Zhonghao Qian

3D Numerical simulation of high current vacuum arc in realistic magnetic fields considering anode evaporation

A time-dependent 3D numerical model considering anode evaporation is developed for the high current vacuum arc (VA) under a realistic spatial magnetic field. The simulation work contains steady state 3D numerical simulation of high current VA considering anode evaporation at nine discrete moments of first half wave of 50 Hz AC current, transient numerical simulation of anode activity, and realistic spatial magnetic field calculation of commercial cup-shaped electrodes. In the simulation, contact opening and arc diffusion processes are also considered. Due to the effect of electrode slots, the simulation results of magnetic field and temperature of anode plate exhibit six leaves shape (SLS). During 6–8 ms, the strong evaporation of anode surface seriously influence the parameter distributions of VA. Ions emitted from anode penetrate into arc column and the axial velocity distribution on the anode side exhibits SLS. The ions emitted from anode surface have the same temperature with anode surface, which cool...

Experimental Investigation on the Initial Expansion Process in a Drawn Vacuum Arc and the Influence of Axial Magnetic Field

The initial expansion process in a drawn vacuum arc and the influence of axial magnetic field (AMF) were investigated experimentally in a demountable vacuum chamber. Arc characteristics were investigated with the aid of a high-speed digital camera with an exposure time of 2 μs . In a drawn vacuum arc, the arc sequence begins with the bridge column arc formed after the rupture of the molten metal bridge. This column evolves into the transition mode, which consisted of a central column with few or no cathode spots (CSs) outside the column, and then into the fully diffuse mode. Experimental results indicated that in transition mode arc, the expansion process could be characterized by the appearance of CSs outside the central column, and could be classified into two patterns, “slow” expansion and “quick” expansion according to the characteristics of the formation and motion of new CSs (conducting channels) outside the central column of arc. The influence of AMF and its distribution on the expansion process was also investigated. Investigation results indicated that AMF had two contrary effects, i.e., inhibiting effect and prompting effect, on the initial expansion stage of drawn vacuum arc. Furthermore, saddle-shaped AMF could encourage the arc transition into diffuse mode more effectively than bell-shaped AMF.

Anode Activity in a High-Current Vacuum Arc: Three-Dimensional Modeling and Simulation

In this paper, a transient 3-D model of the anode activity in a high-current vacuum arc (HCVA) was established. The melting, flow, evaporation, and solidification of the anode material were included in this model. Based on this model, the thermal and flow process of the anode in an HCVA under axial magnetic fields was simulated and analyzed. Simulation results showed that the maximum anode temperature appeared near 7 ms. The maximum rotational velocity and melting radius appeared near 9 ms. This meant that the anode was still in the melting, flow, and evaporation status near current zero moment, which was easier to cause reignition of vacuum interruption. Through 3-D modeling and simulation, a more visualized anode process can be understood. In the future, unsymmetrical anode phenomena will be researched and analyzed by this 3-D model.

3-D Simulation of Plasma's Rotation Behavior in High Current Vacuum Arcs Under Realistic Spatial Magnetic Field Profile

Based on a 3-D steady magnetohydrodynamic model, the high current vacuum arc subjected to a realistic spatial magnetic field generated by the cup-type contacts was simulated. The influence of the realistic spatial magnetic field profile on plasmas rotation behavior was analyzed. The distribution of the azimuthal velocity on anode side under realistic spatial magnetic field profile was quite different from that under uniform axial magnetic field.

Experimental Investigation on Vacuum Arc Behaviors Subjected to Larger Diameter Cup-Shaped and Coil-Shaped Axial Magnetic Field Electrode

In this paper, high-current vacuum arc (VA) breaking experiments in vacuum interrupters were conducted using two kinds of high-capacity (larger diameter) axial magnetic field (AMF) electrodes with contact gap distance changing from 10 to 40 mm. Triggering arc mode and fixed gap distance were adopted. Experimental results showed that during the initial diffusion stage, the arc voltage of high-current VA got a peak value. The change in high-current VA appearance would also affect the curve of arc voltage. With the increase of contact gap distance, the voltage of high-current VA increased and the VA near the anode appeared contracted, and its shape was changed from cylinder to cone. In the shorter gap distance, for cup electrodes, the plasma jets mix with each other and the arc column appears like a bright cone, whereas for coil electrodes, the plasma jets are clear and the column appears like a cylinder, and the transfer of arc appearance affects arc voltage distribution. Moreover, arc twist and rotation often occur under longer gap and higher current, which is due to the larger radial magnetic field component generated by AMF coils, and arc voltage appeared high-frequency and high-amplitude noise, and ultimately resulted in the distortion of the arc current in the current experimental conditions (lower charging voltage of capacitor).

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.

The 3D simulation of high-current vacuum arc under combined effect of actual magnetic field and external transverse magnetic field

Based on a steady 3D Magneto-Hydro-Dynamic (MHD) model, the high-current vacuum arc (HCVA) under combined effect of actual magnetic field (MF) and external transverse magnetic field (ETMF) is simulated. The actual MF is generated by cup-type AMF contact system. 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 3eV. So the MHD model is simplified 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 magnetic field 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 found which may be helpful to understand the mechanism of the contact deflected erosion.

Numerical Simulation of Thermal Characteristics of Anodes by Pure Metal and CuCr Alloy Material in Vacuum Arc

Anode material seriously influences the characteristics of vacuum arc and further affects the performance of medium-voltage vacuum switches when the interruption current is high. There are many materials used for electrode manufacture, and different materials are selected for different switches. For a pure metal, its performance usually cannot satisfy the actual requirement. To improve switchs performance, an alloy is usually used as an electrode material. In this paper, thermal processes of six kinds of metal anodes (including pure metal and alloy anodes) are simulated and researched. The physical parameters of the pure metals all come from experiment results directly or are fitted by the experimental data. The physical parameters of the CuCr alloys are derived from Cu and Cr parameters. Two kinds of temperature calculation methods are used, which are called melting and solidification model and equivalent specific heat method, respectively. Simulation results show that W and Mo anodes have the higher temperature than Cu, Cr, CuCr25, and CuCr50 anodes. A pure Cr anode has the largest melting width and highest saturated vapor pressure and evaporation energy. A Cu anode has the biggest melting depth. A W anode has the smallest melting width and depth. Axial temperature gradient is related to the thermal conductivity, and the Cr anode has the largest axial temperature gradient. The thermal characteristics of CuCr25 and CuCr50 anodes are located between the pure Cu and Cr anodes. There are two melting points appearing in the results of CuCr alloys, and between the two melting points, the alloy anodes are in solid-liquid mixture state.

The Influence of Ignition Position on Arc Characteristics in the Transition Mode of a Drawn Vacuum Arc in Cup-Shaped AMF Contacts

In a drawn vacuum arc, the arc sequence begins with the bridge column arc formed after the rupture of the molten metal bridge. This column evolves into the transition mode, which consists of a central column with few or no cathode spots outside the column, and then develops into the fully diffuse mode. In this paper, the influence of arc ignition position on arc characteristics in the transition mode of a drawn vacuum arc was investigated experimentally in a demountable vacuum chamber. Practical cup-shaped axial magnetic field (AMF) contacts were used in the experiments. Arc characteristics were investigated with the aid of a high-speed digital camera with an exposure time of 2 μs. Experimental results indicated that the characteristics of the arc and arc voltage, particularly the peak in the arc voltage waveform, in the transition mode of a drawn vacuum arc in cup-shaped AMF contacts were closely related to the arc ignition position.

Experimental investigation of cathode spots and plasma jets behavior subjected to two kinds of axial magnetic field electrodes

In this paper, cathode spot plasma jet (CSPJ) rotation and cathode spots behavior subjected to two kinds of large diameter axial magnetic field (AMF) electrode (cup-shaped and coil-shaped) are studied and analyzed based on experiments. The influence of gap distances on the CSPJ rotational behavior is analyzed. Experimental results show that CSPJ rotational phenomena extensively exist in the vacuum interrupters, and CSPJ rotational direction is along the direction of composite magnetic field (mainly the combination of the axial and azimuthal components). For coil-shaped and cup-shaped AMF electrodes, the rotational or inclination phenomena before the current peak value are much more significant than that after current peak value (for the same arc current), which is related to the larger ratio of azimuthal magnetic fieldBt and AMF Bz (Bt/Bz). With the increase of the gap distance, the AMF strength decreases, when the arc current is kept as constant, the azimuthal magnetic field is kept invariable, the ratio between azimuthal magnetic field and AMF is increased, which results in the increase of rotational effect. For cathode spots motion, compared with cup-shaped electrode, coil-shaped electrode has the inverse AMF direction. The Robson drift direction of cathode spots of coil-shaped electrode is opposite to that of cup-shaped electrode. With the increase of gap distance, the Robson angle is decreased, which is associated with the reduced AMF strength. Erosion imprints of anode and cathode are also related to the CSPJ rotational phenomena and cathode spots behavior. The noise of arc voltage in the initial arcing stage is related to the weaker AMF.