Haoran Wang

Deformation of contact surfaces in a vacuum interrupter after high-current interruptions

The molten contact surfaces, which are caused by interrupting high currents in a vacuum interrupter (VI), can be induced to form Taylor cones by electrostatic force after current zero. Moreover, the deformation caused by the Taylor cones can further enhance the local electric field at their tips, which lead to breakdowns during post-arc phases, and make the interruptions fail. The objective of this paper is to simulate how the Taylor cones are brought out, considering influences of initial shapes, electric fields. An electrohydrodynamic (EHD) model was adopted, and the interface between vacuum and liquid metal region was traced by the level-set method. The results show that the presence of a protuberance indeed can transform to a local field enhancing Taylor cone if the liquid metal surface is exposed to a large enough electric field. Besides, the threshold for Taylor cone formation decreases when temperature of electrode increases. Consequently, the possibility of breakdown in a VI after high-current interruptions increases as well.

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.

Fully kinetic model of breakdown during sheath expansion after interruption of vacuum arcs

Research on sheath expansion is critical to the understanding of the dielectric recovery process in a vacuum interrupter after interruption of vacuum arcs. In this paper, we investigated how residual plasma affects breakdown in the sheath expansion period after the current zero. To simulate sheath expansion and breakdown, we developed a fully kinetic particle-in-cell Monte Carlo collision model with one spatial dimension and three velocity dimensions. The model accounted for various collisions, including ionization, excitation, elastic collisions, charge exchange, and momentum exchange, and we added an external circuit to the model to make the calculations self-consistent. The existence of metal vapor slowed the sheath expansion in the gap and caused high electric field formation in front of the cathode surface. The initial residual plasma, which was at sufficiently low density, seemed to have a limited impact on breakdown, and the metal vapor dominated the breakdown in this case. Additionally, the breakdown probability was sensitive to the initial plasma density if the value exceeded a specific threshold, and plasma at sufficiently high density could mean that breakdown would occur more easily. We found that if the simulation does not take the residual plasma into account, it could overestimate the critical value of the metal vapor density, which is always used to describe the boundary of breakdown after interruption of vacuum arcs. We discussed the breakdown mechanism in sheath expansion, and the breakdown is determined by a combination of metal vapor, residual plasma, and the electric field in front of the cathode surface.

The influence from the residual magnetic field on the plasma dissipation in the post-arc phase in a vacuum interrupter

Axial magnetic field (AMF) contact structures are often adopted for promoting interruption ability in the vacuum interrupters. After current-zero, there still exists residual magnetic field in the contact gap result from the eddy current. As a consequence, the plasma dissipation process which recovers the dielectric strength of the vacuum interrupter, was influenced immediately in the post-arc phase. The objective of this paper is to investigate the influence from the residual magnetic field on the plasma dissipation in the post-arc phase. A full kinetic 1D3V PIC model was built. From the results, it was found that the influence from the axial magnetic field component Bz on the plasma dissipation could be neglected. While the transverse magnetic field component Bt could trap the residual plasma and decelerate the plasma dissipation process.

Experimental investigation of typical partial discharge signals in medium-voltage switchgear

Partial discharges (PDs) phenomena in medium-voltage switchgears usually exist and are worth being paid more attention. In this paper, a kind of special switchgear cabinet and two kinds of typical PD sources is designed for the PD measurements. Based on the two typical PD models, measurement sensitivity and influence factors of above four kinds of detection methods are researched and analyzed. The experimental results show that Pulsed current (PC) method is the most sensitive to PD signals but it is also more sensitive to the electromagnetic disturbance. Holes on the switchgear cabinet are important for Acoustic Emission (AE) method. The cabinet materials and sensor positions have no significant influence on Transient earth voltage (TEV) signal. Impermeability magnetic material has more significant influence on preventing Ultra-high-frequency (UHF) signals transmission than the permeability magnetic material, which is unfavorable for detection of UHF signal. Larger holes on switchgear cabinet are beneficial to the measurement of UHF method. For the needle-plane model, the phase-resolved partial discharge (PRPD) graph changes from “ball” shape to the “M” shape as the voltage increases. Different from needle-plane model, PRPD graph of suspended-metal model exists significant stratification phenomena.

Simulation on breakdowns in the post-arc phase of a vacuum interruption

Breakdowns in the post-arc phase are affected by a combination effect of both plasma and metal vapor. The objective of this paper is to study the breakdown boundary as a function of plasma and metal vapor density. A fully kinetic PIC-MCC model was developed to study breakdowns in the post-arc phase. Relevant collisions were taken into consideration, and an external circuit was added to the model to make the calculations self-consistent. From the results, we obtained the breakdown boundary in the post-arc phase under effect of both metal vapor and residual plasma. The residual plasma does not play a major role in breakdowns if the density was below 1017 m-3. However, if the residual plasma density exceeded 1017 m-3, the critical metal atom density for a breakdown steeply decreased with the increase of the plasma density. Therefore, a sufficiently high plasma density gives rise to a higher probability of breakdowns. For this reason, the threshold of metal vapor density could be overestimated if the residual plasma is not considered in the simulations.

DC current interruption by a combination of electric fuse and vacuum switch

The objective of this paper is to experimentally investigate the DC current switching characteristic of a combined DC circuit breaker. The DC circuit breaker, which is a combination of a DC switch and a fuse, has a significant advantage in full-range DC current switching. When a load current or an over-load current emerges, the DC load switch has the ability to clear the current. However, when a short-circuit current is encountered, which would have been exceeded the ability of the DC switch, the fuse is used to decrease the fault current to a lower value, then the DC switch is used to clear the tail current. In this case, it is easier for the combined DC circuit breaker to withstand a transient recovery voltage (TRV) after current zero, because the TRV of power lines stresses on both terminals of fuse and DC switch. In a current making operation of DC circuit breaker, the DC switch makes the current individually. A diode is connected in series with the LC discharging circuit to generate a test DC current of 3kA. Another LC discharging circuit is used to generate a high-frequency counter current of 3.1 kA, which is injected into the DC current to generate an artificial current-zero. Experimental results showed that the combined DC circuit breaker can interrupt the test DC current under 1 kA in a range from 3ms to 4ms. Fuse can interrupt the short circuit current of 3 kA in 20ms.

A PIC-MCC modeling of sheath recovery processes after current zero

A sheath recovery process dominants the initial stage of a dielectric recovery process after interrupting a vacuum arc current. The objective of this paper is to model a post-arc sheath recovery process by using PIC-MCC method. Second electrons, external circuit, and fundamental collisions between particles were taken into considerate in the model. The result shows that the amplitude of post-arc current have a proportional relationship with initial density of plasma. The sheath recovery time turned slowing down after a quick increase with the increase of initial plasma density.

Numerical simulation of anode thermal processes of different materials in vacuum arc

Electrode material seriously influences the characteristics of vacuum arc and further affects the performance of switches. In this paper, thermal processes of six kinds of metal anodes (including pure metal and alloy anodes) are simulated and researched. Two kinds of temperature calculation methods are used. Simulation results show that W and Mo anodes have the higher temperature than Cu, Cr, CuCr25 and CuCr50 anodes. Pure Cr anode has the biggest melting width, and highest saturated vapor pressure. Cu anode has the biggest melting depth. W anode has the smallest melting area. Axial temperature gradient is related to the thermal conductivity, 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 appear in the results of CuCr alloys, between the two melting points, the alloy anodes are in solid-liquid mixture state.