MOISTURE EFFECT ON THE CREEPAGE DISCHARGE PHENOMENA ON CELLULOSE- BASED PRESSBOARD IMMERSED IN PALM FATTY ACID ESTER

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1 Full Paper MOISTURE EFFECT ON THE CREEPAGE DISCHARGE PHENOMENA ON CELLULOSE- BASED PRESSBOARD IMMERSED IN PALM FATTY ACID ESTER Nur Amirah Othman 1, Hidayat Zainuddin 1*, Aminudin Aman 1, Sharin Abd Ghani 1, Mohd Shahril Ahmad Khiar 1, Imran Sutan Chairul 1, Mohd Aizam Talib 2 1 Research Laboratory of High Voltage Engineering, Centre for Robotics and Industrial Automation, Faculty of Electrical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka, Malaysia. 2 TNB Research Sdn Bhd., No. 1, Lorong Ayer Itam, Kawasan Institusi Penyelidikan, Kajang, 43000, Kajang, Selangor, Malaysia *Corresponding Author hidayat@utem.edu.my Graphical abstract Abstract Creepage discharge at oil-pressboard interface tends to cause damage on the solid insulation which lead to catastrophic failure of a power transformer under normal operating condition. Excessive moisture in cellulose insulation is one of the major defects at the inter-phase region of high voltage transformer that may cause the creepage discharge along the pressboard surface. Previously, there are extensive research investigating the moisture effect on creepage discharge along surface pressboard immersed in mineral oil. However, none has been found considering on the effect of moisture at ester oil-pressboard interface. Thus, this paper attempts to present the effect of moisture on the creepage discharge phenomena at palm fatty acid ester (PFAE) oilpressboard interface in order to compare its behavior with mineral oil reported in the previous research. The characteristic of creepage discharge are analyzed by correlating between the visual records of creepage discharge and the partial discharge (PD) results. Keywords: Creepage discharge; transformer; oil - pressboard interface Abstrak Keretakan permukaan pada permukaan minyak pressboard cenderung untuk menyebabkan kerosakan pada penebat pepejal yang membawa kepada kegagalan pengubah kuasa daripada operasi biasa. Kelembapan yang berlebihan dalam penebat selulosa adalah salah satu kegagalan yang besar antara fasa-pengubah voltan tinggi yang boleh menyebabkan keretakan permukaan di sepanjang permukaan pressboard itu. Sebelum ini, terdapat kajian yang meluas dalam menyiasat kesan kelembapan pada keretakan permukaan pressboard yang direndam dalam minyak mineral. Walau bagaimanapun, tiada lagi kajian yang dijumpai membincangkan tentang kesan kelembapan di permukaan pressboard di dalam minyak ester. Oleh itu, kertas kerja ini cuba untuk membincangkan tentang kesan kelembapan pada fenomena keretakan pemukaan diantara asid lemak sawit ester (PFAE) dan pemukaan pressboard. Ia bertujuan untuk membandingkan fenomena yang menggunaan minyak mineral yang direkod dalam penyelidikan sebelumnya. Ciri-ciri keretakan permukaan dianalisis dengan menghubungkaitkan antara rekod visual keretakan permukaan dan hasil kajian PD. Kata kunci: Keretakan permukaan; pengubah kuasa; permukaan minyak-pressboard 458

2 1.0 INTRODUCTION Basically, the insulating systems of high-voltage power transformer comprises the combination of liquid insulating system and solid material. This composite insulation system could give a design benefit which are reduction of manufacturing cost and transformer size as well as to increase the lifetime of the transformer. In power transformer, the use of pressboard barriers is to divide the large oil volume into the small gap in order to enhance the dielectric strength of the oil gap [1]. Cellulose pressboard insulation is made of the layers of laminated cellulose paper. This pressboard insulation is used for the electrical purposes, which is produced from unbleached cellulose that mainly obtained from softwood using the Kraft process [2]. The oil- pressboard interface is known as the weak point, involving electric charges deposition and boundaries which can cause creepage discharge that leads damages to the insulation system [3, 4]. Figure 1 shows the loose structure of cellulose fibers as a weak point in pressboard. However, based on the microscopic view, the cellulose pressboard is suggested by Mitchinson that the inhomogeneous and porous condition is increases near to the bulk oil and this condition can be maximized on the impregnated pressboard surface [4, 5]. Figure 2 shows the example of inhomogeneous condition of pressboard surface. usually will result in the catastrophic failure under normal operating condition. This failure can continue for a minute to a month, even a year if there is no proper and effective monitoring on the transformer condition. Figure 3 shows an example of catastrophic failure along barrier board. Figure 3 Catastrophic failure along barrier board [8] In site measurement, Sokolov has classified that excessive moisture content in the cellulose insulation is one of the major defects that leads to the creepage discharge failure [9]. On the other hand, in laboratory work, this is also supported by Mitchinson, Yi et al and Zainuddin which is concluded that excessive moisture content in pressboard insulation immersed in mineral oil affect to the creepage discharge failure [3, 8, 9]. White mark, carbonized mark and full discharge events are usually observed during creepage discharge [2 4]. These events can be observed for several minutes to hours dependent on the pressboard moisture level. In previous work using mineral oil, [5] it is found that it is difficult to observe the propagation of white mark when dry pressboard are used. Zainuddin [5] has observed that the period of time for the propagation Figure 1 Loose structure of cellulose fibers [6] of white mark until first appearance of full discharge event took about 5 hours at constant voltage of 30kV. As for [11], Yi et al also found that white mark are hard to be seen when dry pressboard is used along with mineral oil. There is no growing white mark was observed until 200 minutes at 52.0 kv until the flashover is occasionally occur. The difference in observation between [5] and [11] is probably because of difference of applied voltage and needle electrode configuration used to initiate the discharge. The results in mineral oil are contradict with the previous research using ester oil that observed the white mark could be easily initiated on dry pressboard Figure 2 Inhomogeneous condition of pressboard structure [4] In power transformer, creepage discharge or at relatively low voltage [11]. The white mark is sometimes observed after applying the voltage for more than 10 minutes. surface discharge along the oil-pressboard interface may cause damage to the pressboard surface. Creepage discharge can be classified as the main failure mechanism for the outdoor insulator [7]. It is In the case of wet pressboard using mineral oil, at 2.7 % of moisture contentt in pressboard, Dai et al [14] have observed that the white mark appears 459

3 after 2 hours of applying 45kV at the needle electrode. Another works by Zainuddin [5] has observed that, when 3 % of moisture content is used, the period of time for white mark to reach the earth electrode under 30kV constant stresss is about 45 minutes. These results clearly show that excessive moisture content in solid insulation will promote the discharge on the pressboard insulation and subsequently leads to the insulation failure. Nowadays, ester oils are being considered as potential alternative dielectric liquid for mineral oil due to the better performances associated with environmental impact (readily biodegradable), non toxic, non copper sulfide, high relative permittivity and higher flash point. Ester oils including synthetic ester and natural ester have been widely used for distribution transformer. However, the application of ester oils is quite slow due to lack of understanding on its performances for a higher voltage level especially when combined with the pressboard insulation. Thus, before applying esters in large power transformer, it is important to understand the dielectricc performance of ester oil-pressboard interface especially its capability to withstand the long-term creepage discharge. The effect of moisture also need to be considered on the creepage discharge phenomenon which is not has been found considering by the previous research. In view of the foregoing discussions, this paper is therefore aimed to present on how the moisture content in cellulose pressboard affect the behaviors of the creepagee discharge at PFAE oil-pressboard interface. 2.0 EXPERIMENTAL DESCRIPTION Figure 4 Experimental setup for surface discharge The surface discharge is conducted in oil test chamber filled with approximately 5 litres of PFAE oil. The oil sample is obtained directly from the barrels and is changed for every. For needle electrode configuration, medical needle is used as the point electrode to initiate the discharges at the cellulose pressboard interface. Medical needle with tip radius of 20 µm is placed at an acute angle to the horizontal of the pressboard surface as shown in Figure 5. The distance between the needle tip and the earth bar is fixed to 30 mm. This configuration is to ensure that the charges built around the needle tip will distribute along the surface pressboard rather than the bulk of the pressboard [9, 12, 13]. The gap distance between the needle tip and earth bar was calibrated before start the in order to ensure a constant distance in each of the. 2.1 Test Configuration The surface discharge is carried out in the laboratory under AC stress using needle-bar configuration [5, 12, 13]. Figure 4 shows the al setup to measure the PD activities during surface discharge. All the important events are recorded using a digital camera 60 fps in order to correlate the surface discharge degradation on the pressboard surface with PRPD patterns. The coupling capacitor and oil test chamber are connected to high voltage supply using copper pipes elsewhere in the system except from the needle electrode to avoid corona discharge from the sharp edges. 460 Figure 5 Needle setup configuration 2.2 Sample Pressboard Preparation In this work, dry and wet cellulose pressboard insulation are prepared for the creepage discharge. The percentage of the moisture content by weight in the pressboard samples are 0.5 % for dry condition while 3.5 % for wet condition. The pressboard samples are firstly dried in air circulating oven at 105 ºC for 48 hours. The pressboard samples are considered dry when it is in constant mass at variation of ± 0.5 % between two successive drying

4 to comply with the standard (BS EN : ) [15]. For the moisture content of 3.5 % in pressboard samples, the samples are left in the laboratory under atmospheric condition ( T = 27-31, RH = 60 % - 70 % ) in order to absorb moisture until the required mass is achieved. Afterwards, all the samples are impregnated in PFAE oil under vacuum condition of 0.09MPa for 48 hours. Figure 6 shows the dry and wet pressboard samples after impregnated with PFAE oil under vacuum condition. noise level during the inceptionn voltage, i.e. noise free up to 2.5 pc and the PD threshold is set twice the background as reported in other literature [14]. 3.0 RESULTS AND DISCUSSION The following sections discuss the al results by correlating the visual observation using digital camera and partial discharge data. 3.1 Surface discharge process at 26Kv and 28 kv Visual Observation It appears that no visible white mark and carbonized mark were observed under 26 kv is applied for a period of 2 hours and 28 kv for another 2 hours. These results might be due to insufficient energy from the voltage applied for white mark to be initiated along the pressboard surface. These are shown in Figure 7 and Figure 8. Figure 6 Impregnation of dry and wet pressboard samples 2.3 Partial Discharge Measurement OMICRON Mtronix Partial Discharge measurement is used to monitor and record the partial discharge (PD) patterns during the. Before the surface discharge is started, the PD inception voltage and breakdown voltage are recorded in order to ensure there is no immediate breakdown during the. Thus, the range of applied voltage can be determined. Table 1 shows the inception voltage and breakdown voltage for dry and wet pressboard samples. Table 1 Inception voltage and breakdown voltage for dry and wet pressboard sampless Dry sample Wet sample Inception voltage 15kV 14kV Breakdown voltage 35kV 33kV From the range between the inception voltage and breakdown voltage for each pressboard sample, the increment voltage by a step of 2kV at every 2 hours is applied in this. The applied voltage used in this are 26kV, 28kV and 30kV. The PD threshold values for inception voltage and surface discharge used in this are 5 pc and 25 pc respectively. The difference in the threshold values between both s is due to a higher level of noise during the surface discharge is conducted i.e. noise free up to 20 pc, thus the PD threshold value is set by considering +5 pc contingency. Thanks to the low 461 (a) At 26 kv, after 2 hours of (b) At 28 kv, after 4 hours of Figure 7 Visual observation on dry pressboard sample (0.5 %) (a) At 26 kv, after 2 hours of (b) At 28 kv, after 4 hours of Figure 8 Visual observation on wet pressboard sample (3.5 %) Partial Discharge Dataa Table 2 shows the PD number as a result of surface discharge on the dry and wet pressboard samples for a period of 30s. The time column indicates the time at which the discharges are started to be counted for a period of 30s. The time is given as the cumulative time from the beginning of the surface discharge. Based on findings in Table 2, the number of discharges for dry pressboard sample is higher than the number of discharges when wet pressboard sample is used. Such results are contradict to the results that one can expect. This condition can be explained based on the following arguments. Firstly, it

5 is important to note that the maximum discharge magnitude during PD inception for dry pressboard is higher compared to the wet pressboard. This is shown in Table 3 whereby the PD magnitude during inception with PD threshold of 5 pc for the dry pressboard is distributed up to 1.0 nc, whilst it is just up to 25 pc for the wet sample. In addition, the table also shows that the average number of PD events for wet pressboard is higher than the dry pressboard. Secondly, as mentioned earlier, the PD threshold for the surface discharge is higher than the PD inception test due to unwanted noise during the was conducted, i.e. 25 pc. Such high threshold has caused a large number of PD events not been recorded by the measurement system particularly for the wet pressboard whereby the PD events are concentrated at a lower PD magnitude. These arguments can also be supported by the higher number of PD events when the wet pressboard is used compared to the dry one that will be discussed in the following section. Table 2 Number of discharge events for a period of 30s Experiment Number of discharge event Voltage time (PDs) (kv) (h) Dry sample Wet sample After After After After Table 3 Maximum discharge magnitude and number of PD events during PD inception for a period of 30s Pressboard sample Maximum value of discharge, Qmax Number of PD events (PDs) Dry sample 1.0 nc Up to 59 Wet sample 25 pc Up to 69 Figure 9 Arc discharge after 15 minutes of applied voltage Figure 10 shows the growing of white mark in the form of bush-like pattern after 1 hour of 30kV voltage stress. Meanwhile, at this stage, the carbonized mark or char mark are also observed but not clearly visible. The carbonized mark in the form of tracking line are clearly visible after 2 hours of 30kV voltage stress Figure 11. Figure 10 Formation of white mark and carbonized mark after 1 hour of applied voltage 3.2 Surface discharge process at 30kV Visual Observation This section discusses the results of surface discharge when the voltage is further increased to 30 kv for a period of another 2 hours. At the early stage of applying this level of voltage, i.e. after 15 minutes of applying 30 kv, the discharge activity is usually observed in the form of intermittent arc or glow discharges at the needle tip. This discharge activity as shown in Figure 9 suggests that ionization processes are taking place at the vicinity of the needle tip. As time progresses, white mark is developed from the needle tip and propogate towards the earth electrode. It is also observed that the arc or glow discharges are visible throughout the al period, but with a longer interval time between one discharge to another as time progresses. It should be noted that similar findings are also reported in [4, 10] 462 Figure 11 Formation of white mark and carbonized mark after 2 hours applied voltage In the case of wet pressboard, the appearance of white mark is observed earlier compared to the dry pressboard. The white mark is also wider than those obtained on dry pressboard. Figure 12 shows the white mark on wet pressboard after 15 minutes of increasing the voltage from 28 kv to 30 kv. The figure also showss an intermittent arc or glow discharge at the needle tip as observed on the dry pressboard and other literature [4, 5]. After 1 hour of 30kV voltage stress, the carbonized mark starts to grow on the pressboard surface as shown in Figure 13. However, the carbonized mark is not clearly visible but it is darker

6 than on the dry pressboard (compare between Figure 13 and 10). Arc discharge splitting the oil promotes the formation of hydrocarbons and subsequently lead to the formation of carbonized mark [9]. Figure 14 Formation of white mark and carbonized mark after 2 hours of applied voltage Throughout the, after the white mark appears on the wet pressboard, small gas bubbles are randomly observed dissipated freely into the insulating liquid. These bubbles are seen move out from the pressboard surface around the needle tip. Figure 12 Formation of white mark and arc discharge after 15 minutes of applied voltage Figure 13 Formation of white mark and carbonized mark after 1 hour of applied voltage Figure 14 shows the white and carbonized mark are clearly observed after 2 hours of applying that 30kV voltage stress. The results show that the growing of carbonized mark is much intense than those observed on the dry pressboard surface (see in Figure 11). This might be due to the excessive water present in the transition region of the wet pressboard. The presence of water might be due to the decomposition of cellulose and insulation oil during the surface discharge [16]. This moisture allowed the ionization of polar molecules which then provide more source of charge transported through interfacial region [4]. The moisture also promotes discharges to occur at a concentrated region which involve evaporation of moisture under higher heat energy from a more concentrated PD activity [5] Partial Discharge Dataa Table 4 depicts the number of PD events during surface discharge for a period of 30s at certain al period after 30 kv of voltage is applied. In overall, the PD numbers show an increasing trend during the 2 hours of applying 30 kv of applied voltage on both dry and wet pressboard samples. Such increment is probably due to the degradation of pressboard surface that involves decomposition of cellulose and oil molecules which further increase the amount of water level and thus, enhance the PD activity [4, 20] ]. The results from the table also show that the wet pressboard experienced a higher number of PD events compared to the dry pressboard suggesting that water content plays an important role in the discharge activity at the oil- noting that, at pressboard interface. It is worthwhle this level of voltage, the PD events are distributed with high density at a much larger PD magnitude (up to 4.0 nc) compared to those obtained at 26 kv and 28 kv. This, thus, support the hypothesis of wet pressboard is expected to cause higher PD events as mentioned earlier in the previous section. Table 4 Number of discharge events at 30 kv for a period of 30s Number of discharge Voltage (kv) Experiment event (PDs) time Dry Wet sample sample 30 After 15min After 1h After 2h 16,053 44,308 66,403 29, , , CONCLUSION A surface discharge has been conducted using needle-bar electrode configuration to study the effect of moisture content in pressboard immersed in PFAE oil. The resultss suggest that moisture content plays important role in the formation of white mark whereby the white mark appears earlier when wet pressboard is used compared to the dry pressboard. The moisture levell also enhances the PD activity at the oil-pressboard interface based on the higher number in PD events recorded. Thus, it is clearly shows that the moisture content for the ester oilcreepage discharge pressboard affect the phenomenon. This observation is also agreed with those reported in the literature using mineral oil. The

7 work also suggests that, a sufficient voltage stress is required to initiate the white and carbonized mark on the pressboard surface. Acknowledgement The authors wish to extend their thanks for the encouragement and financial assistance of Universiti Teknikal Malaysia Melaka and Ministry of Education. This work is supported by Ministry of Education, Malaysia under Fundamental Research Grant Scheme (FRGS/1/2014/TK03/FKE/02/F00216). [15] BS EN :2004, Pressboard and Presspaper for Electrical Purposes - Part 2: Methods of Tests.. [16] A. A. Suleiman, N. a. Muhamad, N. Bashir, N. S. Murad, Y. Z. Arief, and B. T. Phung, Effect of moisture on breakdown voltage and structure of palm based insulation oils, IEEE Trans. Dielectr. Electr. Insul., vol. 21, no. 5, pp , Oct References [1] J.Dai, Z. D. Wang, and P.Jarman, Moisture and Aging Effect on the Creepage Discharge at the Oil/Transformerboard Interface under Divergent Field, in Annual Report Conference on Electrical Insulation Dielectric Phenomena, [2] M. M. Tshivhilinge, A Comparative Study of The Effect of Surface Discharge on The Impulse Breakdown Voltage of oil-impregnated pressboard Insulation, University of the Witwatersrand, Johannesburg, [3] X. Yi, Characteristics of Creepage Discharge Along Ester- Pressboard Interfaces under AC Stress,, PhD thesis, University of Machenster, [4] P. M. Mitchinson, Surface tracking in the tracking in the inter-phase of large transformers,, PhD thesis, University Of Southampton, [5] H. Zainuddin, Study of Surface Discharge Behaviour at the Oil-Pressboard Interface,, PhD thesis, University Of Southampton, [6] K. Giese, The Effects of Cellulose Insulation Quality on Electrical Intrinsic Strength, IEEE Electrical Insulation Magazine, vol. 10, no. 5, pp , [7] C. L. Wadhwa, High Voltage Engineering, vol. 177, no [8] J. A. Lapworth and A. Wilson, Transformer Internal Over- Voltages Caused by Remote Energisation, 2007, no. July, pp [9] V. Sokolov, Z. Berler, and V. Rashkes, Effective Methods Assesment of Insulation System conditions in Power Transformers : A View Based on Practical Experience, in Proceedings of the Electrical Insulation Conference and Electrical Manufacturing & Coil Winding Conference, Cincinnati, Ohio, USA, 1996, pp. pp [10] P. M. Mitchinson, P. L. Lewin, and B. D. Strawbridge, Tracking and Surface Discharge at the Oil-Pressboard Interface, IEEE Electrical Insulation Magazine, vol. 26, no. 2, pp [11] X. Yi and Z. D. Wang, Surface Tracking on Pressboard in Natural and Synthetic Transformer Liquids under AC Stress, IEEE Trans. Dielectr. Electr. Insul., vol. 20, no. 5, pp , [12] P. M. Mitchinson, P. L. Lewin, G. Chen, and P. N. Jarman, A new approach to the study of surface discharge on the oilpressboard interface, in IEEE International Conference on Dielectric Liquids (ICDL), pp [13] H. Zainuddin, P. L. Lewin, and P. M. Mitchinson, Partial Discharge Characteristics of Surface Tracking on Oilimpregnated Pressboard under AC Voltages, in IEEE International Conference on Solid Dielectrics, Bologna, Italy, June 30 July 4, 2013 It, 2013, pp [14] J. Dai, Z. D. Wang, and P. Jarman, Creepage Discharge on Insulation Barriers in Aged Power Transformers, IEEE Trans. Dielectr. Electr. Insul., vol. 17, no. 4, pp ,