Degradation of Cross Linked Polystyrene by Repetitive ... - IEEE Xplore

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S. Li et al.: Degradation of Cross Linked Polystyrene by Repetitive Impulse Surface Flashovers in Vacuum

Degradation of Cross Linked Polystyrene by Repetitive Impulse Surface Flashovers in Vacuum Shengtao Li, Weiwang Wang, Shihu Yu and Jianying Li State Key Laboratory of Electrical Insulation andPower Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China

ABSTRACT In order to understand aging mechanism of solid insulating materialscaused by surface flashover, this paper presents the electrical degradation characteristics of cross linked polystyrene (XLPS) by repetitive impulse surface flashovers in vacuum. The surface of samples underwent flashover repeatedly under different impulse voltage situations. Each flashover voltage and current was recorded, and the flashover energy was calculated to present flashover properties under each condition. Experimental results indicate that there are different degradation phenomena of XLPS after undergoing those impulse flashovers. The flashovers with small current have positive effect on surface flashover property of insulating material. However, the large currents when flashovers occurred play a destruction role in surface properties of XLPS. It is said that finite numbers of repetitive impulse flashovers under small flashover currents removed the contaminants, water and gas on the sample surface. In addition, it represents a “conditioning” effect and stable situation of flashover properties under these situations due to removal of emission sites, micro protuberance of electrodes and sample surface, especially at the cathode triple junction. However, repetitive flashovers under high flashover current (>1 kA) led to surface degradation. This phenomena attributes to the high flashover energy changed the sample surface state, destroyed the sample surface and formed apparent carbonized channels and regions on sample surface. So flashover becomes unstable in this situation. Through summarizing and analyzing the experimental results, the flashover energy was proposed as a main factor to characterize the phenomenon of degradation by repetitive impulse flashovers in vacuum. Index Terms—Electrical degradation, repetitive impulse flashover, XLPS, flashover energy, flashover current.

1

INTRODUCTION

VACUUM surface flashover which acted as a prolonged research topic has been investigated for several decades in high voltage engineering and electrical insulation fields, especially for the development of high voltage pulsedpower device and particle accelerator [1]. Flashover has become a main restricted factor to develop those high voltage facilities in vacuum. In order to improve the surface flashover properties, various methods were applied. Such as applying magnetic fields to steer secondary electrons away from the insulating material surface [2]; changing the surface roughness [3, 4]; developing some coatingmaterial on the insulator surface [5]; and adjusting the trap level to prevent electron transmission [6-9]. Majority literatures show that flashover initiates from electrons emission at the vacuum-insulator-cathode triple junction (CTJ) by field or thermal emission. At present, two major mechanisms about the flashover development are popularly accepted, Manuscript received on 7 January 2013, in final form 26 May 2013.

including the secondary electron emission avalanche (SEEA) theory [10-13] and the electronic triggered polarization relaxation (EPTR) theory [14-16]. Cross linked polystyrene (XLPS) has great potential applications due to its excellent properties. For example, the favorable dielectric properties, small and stable permittivity (2.53), low dielectric loss; prominentlyoptical and electromagnetic performance (high light transmittance). In addition, it is good for making microwave window material due to its excellent high frequency and high voltage properties. XLPS which used in those application fields is easilysubjected to surface flashover, so it is necessary to investigate the flashover characteristics of XLPS in vacuum. Some researchers have investigated the flashover properties of XLPS [16], and results of Rexolite® 1400 showed the best UV hold-off properties. Roth et al [5] also found that cross linked polystyrene (Rexolite) presents higher flashover strength compared with that of polymethylmethacrylate (PMMA). Recently, the aging of insulating materials by repetitive flashovers attracts researchers’ attentions [17-19]. Degradation of Ethylene Propylene Rubber (EPR) and Cross Linked

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IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 20, No. 5; October 2013

Polyethylene (XLPE) cables occurred when the applied switching impulses excess some certain values [20].The increasing partial discharge activities and the significantly reducing breakdown voltage after thousands of applied impulses provide the most forceful evidence for degradation. Some researchers focused on hold-off voltage and delay time to breakdown vary with an increasing number of flashovers and the changing degree of insulator degradation due to an increasing numbers of surface flashovers in pulsed power research [21]. It is found that Rexolite shows the greatest vulnerability to surface flashovers, and the photographs which give best view of damage to surface due to electrical treeing were also listed. W. A. Stygar et al [22] have also discussed the electrical degradation of insulator and electrodes caused by a single flashover. However, few researchers draw their attentions on electrical degradation of insulator by repetitive surface flashovers. In addition, the characterized methods of the degradation are still deficient. In our previous work, some methods were adopted to improve the surface flashover properties. For example, using a new organic insulation system [23-24] and changing insulation structure [25-27]. However, the degradation of insulating material by repetitive flashovers are still lacking. This paper focuses on the degradation of XLPS by repetitive impulse flashovers and presents the results of vacuum surface flashover voltages, currents and flashover energies of XLPS under impulse voltages of different steepness. The flashover voltages as a function of flashover times and dependence of flashover voltages on impulse steepness were shown to character the flashover properties under different impulse situations. SEM images of sample surface under high flashover current were presented to analyze the degradation characteristics, and a ratio k was proposed to discuss the stability of flashover under different impulse situations. Through illustrating the formation of carbonized channels, the flashover energy was proposed as an important parameter for interpreting surface degradation of XLPS qualitatively.

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material is 2.53. The materials were prepared into laminar samples with thickness of d=2 mm and Φ=50 mm in diameter for vacuum surface flashover measurement, as shown in Figure 1. The samples were prepared by machining process to guarantee the same surface roughness, so as to possess comparability among the experimental results. Prior to measurement, the samples were also ultrasonically cleaned in absolute ethyl alcohol for 10 min, then in deionized water for 10 min, and baked at temperature of 100 oC for 6 h. Finger electrodes which were made of stainless steel were polished to attain mirror finish. The radius of curvature at the tip of electrode was 10 mm. The electrode gap spacing was set to d =10 mm and remained unchanged throughout the whole experiment. Samples were placed on a holder made of PTFE between the two electrodes. After polishing, the electrodes were ultrasonically cleaned in absolute ethyl alcohol for 10 min, and then in deionized water for 10 min. In order to evaporate water completely, the electrodes were baked at a temperature of 120 oC for 12 h.

Figure 2. Electrical field distribution of the finger electrodes.

2

SAMPLES AND EXPERIMENTAL SETUP

2.1 SAMPLE PREPARATION AND ELECTRODE STRUCTURE In this paper, Rexolite 1422 which is purchased by C-Lec Plastics, Inc. was used as tested material. Permittivity of this

Figure 1. Photograph of prepared sample (d=2 mm and Φ=50 mm).

Simulated software ANSYS was used to calculate the field distribution of finger electrodes. The simulated result is shown in Figure 2. It indicated that the maximum electric field was located at the cathode or anode triple junctions (ATJ). From the calculated results, the effective field enhancement factor was estimated. k=Emarx/Eav=5.3. Where Emax is the maximum electric field of the electrode system and Eav is the average electric field. The field enhancement factor is high enough, which is beneficial for electron emission and flashover initiation. 2.2 IMPULSED VACUUM FLASHOVER EXPERIMENTAL SYSTEM The unipolar impulse generator was set up with six stage switches and capacitors, having a charging capacitance C=5.6 nF. The controlled resistance Rload and the loaded resistance Rlimit were applied to adjust the parameters of output impulse waveform. In this paper, through changing the Rload and Rlimit, eight impulse waves with different rise time and the full widths at half maximum (FWHM) were acquired. Those

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S. Li et al.: Degradation of Cross Linked Polystyrene by Repetitive Impulse Surface Flashovers in Vacuum

different output impulse voltage waveforms were conducted for investigating the flashover properties of XLPS by repetitive flashovers. The concrete information about the output waves are listed in Table 1, and the steepness of impulse voltage waveforms were also calculated.

Conditions 1

Table 1. Information of output waveforms. Parameters of waveforms Rlimit Rload Voltage (Ω) (Ω) amplitude Parameters (kV) 690 3000 38kV 50ns/1.2μs

Steepness dU/dt (kV/ns) 0.76

2

2300

2000

75kV

50ns/1μs

1.50

3

767

800

85kV

40ns/500ns

2.12

4

567

800

80kV

30ns/500ns

2.67

5

200

1000

75kV

25ns/650ns

2.98

6

80

1000

80kV

23ns/650ns

3.48

7

60

1000

85kV

20ns/650ns

4.25

8

50

1000

90kV

20ns/500ns

4.50

A voltage divider of metal-film resistors is employed for flashover voltage measurement and its divided ratio is 11500:1. The flashover current was tested by Rogowski current coil (1 V/A, 250 MHz bandwidth). All flashover voltages and currents waves were recorded by the Tektronix DPO4032 oscilloscope, as shown in Figure 3. During the flashover experiments, the vacuum level of 5×10-3 Pa was kept during the whole measurement. The vacuum flashover characteristics of XLPS were investigated using the different output impulses which are derived from Marx generator. During experiments, we ensured that all flashovers occurred at the rise time of flashover voltage waveform. 30 consecutive flashovers at the time interval of 30 seconds occurred on sample surface. Flashover voltages, average value and standard deviation of flashovercurrents were presented. In addition, the flashover energies were also calculated according to integrating the original flashover data that was recorded by oscilloscope. A standard flashover voltage and current waveforms are shown in Figure 4.

Figure 3. Schematic of impulse vacuum flashover system.

Figure 4. Typical waveforms of flashover voltage and current under condition 2.

3

EXPERIMENTAL RESULTS

3.1 IMPULSED SURFACE FLASHOVER MEASUREMENTS Generally speaking, many experimental results indicated that the surface flashover has an obvious “conditioning” effect after repetitive flashovers occurred on the same sample surface. That was to say, once the initial flashover occurred, the subsequent flashover voltages would be sequentially enhanced, and finally it reached a stable value. For example, under condition 1, the initial flashover voltage is 32.02 kV, after 24 flashovers, it reached to 36.93 kV. The number of applied impulse voltage was mainly dependent on the property of material and applied voltage waveforms, and generally several dozen flashovers were required. Predischarge treatment has great influence on surface flashover characteristic of polymer insulator. In this paper, prior to flashover measurement, the samples were underwent predischarge treatments. The method of predischarge is made as follows: before predischarge treatment, Rload= 2300 Ω was series connected in the circuit to ensure the flashover current is limited under several hundred amperes. The sample was subjected to impulse voltage at low voltage, and then progressively increasing the peak voltage in step of 2.0 kV per minutes until the samples underwent 10 flashovers at a certain voltage level. All flashovers occurred at the rise time of voltage waveform. After predischarge treatment, the different Rload (2300 Ω, 690 Ω, 767 Ω, 567 Ω, 200 Ω and 80 Ω) were applied to realize the low flashover current conditions (condition 1, 2, 3, 4, 5 and 6). The applied impulse voltage was progressively increased to a certain value which guaranteed each flashover occurred at rising time of impulse voltage waveform. Then 30 flashovers were carried at this certain voltage level, and the interval time of adjacent applied voltages is 30 s. Here, the flashover voltage as a function of flashover times and the amplitude of flashover currents and corresponding standard deviation are presented, as shown in Figure 5. When Rload was 60 Ω and 50 Ω, the flashover current is high. It is used for measuring flashover properties under high



flashover current (condition 7 and 8). The same method was employed for flashover measurement under these situations. Condition 7and 8 present these flashover characteristics, as shown in Figure 5. Eight samples were selected to do the experiments, and one sample was used under each condition. The experimental environments are the same under each condition to guarantee a good reproducibility of experimental results.

Figure 5. Flashover voltage as a function of flashover numbers. The corresponding flashover currents are 1: 70.22±5.01 A; 2: 171.31± 7.95 A; 3: 295.53±24.02 A; 4: 354.03±15.35 A; 5: 549±18.98 A; 6: 658±39.58 A; 7: 929.18±28.69 A; 8: 1385.42±21.92 A.

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flashover property. Namely, the initial flashover voltage is small (for example, for condition 1, it is 32 kV), and with increasing the flashover numbers, the flashover voltage increases and reaches a stable value (38kV). Figure 5 shows that flashover voltages increase initially by repetitive flashovers, and then remain essentially constant after 30 flashovers. However, with further increasing flashover numbers, flashover voltage decreases due to surface degradation, as shown in condition 2. These phenomena under condition 2, 3, 4, 5, 6 have same “conditioning” properties with condition 1. In addition, flashover currents gradually increase from condition 1 to 8. However, when the flashover current is high enough (>900 A), the “conditioning” effect doesn’t occur. In this situation, the initial flashover voltage is slightly lower than the second and the third ones. It has an obviously decrease trend when further increasing the flashover numbers. Moreover, the large dispersion of flashover voltages occurred compared with that of low flashover currents. Compared with the initial flashover voltage (87 kV), the flashover voltage decrease to 71 kV after 30 repetitive flashovers, which is reduced by 18.4% under condition 8. Flashover voltage gradually decreases with increasing flashover numbers, which is due to obviously surface degradation. The degree of surface degradation depends on the flashover energy during flashover. From experimental results, it is found that the transition from beneficial conditioning into destructive surface degradation due to repeated surface flashover events existed from low flashover current to high flashover current.

Figure 6. Relationship between initial flashover voltage and impulse steepness.

Obviously, the initial flashover voltages have large deviations under different conditions. It indicated that the higher flashover current, the higher flashover voltage. This characteristic is due to the difference of impulse voltage steepness and the amplitude of impulse voltage, as shown in Figure 6. The initial flashover voltage has a liner relation with impulse steepness. Chen et al [28] have investigated the influence of pulse steepness on flashover properties of epoxy resin and presented the same results. In addition, large impulse steepness corresponds to high amplitude of output voltage, which increases the flashover voltage. When flashover current is small (1 kA) dominate the electrical degradation of insulator surface. Repetitive flashover under low flashover current takes a “conditioning” effect on flashover properties, and it can improve the flashover characteristic. Moreover, it takes “self-cleaning” effect on sample surface after finite flashovers. It means that the low flashover energy can remove the impurities and micro protuberance on sample surface, so as to clean the sample surface. It has little influence on surface morphology. Low flashover current represents low flashover energy (low energy of PDs). Flashover can keep a stable state in this situation due to small and homogeneous distribution of surface charge. However, high flashover current leads to flashover voltage decrease. It significantly changed the surface morphology. High flashover energy corresponds to high energy of PDs, which leads to an unstable flashover. In addition, large amount of heat were released during the whole flashover current to form carbonized channels on sample surface. Repetitive flashovers under large flashover current give rise to surface degradation of insulator. These experimental results and characterized methods play significant parts on insulating material application that used in high voltage and insulation system in vacuum. Nevertheless, the degradation mechanisms of insulator by repetitive flashovers of high energy are still needed to study further, including the influence of surface charging, high energy of PDs and so on.

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ACKNOWLEDGMENT The authors wish to thank the National Science Fund for Outstanding Young Scholars of China under Projects No. 50625721, the financial support of the National Basic Research Program (973 Program) of China (Grant No. 2011CB209404). This work was also supported by National Natural Science Foundation of China (Grant No. 11275146, and No. 51221005).

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[28] Y. Chen, Y. H. Cheng, Z. B. Wang, J. B. Zhou, G. D. Meng, K. Wu, and S. T. Li, "Influence of Pulse Steepness on Vacuum Flashover of Casting Epoxy Resin," Plasma Science and Technology, Vol. 11, pp. 89-93, 2009. [29] M. I. Takeo Ichinokawa, A. Onoguchi and T. Kobayashi, "Charging Effect of Specimen in Scanning Electron Microscopy," Jpn. J. Appl. Phys., Vol. 13, pp. 1272-1277, 1974. [30] S. Grzybowski, J. E. Thompson and E. Kuffel, "Electric Surface Strength and Surface Deterioration of Thermoplastic Insulators in Vacuum," IEEE Trans. Dielectr. Electr. Insul., Vol. 18, pp. 301-309, 1983. [31] C. H. De Tourreil and K. D. Srivastava, "Mechanism of Surface Charging of High-Voltage Insulators in Vacuum", IEEE Trans. Dielectr. Electr. Insul., Vol. 8, pp. 17-21, 1973. [32] I. D. Chalmers, J. H. Lei, B. Yang, and W. H. Siew, "Surface charging and flashover on insulators in vacuum", IEEE Trans. Dielectr. Electr. Insul., Vol. 2, pp. 225-230, 1995. Shengtao Li, (M’96-SM’11), was born in Sichuan, China, in February 1963. He received the B. S., M. S. and Ph.D. degrees in electrical engineering from Xi'an Jiaotong University (XJTU) in 1983, 1986, and 1990, respectively. He worked at Waseda University, Tokyo, Japan, as JSPS research fellow for 3 months in 1996, and did research at the University of Southampton, UK, as a senior visiting scholar for 6 months in 2001. He was a Lecturer, Associate Professor, and Professor with Xi'an Jiaotong University, China, in 1990, 1993, and 1998, respectively. From 1993 to 2003, he was a deputy director of the State Key Laboratory of Electrical Insulation and Power Equipment (SKLEIPE) in Xi'an Jiaotong University. Since 2003, he has been an executive deputy director of SKLEIPE. He received financial support from the National Science Foundation for Distinguished Young Scholars of China in 2006. He is an Associate Editor of the IEEE Transactions on Dielectrics and Electrical Insulation (TDEI) and a Guest Editor of the special issue of TDEI to Celebrate and Recognize the 60th Anniversary of Research and Developments of Dielectrics and Electrical Insulation in China (June 2014 issue). He can be reached by email at [email protected].

1941 Weiwang Wang was born in Shaanxi, China in 1987. He majored in electrical engineering and received the B. S. degree from Harbin University of Science and Technology in 2010. Currently, he is a graduate student of high voltage and insulation technology in Xi’an Jiaotong University, and studies in State Key Laboratory of Electrical Insulation and Power Equipment for the Ph.D. degree. His research interests is in polymer nanocomposites, insulating materials and insulation system used in vacuum.

Shihu Yu was born in Shaanxi, China, in 1990. She majored in electrical engineering and graduated from Xi’an Jiaotong University with a B. S. degree in 2012. She is now studying in State Key Laboratory of Electrical Insulation for the Master degree of high voltage and insulation technology in Xi’an Jiaotong University. Her main research interest is nanocomposites in insulating materials.

Jianying Li was born in Shaanxi, China in 1972. He received the B. S, M. S and Ph.D. degrees in electrical engineering from Xi'an Jiaotong University, China in 1993, 1996 and 1999, respectively. He is now a professor at Xi’an Jiaotong University. His major research fields are high voltage insulation and dielectrics.