International Journal of Advanced Research in Engineering and Technology (IJARET) Volume 12, Issue 1, January 2021, pp.962-970 Article ID: IJARET_12_01_088 Available online at http://iaeme.com/Home/issue/IJARET?Volume=12&Issue=1 ISSN Print: 0976-6480 and ISSN Online: 0976-6499 DOI: 10.34218/IJARET.12.1.2020.088 © IAEME Publication Scopus Indexed COMPARATIVE INVESTIGATIONS ON BREAKDOWN VOLTAGE OF PVA AND PVP PRECOMPRESSED PRESSBOARDS USED IN HV POWER TRANSFORMERS- AN EXPERIMENTAL APPROACH Ashok Kumar S and Dr. Mohd. Z. A. Ansari Department of Electrical and Electronics Engineering, Ghousia college of Engineering, Ramanagara, Karnataka, India ABSTRACT The most significant characteristics of high voltage power transformers are capability to withstand breakdown voltage through faults in insulation system. Dielectric breakdown cause degradation of insulation system and study of its performance can give useful data regarding the condition of insulation. In this paper, the experimental investigations on Breakdown voltage were carried out at various temperatures on different thickness of laminated precompressed pressboards prepared by an insulating polymers polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP). A test were conducted on a transformer model insulation comprising of precompressed pressboards impregnated with transformer oil kept between electric field electrodes arrangement for its behavior for high voltage applications. A variable high voltage AC were applied to test samples until breakdown occurs in the form of puncture and the readings were recorded. Investigations were done for different thickness samples at various temperatures and the results obtained shows superiority of PVP over PVA precompressed pressboards in regards to withstanding the breakdown voltage. Key words: Precompressed pressboard (PCPB), Polyvinyl alcohol (PVA), Polyvinylpyrrolidone (PVP), Power transformers, High Voltage (HV), Breakdown voltage (BDV). Cite this Article: Ashok Kumar S and Mohd. Z. A. Ansari, Comparative Investigations on Breakdown Voltage of PVA and PVP Precompressed Pressboards Used in HV Power Transformers- An Experimental Approach, International Journal of Advanced Research in Engineering and Technology (IJARET), 12(1), 2021, pp. 962-970. http://iaeme.com/Home/issue/IJARET?Volume=12&Issue=1 http://iaeme.com/Home/journal/IJARET 962 editor@iaeme.com Ashok Kumar S and Mohd. Z. A. Ansari 1. INTRODUCTION HV power transformers are the main components in power system network. These expensive systems should operate for long periods without any issues and failure of components can cause significant losses in the system. Among these insulation failure will leads to major issue, the insulation system includes kraft paper, presspaper, pressboards and laminated precompressed pressboards made up of cellulose materials [1]. Precompressed pressboards (PCPBs) are considered as one of the main insulating material used in between the windings and spacers in power transformers. It provides not only electrical insulation strength but also the structural strength to the windings. Therefore, structural behaviour of precompressed pressboards has remarkable impact on the arrangement of power transformer windings as shown in Figure 1, even though PCPBs take up a great amount of volume in the solid insulation material and are impregnated in transformer oil. As a result usage of insulation is comparatively easy due to its variable thickness, size of insulating kraft paper and pressboard. However, the laminated precompressed pressboard with increased thickness and size leads to more challenges with regard to its usage and its impregnation procedures. In order to obtain better insulation properties on oil filled transformers, conventional methods of impregnation for a variety of mineral have been very much evolved by tests and long stretch of understanding[2]. Figure 1 Precompressed pressboard insulation arrangement in HV power transformer windings. The cellulose material would be impregnated quicker because of normal comparable ducts would be greater. Since the impregnation time is a function of viscosity of oil. Hence for faster impregnation lower viscosity of oil is preferred. Most of the failures in high voltage power transformer are due to insulation faults [3]. To avoid these faults, continuous evaluation of the state of the transformer insulation is a major concern. The reliability and quality of transformer insulations are determined by its breakdown voltage [4]. Therefore it is most important parameter to find the breakdown voltage of insulation which gives further information regarding the life of transformer. The basic essential inputs of the large size power transformers are the insulating materials and it becomes very much important to maintain their state and condition. In the present work, different thickness of insulating precompressed pressboard samples has been selected and prepared and immersed in transformer oil to impregnate. Further, electrical breakdown experiments are carried out on impregnated precompressed pressboard samples in insulating oil at various temperatures. The extension of the voltage stresses due to the insulation breakdown will be important parameters during the design of the insulation system in power http://iaeme.com/Home/journal/IJARET 963 editor@iaeme.com Comparative Investigations on Breakdown Voltage of PVA and PVP Precompressed Pressboards Used in HV Power Transformers- An Experimental Approach transformers and this work can lead to the proper design of the insulation to initiate appropriate corrective methods for the outages and failure of large power transformers in operation. 2. PRECOMPRESSED PRESSBOARDS (PCPBS) Precompressed pressboards are widely used insulating material for variety of components in electrical, mechanical and thermal design of transformers. It is a cellulose-based material, manufactured by laminating and pressing of multi-layer of pressboards together with suitable adhesives under high pressure and temperature. Due to its excellent insulating properties, high strength and friendly nature with transformer oil, it is used for load bearing structures and dielectric insulation for high voltage power transformers. Previously several scholars have studied their performance of such cellulose products. Now a days, the transformer manufactures started to use extensively developed quantities of laminated precompressed pressboards products as an insulation in between the windings and spacers. The different thickness of laminated precompressed pressboards are shown in Figure 2. The main requirements considered for various precompressed pressboard types existing are defined as per IEC specification 60641-3-1. Figure 2 Laminated precompressed pressboards. 3. EXPERIMENTAL ANALYSIS 3.1 Sample Preparation For Experimental investigations, two different water soluble insulating polymers PVA and PVP K-90 were chosen and are classified as synthetic or natural polymers. These polymers has lengthier molecular chain that plays an important role in material bonding, thus relying on their molecular weight. The insulating polymers can be obtained in different grades, densities, molecular weights. Depending upon their molecular weights K values are expressed for PVP which are derived from relative viscosity measurements represents a function of the average molecular weight. The polymers have various uses, predominantly used for bonding of insulating papers and boards utilized as protection of HV power transformers in modern applications. Insulating polymer gel is prepared by adding gradually 15gm of polymer material in 100ml of hot water at about 90o C. A magnetic stirrer is used for stirring at different speed until a homogeneous gel type solution is obtained. The precompressed pressboards are manufactured by using high purity unbleached soft wood fibers mixed with water to form a pulp and it is refined, which is continuously drawn on to a board machine to form pressboard ranging a thickness from 0.5 mm to 6 mm. The wet pressboard are compressed in a hydraulic presser to remove moisture content. The pressboards are laminated to get required thickness by using obtained insulating polymer gel type solution. It is again compressed in presser to get PCPBs. http://iaeme.com/Home/journal/IJARET 964 editor@iaeme.com Ashok Kumar S and Mohd. Z. A. Ansari The PCPBs samples used in this work has been manufactured according to the international standard IEC 60641-3-1. The process of manufacturing PCPBs is represented in flow diagram as shown in Figure 3. Figure 3 Manufacturing Process of PCPB. 3.2 Experimental Setup Figure 4 Schematic diagram of experimental setup. Figure 4 shows the schematic diagram of experimental setup and electrode configuration arrangement. The test samples of different thickness having a dimensions of 100mm x 100mm of PVA and PVP PCPBs were selected to test in the laboratory. The test samples were dried out in vacuum chamber to remove the impurities like moisture and gas. The testing oil was filtered using an oil filter and samples are immersed in transformer oil for 24 hours to impregnate. The impregnated test samples were placed in between electrodes in cell in which the experiments are carried out and the cell is filled with virgin transformer oil. The oil is heated to the required temperature with resistive heating coil provided in cell and temperature of the oil is measured using temperature sensor. The test cell is adequate in size to hold the samples and proper interlocks should be provided to avoid accidental contacts with any electrically energized parts. The test circuit arrangement to determine breakdown voltage was setup according to ASTM D149-09. The two cylindrical electrodes HV and LV are made of copper 25mm and 100mm diameter respectively and 50mm thickness have been used for experimental investigations. The LV electrode on bottom side is grounded whereas the HV electrode is provided with variable High voltage AC. The gap distance between two electrodes can be adjusted by a set of thickness http://iaeme.com/Home/journal/IJARET 965 editor@iaeme.com Comparative Investigations on Breakdown Voltage of PVA and PVP Precompressed Pressboards Used in HV Power Transformers- An Experimental Approach gauges. Precompressed pressboard samples were placed in between the electrodes in tight contact with it. The test samples were meant to represent the transformer inter turn and inter coil insulation. Figure 5 Electrode configuration cell arrangement. Figure 5 shows the electrode configuration cell arrangement of experimental setup used in the experiment. After setting up the test equipment, the breakdown voltage is measured at three different temperatures like 28o C (room temperature), 40o C and 90o C. The test voltage was applied at the rate of 2 kV/sec until the breakdown occur. The failure of the PVP PCPB test sample in the form of puncture at the point of contact of electrodes is shown in Figure 6. Figure 6 Failure observed in PVP PCPB sample. 4. RESULTS AND DISCUSSIONS The experimental analysis was carried out to determine breakdown voltage of insulating PVA and PVP PCPBs of variable thickness. Temperature of the transformer oil in which these PVA and PVP PCPBs samples were immersed was maintained at 28 o C, 40 o C and 90o C. Before conducting the test, the samples are impregnated by immersing in transformer oil and should be kept in hot oven for 24 hours at 100o C and the tests on PVP and PVA PCPBs samples were carried out separately. A variable AC voltage in steps is applied to the test cell consisting of thickness of different PVP and PVA PCPBs samples in transformer oil at 28o C and 40 o C. The meter readings of the high voltage that was applied to both PVA and PVP PCPBs samples until the breakdown occur http://iaeme.com/Home/journal/IJARET 966 editor@iaeme.com Ashok Kumar S and Mohd. Z. A. Ansari was recorded as shown in Table 1 and Table 2 respectively. At 28o C, It is observed that the breakdown voltage of PVP PCPBs is more than PVA PCPBs for different thickness. It means that PVP PCPBs withstands more voltage than PVA PCPBs. It is also seen that average increase in percentage of BDV in PVP PCPBs for variable thickness is 39.66%. Table 1 BDV of PVA and PVP PCPBs in transformer oil at 28o C. Precompressed pressboard thickness size in mm 8.5 9.65 11.2 12.1 BDV in kV Percentage increase in BDV Average BDV in % 66.08 75.29 79.80 82.9 46.31 44.23 36.73 31.37 39.66 PVA PCPB PVP PCPB BDV in kV 45.162 52.20 58.36 63.1 Breakdown voltage in kV BDV of PVA and PVP PCPBs at 28o C 90 80 70 60 50 66.08 63.1 58.36 52.2 45.162 82.9 79.8 75.29 40 30 20 10 0 1 2 3 4 Precompressed Pressboard Sample thickness in mm PVA PCPB PVP PCPB Figure 7 BDV of PVA and PCPBs in transformer oil at 28o C. The percentage increase in average BDV at 40o C of oil temperature in PVP PCPBs is 38.96%. Which is almost similar to that at 28 o C as recorded in Table 2. Table 2 BDV of PVA and PVP PCPBs in transformer oil at 40oC. Precompressed pressboard thickness size in mm 8.5 9.65 11.2 12.1 PVA PCPB PVP PCPB BDV in kV 43.5 52.9 58.0 62.8 http://iaeme.com/Home/journal/IJARET BDV in kV Percentage increase in BDV Average BDV in % 63.2 75.0 79.11 81.5 45.28 44.5 36.37 29.7 38.96 967 editor@iaeme.com Comparative Investigations on Breakdown Voltage of PVA and PVP Precompressed Pressboards Used in HV Power Transformers- An Experimental Approach Breakdown Voltage in kV BDV of PVA and PVP PCPBs at 40 o C 100 80 60 75 63.2 43.5 52.9 81.5 79.11 62.8 58 40 20 0 1 2 3 4 Precompressed Pressboard Sample thickness in mm PVA PCPB PVP PCPB Figure 8 BDV of PVA and PCPBs in transformer oil at 40o C. Figure 7 and Figure 8 shows the BDV for variable thickness of PVA and PVP PCPBs in transformer oil are measured and compared at 28o C and 40o C. Table 3 represents the BDV of PVA and PVP PCPBs in transformer oil at 90o C. the average percentage increase in BDV is 43.31%. Table 3 BDV of PVA and PVP PCPBs in transformer oil at 90 o C. Precompressed pressboard thickness size in mm 8.5 9.65 11.2 12.1 PVA PCPB PVP PCPB BDV in kV 32.8 42.0 49.7 52.1 BDV in kV 57.2 68.4 70.0 71.1 Percentage increase in BDV Average BDV in % 49.31 46.69 40.84 36.4 43.31 Breakdown Voltage in kV BDV of PVA and PVP PCPBs at 90 o C 80 60 40 64.4 54.2 36.3 43.9 70 71.1 49.7 52.1 20 0 1 2 3 Precompressed Pressboard Sample thickness in mm PVA PCPB PVP PCPB 4 Figure 9 BDV of PVA and PCPBs in transformer oil at 90o C. http://iaeme.com/Home/journal/IJARET 968 editor@iaeme.com Ashok Kumar S and Mohd. Z. A. Ansari Figure 9 represents the BDV for different thickness of PVA and PVP PCPBs in transformer oil at 90o C. It is observed that percentage increase in BDV in PVP as compared to PVA is more when the thickness of PCPBs is less irrespective of temperature of the transformer oil. percentage Increase in BDV Percentage increased in BDV 60 50 40 30 20 Room temp. 10 40o C 90o C 0 1 2 3 Precompressed Pressboard Sample Thickness in mm 4 Figure 10 Percentage increase in BDV when PVP PCPB is used. Figure 10 represents the rise in percentage of BDV in PVP PCPBs as compared to PVA PCPBs for different thickness. It is seen that lesser the thickness of PVP PCPB higher is the percentage rise and vice versa. 5. CONCLUSION The selected different thickness of proposed PVP PCPBs are manufactured by using insulating polymer PVP and it is tested to determine the electrical breakdown voltage in transformer oil at 28o C, 40o C and 90 o C. The results are compared with the existing PVA PCPBs of same thickness. The findings from the experimental work can be summarized as follows. • • As load on transformer is increased the temperature of the transformer oil, breakdown voltage in both PVA and PVP PCPBs decreases as shown from the tables above. The percentage increase in breakdown voltage of proposed PVP PCPBs provides higher breakdown strength as compare to PVA PCPBs for the given variable thickness at 28o C, 40o C and 90o C are 31 to 46 %, 30 to 45 % and 36 to 49% respectively indicating improved BDV withstanding capacity, thereby improving the life of insulation in HV power transformers. ACKNOWLEDGMENTS The authors thank the authorities of CPRI, Bengaluru, Global Boards and Research and Development Centre, Department of EEE, GCE, Ramanagara for providing the facilities to carry out this research work. REFERENCES [1] O Girlanda, K Wei, S Ostlund. “ Characertization and modelling of the mechanical properties of pressboard”. 2013 Conference paper, October 2013, DOI 10.1109/CEIDP,2013,6748145. 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"The Aging Characteristics of Laminated Pressboard and Laminated Wood in Oil Cooled Power Transformers", Conference Record of the 2006 IEEE International Symposium on Electrical Insulation, 2006. http://iaeme.com/Home/journal/IJARET 970 editor@iaeme.com