Cup-type Axial Magnetic Field Research Articles

3D transient MHD simulation of DC breaking vacuum arc based on artificial current zero

In the vacuum DC circuit breaker based on the artificial current zero, the change of arc luminosity delays the change of current value significantly in the fast current-zero stage. This phenomenon has impacts on the breaking capacity of the DC vacuum circuit breaker. Therefore, it is necessary to study the dynamic characteristics of the arc plasma under the above condition. In this work, a 3D transient magneto-hydro-dynamic model based on the commercial cup-type axial magnetic field (AMF) contact is established to study this phenomenon. The simulation results show that the fast current-zero stage is too short for the plasma to diffuse. The changes of the physical characteristics of the arc plasma lag the change of current value. Moreover, the magnetic field hysteresis caused by the eddy current on the electrode occurs in this stage, resulting in a continuous stronger AMF. It makes the distributions of the plasma uniform, while the diffusion of the arc plasma reduced at low currents, which is detrimental to the post-arc dielectric recovery stage. In addition, in the fast current-zero stage, the AMF near the contact slot is smaller than in other areas. This indicates that the slotting is effective in suppressing eddy currents and avoiding the negative effect of excessive AMF on plasma diffusion at this stage. The simulation results are consistent with the experimental results.

3-D Dynamic Simulation of the Initial Expansion Process of Vacuum Arc Plasma in DC Interruption

This work investigates the dynamic characteristics of vacuum arc in the initial expansion process of a direct current (dc) circuit breaker. Based on the commercial cup-type axial magnetic field (AMF) contact, a 3-D transient magnetohydrodynamic (MHD) model is established. In the model, transient equations are used to study the transient characteristics of the vacuum arc. The effect of magnetic field hysteresis is also considered in the method. The dynamic mesh technique is adopted to simulate the electrode separation, which can make the simulation results more accurate. The simulation results show that, during the initial expansion process, with the increase in the current, the arc gradually expands in the electrode gap. The ion pressure, ion number density, axial current density, and energy flux density to the anode gradually decrease. Ion rotation can be observed under the influence of the magnetic field, and the rotation speed will increase with the increase in the current. Two different arc ignition positions are also compared. The result shows that igniting the arc at the contact center is more helpful to the success of the interruption of the dc circuit breaker. The appearance of the arc predicted by the model is consistent with the experimental results.

A novel controlled switching strategy of back-to-back capacitor banks considering the prestrike characteristics of vacuum circuit breakers

Controlled switching of capacitor banks is an effective way to suppress the inrush current during capacitive-making operations. The objective of this paper is to determine the influence of the prestrike characteristics of four types of vacuum circuit breakers (VCBs) combined with two types of controlled switching strategies on the suppression effect of the inrush current. The four types of VCBs are cup-type axial magnetic field (AMF) single-break VCB, spiral-type transverse magnetic field (TMF) single-break VCB, cup-type AMF double-break VCB and spiral-type TMF double-break VCB. The prestrike characteristics of the four types of VCBs are investigated by synthetic tests. The two types of controlled switching strategies are the traditional controlled switching strategy (the closing command depends on the mechanical touch instant of the two contacts in VCBs) and the novel controlled switching strategy (the closing command depends on the 50% pre-arcing time ta50 of the VCBs). The results show that the spiral-type TMF double-break VCB combined with the novel controlled switching strategy is the best choice for applying back-to-back capacitor bank controlled switching.© 2017 Elsevier Inc. All rights reserved.

Capacitive making of vacuum circuit breakers: Double-break vacuum circuit breaker is superior to single-break vacuum circuit breaker

Previous studies mainly focused on the prestrike phenomena in single-break vacuum circuit breakers (VCBs) under capacitive switching. However, experimental investigations on prestrike characteristics under capacitive making operations of double-break VCBs were not addressed in literatures, which were important for the strategies of capacitive controlled switching. The objective of this paper is to experimentally investigate the prestrike characteristics of single-break and double-break VCBs in switching capacitor banks, to determine the most suitable type of VCBs in capacitive switching. The double-break VCB consists of two commercial 12 kV vacuum interrupters (VIs) with cup-type axial magnetic field (AMF) contacts, which is the same VI in the single-break VCB. The results show that the 50% prestrike gap d50 of each VI in the double-break VCB, which was calculated based on the complementary Weibull distribution, was significantly reduced by 10.5% compared with the single-break VCB. Due to the voltage-sharing effect in the double-break VCB, scatters in prestrike gaps of the double-break VCB were smaller than the single-break VCB. With the application of the double-break VCB, both the inrush current interruption and the prestrike arcing time can be reduced. The double-break VCB can reduce the erosion caused by the inrush current of the low-voltage side VI. In capacitive switching, the double-break VCB is superior to the single-break VCB, especially in capacitive controlled switching.

Optimizing the Opening Velocity for a Vacuum Circuit Breaker With Cup-Type Axial Magnetic Field Contacts

This article extends the quantitative method of optimizing the opening velocities for a vacuum circuit breaker (VCB) with cup-type axial magnetic field (AMF) contacts. The material of the AMF contact is Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> . We define v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> as the initial opening velocity over half the full contact gap and v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as the average velocity over the full contact gap of 20 mm. We optimize v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> using the transition of anode-discharging modes, of which we combine the footpoint mode and anode spot mode into a large-gap high-current (LGHC) mode. The experiment shows that v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> should be higher than a threshold velocity v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1_th</sub> , which corresponds to a peak critical contact gap d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1_th</sub> for the transitions of an intense arc mode into a diffuse arc mode, in high-current interruption. The higher v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> is than v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1_th</sub> , the faster the intense arc mode evolves into the diffuse arc mode. This helps successfully interrupt the short-circuit current in a short arcing time. Another threshold velocity, v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2_th</sub> , corresponds to the maximum critical contact d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">II_th</sub> for the formation of an LGHC mode in the second arc current half-wave. A high v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> design needs v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> to be lower than v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2_th</sub> . The lower v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is than v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2_th</sub> , the better the VCB avoids the LGHC mode in high-current interruption. This helps interruption over a long arcing time. We propose an intelligent opening and breaking operation for VCBs with cup-type AMF contacts, whereby the VCB opens with different opening velocities according to the short-circuit current level.

Experimental Study on Deflection Behavior of Vacuum Arcs Under the Influence of External Transverse Magnetic Field

In real power system, when the three-phase short-circuit fault occurs, the vacuum arc in one phase will be influenced by the transverse magnetic field generated by neighbor phases and bus bars. This kind of effect is the main cause of the unstable arc and deflected erosion of contact plates, which leads to the failure of vacuum circuit breakers interruption. The objective of this paper is to get more insight into the influence of external transverse magnetic field (ETMF) on vacuum arc’s behavior. The experiments were conducted in a demountable vacuum chamber with pressure about $10^{-4}$ Pa. The cup-type axial magnetic field contacts were used, whose material was pure copper and the diameter is 35 mm. The uniform ETMF in the arc region was generated by two parallel bulk permanent magnets. The experiments were conducted under different ETMFs (0, 15, and 25 mT) and current levels (1-, 2.5-, and 4-kA rms) with different gap distances (6, 8.5, and 11 mm). The videos of arc column were recorded by a high-speed charge-coupled device camera. Under the action of ETMF, the vacuum arc got deflected. Due to the retrograde motion of cathode spots and Ampere force acting on arc column, the deflection behaviors during three typical periods (that is, initial, peak value, and close-to-current-zero period) were different. Moreover, the deflection level of vacuum arc at current peak value moment was greatly impacted by ETMF and gap distance. Larger the ETMF and gap distance were, higher the deflection level at current peak value moment was. The simulation results of arc deflection based on magnetic-hydrodynamic model were in agreement with experimental results in trends.

3-D Simulation of High-Current Vacuum Arcs Under Combined Effect of Actual Magnetic Field and External Transverse Magnetic Field

Based on a steady-state 3-D magnetohydrodynamic (MHD) model, the high-current vacuum arc (HCVA) under combined effect of actual magnetic field (MF) and external transverse MF (ETMF) is simulated. The actual MF is generated by cup-type axial magnetic field contact system commonly used in commercial vacuum circuit breakers. The ETMF may cause the deflection of arc column, which is the main reason of the contact deflected erosion. According to some experimental results, the electron temperature in HCVA is assumed to be uniform and equal to 3 eV. Therefore, the MHD model is simplified by neglecting the electron energy equation to improve the simulation efficiency. With the three conservation equations (mass, momentum, and energy) of ion flow coupling solved, the spatial distributions of some flow parameters can be obtained. The influence of all three components of the MF is inserted by solving the magnetic transport equations sequentially. Proper boundary conditions are set on the cathode and anode side, which separated the cathode spots mixing region and anode sheath region from computation domain, respectively. Under the influence of the ETMF, the deflection of the plasma flow can be predicted, which may be helpful to understand the mechanism of the contact deflected erosion.

Influence of Gap Distance on Vacuum Arc Characteristics of Cup Type AMF Electrode in Vacuum Interrupters

The greenhouse effect of SF6 is a great concern today. The development of high voltage vacuum circuit breakers becomes more important. The vacuum circuit breaker has minimum pollution to the environment. The vacuum interrupter is the key part of a vacuum circuit breaker. The interrupting characteristics in vacuum and arc-controlling technique are the main problems to be solved for a longer gap distance in developing high voltage vacuum interrupters. To understand the vacuum arc characteristics and provide effective technique to control vacuum arc in a long gap distance, the arc mode transition of a cup-type axial magnetic field electrode is observed by a high-speed charge coupled device (CCD) video camera under different gap distances while the arc voltage and arc current are recorded. The controlling ability of the axial magnetic field on vacuum arc obviously decreases when the gap distance is longer than 40 mm. The noise components and mean value of the arc voltage significantly increase. The effective method for controlling the vacuum arc characteristics is provided by long gap distances based on the test results. The test results can be used as a reference to develop high voltage and large capacity vacuum interrupters.