insulator before test is to be cleaned to remove all traces of pollution and it is erected vertically editor@iaeme.com

2 Rajamohan Jayabal and K. Vijayarekha Minimum clearance between any part and earthed object should not be less than 0.5m per 100 kv. In this analysis 11kV insulator is subjected to power frequency test [7] and various solutions are prepared based on ESDD values are sprayed on the insulator surface. 2. EXPERIMENTAL PROCEDURE: An 11 kv porcelain insulator is used as specimen for the experiment under different levels of contamination. Rated voltage for the test is 11 kv, 50 Hz, single phase AC supply. Block diagram for the power frequency test is shown in figure 1. Voltage is increased slowly upto flashover occurs and various parameters are noted [2, 3, 4]. Test is repeated for various pollution levels and their performances are studied. Figure 1 Power Frequency Voltage withstand Test Block Diagram 2.1. Calculation of ESDD values: Equivalent salt deposit density (ESDD) could be defined as the equivalent NaCl weight of dissolved (i.e. the effective) salt in the wet pollution layer per unit area of insulator. The formula and level of pollution [5, 6] is calculate as below. Table 1 shows the Classification of pollutant level based on ESDD values. Table 1 Classification of pollutant level based on ESDD values Pollution severity ESDD value (mg/cm^2) Very light Light Medium High Very high ESDD = (Sa*v)/A Where, v = volume of the solution used and A= area of the suspension. Sa = [{(5.7*10^(-4)}*c(20)]^1.03 c (20) = c (R) [1-b(R-20)] Where, R = Solution temperature and C (R) = conductivity at that particular temperature Considering a sample of Volume=500 ml and salt [NaCl] = 5gm; C (R)[Conductivity in solution temperature] =11.8 ppt TDS = 6.9; 1 micro Siemen=1.56 ppm; 1 ppt =1000 ppm. Conductivity in micro Siemens/cm [C(R)] = editor@iaeme.com

3 Performance Analysis of Overhead Porcelain Insulator Under Different Contaminations Therefore substituting in the formula by considering the b value from the standard C (20)=c(R)[1-b(R-20)] B = ; R = 30 deg Celcius; C (20) = micro Siemen/cm Sa = [{(5.7*10^(-4)}*c(20)]^1.03; Sa = 9.05(kg/m^3) ESDD = (Sa*v)/A ESDD = mg/cm^2. Hence this pollutant is of Medium severity. Similarly other types of pollution is prepared and applied on the specimen. 3. RESULTS 3.1. Case 1: Low level pollution respectively. The voltage obtained across the insulator is the secondary voltage of the three phase step up transformer of rating 300kV, 300KVA respectively. The insulator behaves as a conductor after flashover followed by breakdown. The pollutant added is based on their ESDD (Equivalent Salt Deposit Density) values. The low level pollution has an ESDD value in the range of respectively. Supply voltage is increased gradually in steps and various values are noted. The insulator is corona free, both audio and video, till 28.5kV. The Audio scintillation starts at the voltage of 32.4kV with a hissing noise. When the voltage is further increased to 50kV, 59kV higher audio scintillation and video scintillation at the bottom shed occurs respectively. On increasing to 80.2kV tracking starts at the bottom shed of the insulator. Video scintillation at the top shed starts occurring when attaining a voltage of 88.7kV and when it is further increased to 95.2kV results in tracking at both the surfaces. This phenomenon of scintillation occurs at higher voltages because of the low pollution level. On further increasing of voltage, flashover occurs at 99kV and voltage after reflection rises to 129kV. If the voltage is increased after this value breakdown occurs. Below figure 2 shows the graph of secondary voltage versus secondary current. Figure 2 low level pollution 3.2. Case 2: Medium level pollution respectively. The medium level pollution has an ESDD value in the range of respectively. Supply voltage is increased gradually in steps and various values are noted The insulator is corona free, both audio and video, till 25kV. The Audio scintillation starts at the editor@iaeme.com

4 Rajamohan Jayabal and K. Vijayarekha voltage of 30.2kV with a hissing noise. When the voltage is further increased to 46kV, 50.2kV higher audio scintillation and video scintillation at the bottom shed occurs respectively. On increasing to 62kV tracking starts at the bottom shed of the insulator. Video scintillation at the top shed starts occurring when attaining a voltage of 81.6kV and when it is further increased to 84.9kV results in tracking at both the surfaces. This phenomenon of scintillation occurs at moderate voltages because of the Medium pollution level. On further increasing of voltage, flashover occurs at 88kV and voltage after reflection rises to 98kV. If the voltage is increased after this value breakdown occurs. Pollutant applied is of Medium level, breakdown occurs earlier when compared with low level pollutant condition shown in figure 3. Figure 3 Medium level pollution 3.3. Case 3: Heavy level pollution respectively. The high level pollution has an ESDD value in the range of respectively. Supply voltage is increased gradually in steps and various values are noted. The insulator is corona free, both audio and video, till 22kV. The Audio scintillation starts at the voltage of 25kV with a hissing noise. When the voltage is further increased to 41kV, 49kV higher audio scintillation and video scintillation at the bottom shed occurs respectively. On increasing to 57kV tracking starts at the bottom shed of the insulator. Video scintillation at the top shed starts occurring when attaining a voltage of 63kV and when it is further increased to 76.3kV results in tracking at both the surfaces. This phenomenon of scintillation occurs at lower voltages because of the heavy pollution level. Since dielectric strength is inversely proportional to the pollution level the dielectric strength in this case decreases to almost zero. On further increasing of voltage, flashover occurs at 80kV and voltage after reflection rises to 82kV. If the voltage is increased after this value breakdown occurs. Since the pollutant applied is of heavy level, breakdown occurs earlier when compared with medium and low level pollutant condition shown in figure 4. Figure 4 Heavy level pollution editor@iaeme.com

5 Performance Analysis of Overhead Porcelain Insulator Under Different Contaminations 4. CONCLUSION In this analysis the behaviour of insulator is done. Artificial pollution test is conducted on insulator. Test procedure is repeated for various atmospheric conditions by changing the pollutants used. From the experimental investigations done on 11kV porcelain insulators at normal and different pollution levels with power frequency voltage the following conclusions were drawn: 1. As the pollution increases, the withstand voltage decreases 2. For low level pollution the flash over occurs at 98 kv, whereas for medium level pollution the flash over occurs at 84 kv and for heavy level pollution the flash over occurs at 75 kv. 3. As pollution level increases the leakage current at a particular voltage also increases. 4. Mobility of ions in the solution plays an important role. As mobility increases the surface resistance of the insulator decreases. 5. When the pollution gets dried on the surface of the insulator, conduction stops and a dry band is formed on the surface of insulator. ACKNOWLEDGEMENT The authors thank the Management of SASTRA University and DST-FIST(Sanction order ref: SR/FST/ETI-338/2013(C) dated 10/09/2014) for their motivation and financial support for creating the facilities at Our HV lab of SASTRA, with which the experimental works for this research works were carried out. REFERENCES [1] J.S.T. Looms, Insulators for high voltages, IEE series, [2] R.S. Gorur and H.M. Schneider, Surface resistance measurements on non-ceramic insulators, IEEE Trans. Power Delivery, Vol.16, pp , Oct [3] Cavallini, G. C. Montanari, A. Contin and F. Puletti, A new approach to the diagnosis of solid insulation systems based on PD signal inference, IEEE Electr. Insul. Mag., vol. 19, pp , [4] S. Chandrasekar, C. Kalaivanan, Gian Carlo Montanari and Andrea Cavallini, Partial Discharge Detection as a Tool to Infer Pollution Severity of Polymeric Insulators, IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 1; February 2010.pp [5] IEC 60507, Artificial pollution tests on high voltage insulators to be used on AC systems, [6] IS 8704 (1995) Artificial Pollution on High Voltage Insulators to be Used on AC Systems [ETD 6 Electrical Insulators and Accessories]. [7] IS (1993) High voltage test techniques, Part 1 General definitions and test requirements [ETD 19 High Voltage Engineering] [8] Vinayaka V Rao and Pradipkumar Dixit, Electric Field Computation of 400kv Ac Porcelain String Insulator, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 2, July September (2012), pp [9] NagaRaju Pendam, C. P. Vardhani, Design and Simulation of Optical Power Splitter with S-Bend Using Silicon-on-Insulator Platform and Study the Transmitted Power with Variation of Refractive Index Difference, International Journal of Electronics and Communication Engineering & Technology (IJECET), Volume 5, Issue 8, August (2014), pp editor@iaeme.com