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Abstract- A single break 126 kV vacuum circuit breaker debuted in China is introduced.Configuration of 126 kV vacuum circuit breaker and configuration of its electrode are illustrated. Particularly, silicone oil is used as external insulation of vacuum interrupters. Heat radiators are designed for raising rated current. Permanent magnet actuators with long stroke are developed for improving performance of the vacuum circuit breaker.

I. INTRODUCTION
A project of researching and developing 126 kVsingle break vacuum circuit breaker is being done by cooperation of Xi'an Jiaotong University, vacuum interrupter manufacturers and vacuum switchgear factories in China. It took nearly sixteen months to finish a prototype. Its main technical objectives are shown in Table 1.
 
II. CONFIGURATION OF 126 kV VACUUM CIRCUIT BREAKER
The 126 kV single break vacuum circuit breaker prototype is shown in Fig. 1, which shows three vacuum interrupters are individually assembled in each porcelain bushing 1. Porcelain insulating support 2 is mounted on a horizontal steel bracket 3, and three permanent magnet actuators 4 are separately installed at the end of each insulating porcelain insulating support 2, which are electronically controlled by control box 5, thus synchronization switching could be realized. Two steel vacuum interrupter that is assembled in porcelain bushing,2-porcelain insulation supporters, 3-horizontal steel bracket,4-permanent magnetic actuator, 5-control box, 6-steel supporting stand, 7-steel base supporting stands 6 are welded between horizontal steel bracket 3 and steel base 7. The vacuum circuit breaker is fixed on a cement block. The distance between vacuum interrupters is 1700 mm and the total height of the breaker is approximately 5000 mm.
 
III. ELECTRODE ANALYSIS
A. Axial magneticfield electrode-Type A
Configuration of type A axial magnetic field (AMF) electrode is shown in Fig.2.J1' Magnetic field is generated with current passing through coils and it applies on vacuum arc, which is shown in Fig.3. The axial magnetic flux density distribution on intermediate plane of contact gap at current peak is shown in Fig.4. It showed that maximum axial magnetic flux on the plane is 0.205 T at current peak with current 4OkA rms, contact diameter 100mm and contact gap 60mm. A synthetic circuit test showed that a vacuum interrupter prototype with the type A AMF electrode successfully interrupted a 4OkA rms short circuit current.
 
B. Axial magnetic field electrode-Type B
Fig.5 shows configuration of type B AMF electrode. 21 Current flows through conducting rod 1, then it goes one round in coil 2 and it enters contact 3. By passing through arc, current flows through counterpart contact 4, coil 5 and rod 6. AMF is generated with current passing through coil 2 and coil 5, and its distribution on intermediate plane of contact gap at current peak is shown in Fig. 6. The maximum axial magnetic flux density on the plane at current peak is 0.356 T with current 40 kA rms, contact diameter 100mm and contact gap 60mm, which is 7400 higher than that of type A AMF electrode. Synthetic circuit test also proved that a vacuum interrupter prototype with type B electrode can successfully interrupt a 40 kA rms current.#p#分頁標題#e#
 
C. Axial magnetic electrode-Type C
Fig.7 shows configuration of type C AMF electrode [3].
It is a bipolar AMF electrode. As shown in Fig. 8, current passes through a conducting arm, then it divides into 2 branches in a coil. After that current goes into contact plate and it passes through 2 vacuum arc columns. Thereafter it enters counterpart contact plate, 2 coil branches and conducting arm. So the induced magnetic field have opposite directions at 2 sides of conducting rods. The maximum axial magnetic flux density on intermediate plane of contact gap at current peak is 0.202 T with current 40 kA rms, contact diameter 100 mm and contact gap 60 mm.
 
IV. SILICONE OIL POURING TECHNOLOGY
SF6 is an external insulation candidate in high voltage vacuum circuit breaker. Considering its greenhouse effects, silicone oil was tried as an replacement of SF6 as external insulation of 126 kV vacuum circuit breaker, as suggested by Okubo and Yanabu. 41 Silicone oil is a kind of transparent liquids. Its pouring technology is as following:
1) To clean porcelain bushing cavity with distilled water and then use 100 00 ethyl alcohol;
2) To Vacuumize the porcelain bushing cavity as illustrated in Fig.9. First, open the vacuum valve to vacuumize the cavity to 0.2 Pa. Second, keep vacuum degree at 0.2-0.3 Pa and gradually open the silicone oil valve and let silicone oil freely flow in the cavity till reaching a specified height level, then close the vacuum valve. Third, reopen the vacuum valve to vacuumize the rest space over the oil to 0.5 Pa. Finally, close the vacuum valve and open the N2 gas valve to charge N2 into the space till to 1 Pa.
3) The success of oil pouring technology can be examined by checking the N2 pressure at normal position or not with observing the gauge after 24 hours.

V. RESEARCH ON RAISING RATED CURRENT
Rated current of 126 kV vacuum circuit breaker shown in Fig.1 is 1600 A rms. To raise its rated current, heat radiators are installed on the breaker to improve its rated current level. Fig. 10 shows the layout of heat radiators attached to this breaker. Temperature rise was measured for the improved 126kV vacuum circuit breaker prototype at an outlet terminal, as shown in Fig. 11. Test results showed that temperature rise at the outlet terminal was 30 K with rated current 2000 A rms, which meets the requirements of Chinese standard. An additional test for temperature rise at rated current 2000 1.2=2400 A shows that it still satisfies the Chinese standard and IEC standard. This showed that rated current was increased from 1600A to 2000 A by the heat radiators technology.
 
VI. DEVELOPMENT OF HIGH STROKE PERMANENT MAGNET ACTUATOR
Construction of permanent magnet actuator is quite simple. Its parts number is only 25-40 00 of spring type actuator. Besides, it has merits of high reliable performance and long mechanical life that is approximately 100,000 operations. Since permanent magnet actuator appeared on the market, it has drawn a common interest in vacuum circuit breakers. We developed a high stroke permanent magnet actuator for 126 kV vacuum circuit breaker. Its contact stroke is 60mm and average opening velocity is 3.5-4 m/s. Moreover, synchronization switching can be realized by permanent magnet actuator that can further improve performance of high voltage vacuum circuit breaker. Because performance of permanent magnet actuator is stable, it can be used to control the contacts of vacuum#p#分頁標題#e#
circuit breaker, which open or close at about 500s prior to current zero. With much lower current vacuum arc is easy to extinguish. By my experience this may raise the rated short circuit breaking current by 20 -25%.And in closing operation, rush current can be reduced to nearly zero.
VII. CONCLUSIONS
Design features of single break 126 kV vacuum circuit breaker debuted in China is introduced, which include:
1) Silicon oil is used as external insulation of vacuum interrupters.
2) Rated current of the vacuum circuit breaker was raised to 2000 A with heat radiators.
3) Synchronization switching can be realized by permanent magnet actuator.

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