The Influence of Lagged Axial Magnetic Field on the Distribution of Cathodes Spots in Current

精品论文The Influence of Lagged Axial Magnetic Field on the Distribution of Cathodes Spots in Current-zero Stage of Vacuum Arc5Jia Shenli, Song Xiaochuan, Shi Zongqian, Wang lijun, Zhang Yingkui(the State Key Laboratory of Electrical Insulation and Power Equipment, Xian JiaotongUniversity, Xian 710049)Abstract: The influence of lagged axial magnetic field (AMF) due to eddy current induced in contacts system on the distribution of cathode spots (CSs) in current-zero stage of vacuum arc with different10current falling rates was investigated experimentally. Experiments were conducted with cup-shapedAMF contacts in a demountable vacuum chamber. The characteristic of CSs distribution of triggered vacuum arc in current-zero stage with arc current frequency of 50Hz was investigated at first with the aid of a high-speed digital camera with an exposure time of 2 microseconds. Then the characteristic ofCSs distribution with faster current falling rate, which was realized by injecting a countercurrent with15frequency of 500Hz or 1kHz, was investigated. Moreover, the lagged AMF flux density in current-zero stage with different current falling rates was calculated by ANSYS. Research results showed that CSs located in the center of contact plate tended to extinguish earlier than those at periphery of contact plate in fast current-zero stage due to the stronger lagged AMF in contact center.Keywords: Vacuum arc; Cathode spot; Axial magnetic field200IntroductionVacuum circuit breakers have been being widely used in AC power distribution systems due to its excellent performance, and they are increasingly being used in DC networks. In order to reduce the duration from the onset of fault current to current interruption, a high-frequency current,25which is in opposite direction to the fault current, is used to force the fault current to zero 1, 2.This method is mainly used in DC current interruption 3, and also used in high-speed interruption of AC current 4, 5.Axial magnetic field (AMF) contacts are widely used in vacuum interrupters, since an AMFcan keep vacuum arc in diffuse mode at high current due to its confining effect on electrons and30ions 6. Meanwhile, AMF can distribute cathode spots (CS) over cathode surface more uniformly 7. In the case of practical vacuum interrupter with AMF contacts, an important effect of AMF should be noted. As vacuum arc extinguishes at current zero crossing, AMF does not decline to zero, but with certain strength. The lag between arc current and self-generated AMF results from the eddy current in the AMF contacts structure. The residual AMF across the contact gap after35current zero can keep the residual plasma trapped between the contacts, which can slow the insulation strength recovery speed.In 3 and 5, a high-speed vacuum circuit breaker, which could interrupt a AC fault current or DC current quickly by superimposing a high-frequency countercurrent to force current to zero,was investigated. The arc behavior in the AMF contacts in this high speed interruption processFoundations: Doctoral Fund of the Ministry of Education of China under Grants 20090201110015, and by theNational High Technology Research and Development Program of China (863 Program) under project2013AA050103.Brief author introduction:Jia Shenli was born in Shaanxi, China,in 1968. He received the B.S., M.S., andPh.D.degrees in electrical engineering from Xian Jiaotong University, Xian, China, in 1989, 1993, and1997,respectively.From July 1989 to September 1990, he was with the High Power Laboratory, Xian HighVoltage Apparatus Research Institute, the most authorized certification test laboratory in China, as a Test Engineer. He is currently a Professor with the State Key Laboratory of Electrical Insulation and Power Equipment, Xian Jiaotong University, where he is also the Director of the High Power Laboratory. His fields of interest include vacuum arcs and switches, high-power test technology, and application of arc discharges in nanomaterial fabrication. Dr. Jia is a Member of the Chinese Society for High Power Test Technology. E-mail: sljia- 5 -40was also observed in a demountable vacuum chamber with a high-speed video camera with a frame rate of 2000f/s and an exposure time of 42s. Investigation results indicated that when the arcing time was short and the interrupted current was high, the arc was concentrated during the arcing time, which would lead to the failure of high current interruption 5. However, in their work, the arc characteristics in fast current-zero stage was not investigated due to the relatively45low frame rate of the high-speed video camera.In our previous work 8, the appearance of vacuum arc in the fast current-zero stage was investigated. Since the arc was generated by opening contacts after the onset of the main current, it was difficult to observe the characteristics of CSs in the fast current-zero stage due to the motion of contact plate. Therefore, the evolution of arc column was mainly investigated in our previous50work 8. In this work, using triggered vacuum arc, we mainly investigated the influence of laggedAMF on the distribution of CSs in current-zero stage.1Experimental techniqueExperimental setup is shown in Fig. 1. Experiments were conducted in a demountable vacuum chamber with internal pressure of 510-4 Pa maintained by a turbo-molecular pump. The55vacuum arc was ignited by triggering spark at the center of the cathode. Firstly, the characteristic of CSs distribution of triggered vacuum arc in current-zero stage with current frequency of 50Hz was investigated. The arc current was supplied by L1 and C1. Then, in order to obtain faster current falling rate, a countercurrent with frequency of 500Hz or 1kHz, which was supplied by L2 and C2, was injected by a triggered vacuum switch at current peak to force the arc current to drop to zero.60It should be noted that only the characteristic of CSs distribution in the current-zero stage was investigated in this paper. Arc current and voltage were measured by a Rogowski coil and a high-voltage probe, respectively. Images of CSs were photographed through the observation window with a high-speed digital camera with an exposure time of 2s and a frame rate of56338f/s. A pair of traditional cup-shaped contacts generating bell-shaped AMF distribution wasArc currentArc voltage65used in the experiment. The contact plate diameter was 40 mm. The material of the contact plate was OFHC copper. In all experiments, the contact gap was fixed at 4 mm. Before the test, we washed the contact surfaces by alcohol and acetone to remove the organic adsorption layer and conditioned the contact surface by low-current (5kA) arcing to remove the oxide layer and adsorbed gas.70Fig. 1 Schematic experimental arrangement.2Experimental Results and DiscussionArc current waveforms with different current falling rates are shown in Fig. 2. The subgraph75located on the top right shows the enlarged arc current waveforms in the fast current-zero stage.Experimental results showed that, with frequency of 50Hz, arc current reached the peak at theinstant of 5.5ms and crossed zero at instant of 11ms. The duration time of current-zero stage was 5.5ms. However, with injecting a countercurrent, arc current decreased to zero rapidly, as shown in Fig. 2. The countercurrent with frequency of 500Hz was injected at the instant of805.28ms, arc current crossed zero at the instant of 5.69ms. The duration time of current-zero stage was 0.41ms, which was much shorter than that of 50Hz. The countercurrent with frequency of 1kHz was injected at the instant of 5.36ms, arc current crossed zero at the instant of5.49ms. The duration time of current-zero stage was 0.13ms. These arc current waveforms were used as the current load to compute the transient AMF distributions by commercial software85ANSYS with the effect of eddy current taken into account. The computed Bz distributions with different current falling rates is shown in Fig. 3. From Fig. 3, we can see that Bz distributions with different current falling rates have almost the same value at the instant when arc current began to decrease. However, at current zero crossing, Bz is much stronger in the case of high current falling rate by injecting countercurrent. Moreover, the faster the current falling rate, the stronger the90lagged Bz due to the eddy current induced in contacts system.50Hz500Hz101kHz505.2 5.4 5.6 5.81510curren t (kA)50-5-202468101214time (ms)Fig. 2 Arc current waveforms with different current falling rates. The subgraph located on the top right shows the enlarged curve inside the dotted line box.12010080Bz (mT)6040200-20 -10 0 1020r (mm)1 2 3 4 5 695Fig. 3 Comparison of AMF distribution at the midplane of the intercontact gap with different current falling rates.1-Bz at the time of current peak with arc current frequency of 50Hz. 2, 3-Bz at the instant of countercurrentinjection with frequency of 500Hz and 1kHz, respectively. 4, 5, 6- Bz at the current zero crossing corresponding to1, 2, and 3, respectively.100105110115120Fig. 4 Images of CSs in the current-zero stage with different current falling rates. (a) Image of cathode surface before the experiment. (b1) Images of CSs in the current-zero stage with arc current frequency of 50Hz. (b2)(b3) Images of CSs in the current-zero stage with injected countercurrent frequency of 500Hz and 1kHz, respectively.Fig. 4 shows images of CSs in the current-zero stage with different current falling rates. From Fig. 4 (b2) and (b3) it can be seen that CSs locating in the center of contact plate tend to extinguish earlier than those at periphery of contact plate in fast current-zero stage. This phenomenon was distinct in the CS images with arc current decreased from 5.556kA to zero. However, in slow current-zero stage, as shown in Fig. 4(b1), the number of CSs burning in the center of contact plate was larger than that in (b2) and (b3) at same instantaneous arc current. In our opinion, above difference of CSs distribution characteristics in current-zero stage should be related to the influence of lagged AMF.On the one hand, it has been revealed that 9 the plasma flux generated by a single CS expanding toward the anode showed an anisotropic angular distribution with maximum plasma density at discharge axis. Since the AMF could confine the electrons and the ions 6, the plasma flux would be confined along the axis of individual CS jets under AMF. Consequently, the density of plasma ions back to the cathode, which was important for the ignition of new CS 10, was decreased. On the other hand, the macroscopic motion of CS is, in nature, the formation of a new CS with the extinguishment of its predecessor. According to Juttner and Klebergs explanation forretrograde motion of CS 11, as the plasma jet expanding with velocity v in the transverseuv v uvmagnetic field Bt, the electric fieldE = v Btwould be generated. This electric field E, which125130could drive additional plasma ions to the surface, and therefore enhance the space charge filed on the retrograde side, was very import for the ignition of new CS. Here, v had the direction parallel to the cathode surface. Its value could be decreased by AMF due to the confining effect of AMF on the electrons and the ions. In other word, AMF could decrease the electric field E. In addition, in the center of contact plate, the flux density of transverse magnetic field Bt generated by the arc current was small. As shown in Fig. 3, the AMF was much stronger in the center region at the current zero crossing with countercurrent frequency of 500Hz and 1kHz than that without injecting countercurrent. Thus, new CSs would have a lower probability to be ignited in the center region due to the lower electric field E. Therefore, CSs located in the center region tended to extinguish earlier than those at periphery of contact plate in fast current-zero stage.1351401453ConclusionIn this paper, the influence of lagged AMF on the distribution of CSs in current-zero stage of triggered vacuum arc with different current falling rates was investigated experimentally. Research results indicated that CSs located in the center of contact plate tended to extinguish earlier than those at periphery of contact plate in fast current-zero stage due to the stronger lagged AMF in contact center, which reduced the ignition probability of new CS in the center region caused by the decrease of the density of plasma ions back to the cathode and of the electric field E generated as the CS plasma jet expanding in transverse magnetic field.AcknowledgementsThis work is supported by Doctoral Fund of Ministry of Education of China (20090201110015), and by the National High Technology Research and Development Program of China (863 Program) under project 2013AA050103.References1501551601651701751801 R. Shimada, K. Tani, H. Kishimoto et al., Development of high dc circuit breaker for large tokamak fusion device, IEEE Transactions on Power Apparatus and Systems,1979, vol. 98, no. 1, pp. 11-11.2 S. Tamura, R. Shimada, Y. 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Kleberg, “The retrograde motion of arc cathode spots in vacuum,” J. Phys. D: Appl. Phys.,2000,vol. 33, no. 16, pp. 2025-2036.纵向磁场滞后对快速过零阶段真空电弧阴极斑点分布 的影响贾申利，宋晓川，史宗谦，王立军，张营奎（西安交通大学，电力设备电气绝缘国家重点实验室，西安 710049） 摘要：本文通过实验研究了电极系统中涡流引起的纵向磁场滞后对快速过零阶段真空电弧阴 极斑点分布的影响。实验在可拆卸真空灭弧室内进行，采用杯状纵磁电极。首先，采用高速 相机拍摄了工频下的触发真空电弧在电流过零阶段阴极斑点分布的图片，高速相机的曝光时 间为 2 微秒。其次，通过投入频率为 500Hz 和 1kHz 的高频反向电流，研究了电流快速过零 阶段内阴极斑点分布的特性。此外，利用 ANSYS 计算了快速过零阶段电极间滞后的纵向磁 场的分布。研究结果表明，由于在电极中心区域滞后的纵向磁场较强，在电流快速过零阶段， 位于电极中心区域的阴极斑点比位于电极边缘区域的阴极斑点先熄灭。 关键词：真空电弧；阴极斑点；纵向磁场中图分类号：TM501+.2