Transferred charge: indicator for vacuum applicability

The distribution and dissipation of neutral atoms are crucial for understanding the dielectric recovery process after interrupting direct current (DC) vacuum arcs. This article aims to investigate the dissipation of copper atoms after a forced extinction of the vacuum arc experimentally, adopting the plane laser-induced fluorescence method. The change in the 2-D distribution of copper atoms with time is presented. The results show that the magnetic fields, the axial magnetic field (AMF), and the transverse magnetic field (TMF) have a limited effect on the initial density of copper atoms. For both the AMF and the TMF, the copper atom densities at current zero (CZ) vary in the range (6– $8) \times 10^{17}\,\,\text{m}^{-3}$ when 3-kA vacuum arcs are forced to 0 in 0.2 ms. Contrary to the TMF case, in the AMF case, the evaporation on the anode after the CZ results in long-existing atoms near it. Consequently, the atom density of the TMF decays faster than that of the AMF, which indicates that a vacuum interrupter with the TMF contacts has a better performance when interrupting a DC load. The difference between the two magnetic fields originates from the arc control patterns. Namely, the AMF tends to keep vacuum arcs in a stable mode, which is unfavorable for a DC interruption; on the contrary, the TMF drives the vacuum arcs to move at high velocities resulting in faster dissipation of copper atoms after the CZ. In addition, the composition proportion of CuCr contacts has a limited effect on the diffusion of copper atoms when a low-vacuum arc is interrupted.

Vacuum interrupters are required to perform a wide variety of roles within vacuum circuit breakers. One duty is to pass short-circuit currents with the vacuum interrupters' contacts closed for a period of time (1 to 4 seconds), after which the circuit breaker's mechanism must be able to open the contacts, Thus the possibility of contact welding must be minimized. The flow of current through practical contacts generates a repulsive blow-off force, which has to be balanced by a closing force from the circuit breaker mechanism plus the force from atmospheric pressure acting on the vacuum interrupter's bellows. The magnitude of the applied closing force is an important parameter in a vacuum circuit breaker's design. Axial magnetic field (AMF) vacuum interrupters have an additional attractive force because of the parallel currents flowing in the two AMF coils. This force is calculated using three-dimensional finite element analysis (FEA) for practical AMF designs using contact diameters ranging from 62-100 mm. These results are then compared to two dimensional FEA models and analytic formulas, including the effect of the current frequency on the results (DC vs. 50 Hz). These attractive forces can then be combined with the other forces acting on the closed vacuum interrupter contacts to calculate the threshold welding current: the current above which the contacts will form a weld. Calculations of the total closing force compare the difference in the threshold welding current between AMF and other VI contact designs.

The capabilities and advantages of vacuum interrupters have been widely recognized on switching and controlling the fault currents in medium voltage level, and there have been doing various research and developments for using vacuum interrupters in transmission and distribution lines due to its environmental friendliness. Axial magnetic field electrodes are widely used in vacuum interrupters because the axial magnetic field can keep vacuum arc in a diffuse arc mode at high current level. It can be created by the design of electrodes itself or by external coils of a vacuum bottle where current flows. The interruption capacity of the vacuum interrupter can be increased by the magnitude of the field since the arc gets a diffused mode when the field between a contact gap is enough sufficiently. In this study, three-dimensional coupled-fields analysis of high-current vacuum arcs with an axial magnetic field which is produced by a cup-type electrode itself was conducted. We used a commercial multiphysics package, ANSYS, for coupling the fields which are connected with electromagnetics and hydrodynamics sequentially. It was found that arc constriction with the magnitude of applied currents could be predicted well with the temperature distribution of a contact gap.

A uniform axial or transverse magnetic field is applied on the silicon oil based ferrofluid of high Prandtl number fluid (Pr ? 111.67), and the effect of magnetic field on the thermocapillary convection is investigated. It is shown that the location of vortex core of thermocapillary convection is mainly near the free surface of liquid bridge due to the inhibition of the axial magnetic field. A velocity stagnation region is formed inside the liquid bridge under the axial magnetic field (B = 0.3-0.5 T). The disturbance of bulk reflux and surface flow is suppressed by the increasing axial magnetic field. There is a dynamic response of free surface deformation to the axial magnetic field, and then the contact angle variation of the free surface at the hot corner is as following, ?hot, B = 0.5 T = 83.34? > ?hot, B = 0.3 T = 72.16? > > ?hot,B = 0.1 T = 54.21? > ?hot, B = 0 T = 43.33?. The results show that temperature distribution near the free surface is less and less affected by thermocapillary convection with the increasing magnetic field, and it presents a characteristic of heat-conduction. In addition, the transverse magnetic field does not realize the fundamental inhibition for thermocapillary convection, but it transfers the influence of thermocapillary convection to the free surface.

The objective of this paper is to investigate how strong the axial magnetic field (AMF) is needed for a 126kV vacuum interrupter at different contact gaps. The AMF was generated by a pair of 2/3 coil AMF contacts. First, the accuracy of the three-dimensional finite element simulation of AMF was validated by an AMF measurement at contact gap 30mm. After that high-current switching experiments with the AMF contacts were carried out at different contact gaps of 20mm and 50mm at 40kA rms. Then the AMF at the contact gaps of 20mm and 50mm was simulated to analyze the interrupting experimental results. The results showed that there was a successful interruption at 40kA with an AMF of 235mT at contact gap 20mm. And current interruption failures occurred with an AMF of 146mT at contact gap 50mm.

The irreversibility of dielectric strength of commercial vacuum interrupters after a large number of consecutive short-circuit current interruptions is investigated in order to find out how dielectric performance of a vacuum interrupter degrades during its service life. Breakdown voltage data were measured in appropriate experiments in which the types of tested interrupters and arcing conditions were varied. It was found that for interrupters with poor contact material the dielectric strength can deteriorate severely, falling below limits required by standards. It is concluded that the most important for the irreversible decrease in the dielectric strength is the contact material, i.e. its erosion properties. It appears that contacts with axial magnetic field are less susceptible to irreversible changes in dielectric strength after short-circuit current interruption than contacts with transverse magnetic field.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Circuit breakers using vacuum interrupters (VI) are often tested according to the IEC standard 62271-100. This standard specifies a series of current interruption tests to verify the operation of the circuit breaker under a variety of different failure conditions. The IEC 62271-100 standard will be updated at the end of 2016 to include additional new tests for medium voltage circuit breakers, designed to verify successful interruption in electrical power systems with an effectively earthed neutral. In the current version of the standard, three-phase tests are performed in the non-effectively earthed neutral case. This situation is referred to as k pp = 1.5, where k pp is the first-pole-to-clear factor. In the updated standard, additional three-phase tests are performed with an effectively earthed neutral, referred to as k pp = 1.3. This setup leads to higher stress on the second phase to clear. In order to evaluate the effect of the new tests, two different VI designs, in regular commercial use which previously passed the 62271-100 standard, were tested according to the new requirements. One VI style was an axial magnetic field (AMF) design for 40.5kV/31.5kA, and the other a radial magnetic field (RMF) design for 17.5kV/31.5kA. Both VI designs were first tested with symmetric and asymmetric close/open operations for k pp = 1.5. The VI's were then tested with symmetric operations (T100s) and a large number of asymmetric operations (T100a) with k pp = 1.3. Both VI types passed these test duties, which greatly exceeded the requirements of the updated IEC 62271-100. This performance indicates that well-designed VI's that pass the current version of IEC 62271-100 will also be able to pass the new k pp = 1.3 tests for systems with an effectively earthed neutral.

To improve the limiting current interruption capability and minimizing vacuum interrupter with axial magnetic field (AMF) electrodes, it is significant to investigate the vacuum arc behaviours between the contacts. AMF distributions of the slot type electrodes were studied by both numerical analysis and experiments. Furthermore, the behaviours of vacuum arcs for different parameters of the slot type AMF electrodes were investigated by using high-speed CCD camera. The influences of gap distance, contact diameter and phase shift time between AMF and arc current on the vacuum arc were investigated. The results provide a reference for research and development of vacuum interrupters with slot type or other types of AMF electrode.

The objective of this paper is to determine optimal contact parameters of a 126 kV horseshoe type axial magnetic field (AMF) vacuum interrupter. By a combination of three design methods, including design of experiment (DOE) method, finite elements method and statistical analysis method, the optimal AMF characteristics of the contacts were determined by single factor analysis and orthogonal regression analysis. Four optimized parameters of contacts were chosen as follows: Outer diameter of iron plate (d), the difference between outer and inner diameter of iron plate (s), height of iron plates group (h) and slot angle of iron plate (θ). Here, an effective area was defined as a region in which the axial magnetic flux density was higher than 80% of the maximum value. In addition, three indexes of AMF characteristics, which were axial magnetic flux density at current peak (B y ), width of major magnetic area (D) and phase shift time (t), were chosen as optimization objectives. The optimization determined contact parameters of d, s, h and θ as 90 mm, 55 mm, 15 mm and 70°, respectively. The optimized B y , D, and t are 717.53 mT, 26.07 mm and 0.81ms, respectively.

A statistical study of dc vacuum arc extinction in a vacuum interrupter (VI) with transverse axisymmetric magnetic field has been carried out at various external circuit parameters. The relationship between the maximum current that a VI can switch off and the magnetic field, the circuit inductance, and the VI shunt capacitance has been found. A statistical study has allowed us to define this maximum switched-off current with specified probability of failure. The possible causes of a VI failure are discussed.

In the case of switching back-to-back capacitor banks, there will be inrush current while closing the circuits which will damage the contact surface of vacuum interrupters (VIs). The objective of this paper is to investigate the characteristics of the prestrike arc under the influence of axial magnetic field with different amplitudes of inrush current. Two groups of VIs with a rated voltage of 12kV and glass insulating shell have been tested while the metal shield in VIs has been removed. The prestrike arc is videoed by a high speed CCD. The contact travel, the voltage and current value of the prestrike arc are also measured and recorded with an oscilloscope. The experimental results may indicate the reason for the damage to contacts of VIs caused by prestrike arc. For the cup-type AMF contacts, when the inrush current is 5kA-10kA, slots on the contacts has a significant impact on the prestrike arc. It could make the electric field distribution much more nonuniform than that of the butt-type contact. When the inrush current become stronger, especially in 20kA, the axial magnetic field plays a major role. The prestrike arc is diffused for the most of time, and the contact surfaces are less damaged.

When vacuum interrupters are connected in series, some merits of this structure will be seen and a lot of improvements are expected. This paper describes our investigations on the reignition problem and interruption characteristics of series connected vacuum interrupters. In vacuum interrupters, CuCr (Cr: 25%) is not sufficient at the voltage level <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${&gt;}{\rm 120}~{\rm kV}$</tex></formula> because of saturation of material. Thus, we should add some material or use completely new material to replace it. By connecting two interrupters in series, the reignition problem sufficiently reduced. Simultaneously, compared with spiral electrode, axial magnetic field has an advantage on voltage sharing because it has a smaller fluctuation of postarc current. In addition, this was confirmed by the experiments with practical ac vacuum interrupters and nuclear fusion experiment dc interrupters. For dc circuit breakers (CBs), because of special characteristic, insulation recovers instantly, and this can be used for dc current breaker for subway and Tokamak nuclear experimental devices. For the application of ac CBs, as with series connection, no reignition happens, and it can be used in the Japanese market for the section separator in super express train.

A new type of “dual-contact” structure of vacuum interrupter is proposed in the paper. It consists of main contact and arc contact, which are used to break short circuit current and carry rated current, respectively. Using double-bellows, the contact bounce can be substantially reduced. In order to increase short circuit current breaking capacity of large-current vacuum interrupter, an optimal design of “iron magnetic ring” is put into arc contact to improve its magnetic performance. In terms of the new type large current vacuum interrupter, simulation results show that it is excellent in axial magnetic density and radial magnetic uniformity at current peak, and the contact area is effectively utilized.

In this paper a novel coil axial magnetic contact for vacuum interrupters is designed and its magnetic field distribution has been analyzed. This asymmetric contacts structure have a static contact with two coils in series and a moving contact without any coil. The structure of the moving contact is simplified. Its strength will be enhanced, and its weight will be reduced, which is helpful to raise the breaking speed of the vacuum interrupter. Using finite method the 3-D electrode model has been built and the simulation carried out. With the current value at maximal and zero, the axial magnetic field distribution at static contact surface, at moving surface, and at the middle surface between the static and moving contacts have been calculated. And the phase delay time of the axial magnetic field is considered as well. The simulation results provide that this asymmetric axial magnetic field structure can generate strong and uniform axial magnetic field. The residual magnetic field is small and its phase delay time after the current zero is short.

The objective of this paper is to determine the vacuum arc characteristics of a novel axial magnetic field (AMF)–transverse magnetic field (TMF) composite contact under different opening velocities. The AMF–TMF composite contact consists of a pair of outer fixed AMF contacts and a pair of inner movable TMF contacts. Drawn arc experiments were carried out in an L–C discharging circuit with this composite contact system to observe the differences in the arc development process under different opening velocities. The opening velocity of the inner movable contact was set to 1.3, 1.8, and 2.3 m/s. The arc current was up to 20 kA rms. In the experiments, the arc behavior was recorded using a high-speed camera. The arc current of each contact and the arc voltage were measured. Then the arc characteristics, including the arc current transfer characteristics, diffused arc duration, and mental droplet distribution before zero current, were quantitatively analyzed. Furthermore, magnetic field calculation was carried out to explain why the arc characteristics are different under different opening velocities. Experimental results show that with an increase in the opening velocity from 1.3 to 2.3 m/s, the diffused arc duration before zero current increases from 1.05 to 1.75 ms when the cathode is on the fixed side and increases from 1.30 to 1.90 ms when the anode is on the fixed side. In addition, the quantity of mental droplets between contacts before zero current is less with high opening velocity. It indicates stronger short-current breaking capacity under higher opening velocity. From the simulation results, it can be seen that during the arcing period, more current transfers to the fixed AMF contact under higher opening velocity and generates a stronger AMF. It could explain the earlier diffusion of the vacuum arc column under higher opening velocity.