DC vacuum circuit breaker adopts the method of artificial current zero crossing to break the fault current. During the opening process, the coordination mechanism of pre-zero current drop rate and post-zero voltage rise rate will cause changes in the insulation performance of the DC vacuum circuit breaker, which will directly affect the breaking performance. For improving the breaking performance under the coupling of pre-zero current drop rate and post-zero voltage rise rate, this paper study the variation of the number density of mixed metal vapor particles in post arc. Moreover, based on the drift diffusion equation and the electric field Poisson equation, the development model of the post arc sheath is established to simulate the dissipation of charged particles between the electrodes in post arc, and to study the cathode electric field strength variation. Furthermore, the evaluation criteria of the post-arc breakdown endurance performance under the coupling of pre-zero current drop rate and post-zero voltage rise rate are proposed, and the group of pre-zero current drop rate and post-zero voltage rise rate for reliable breaking the fault current has been obtained.
In vacuum interrupters, plasma remains between the electrodes after the current zero due to arcs generated by the interruption of short-circuit current. This residual plasma affects the insulation recovery characteristics after current interruption. The post-arc current is a parameter that correlates with this ion density. It is also affected by the secondary electrons generated when the ions hit the high-temperature electrode. In this study, a prototype spiral electrode was mounted in a vacuum chamber, and the electrode surface temperature and the post-arc current were measured. The electrode was made of Cu-Cr material, and the interruption current of 10-24 kA was applied. The electrode surface temperature was measured two-dimensionally using a two-color pyrometer method. The results showed that the higher the electrode surface temperature, the higher the post-arc current and post-arc charge after current zero. (Abstract)
In scanning electron microscopy (SEM), restricting surface flashover is necessary to achieve stable insulation properties with a compact structure at high acceleration DC voltages. The flashover discharge model is regarded as an avalanche of field-emitted electrons caused by the concentration of electric fields and the high secondary electron yield on the insulating surface of alumina ceramics. An effective way to improve this phenomenon is by suppressing field emission electrons at the initial stage of the flashover phenomena. Herein, the electric field concentration was relaxed by covering the cathode edge with a vanadate glass to obtain favorable results. Vanadate glass has a conductive dielectric property and can be sintered to an amorphous and smooth surface that can be used to cover the cathode terminal edge to hide the cathode edge and smooth any microprojections. A vanadium-based vanadate glass was used as this has a glass transition temperature of $350^{\circ}C$, crystallization temperature of $450^{\circ}C$, resistivity of 10$^{7}-10^{8} \Omega$ -cm, and relative permittivity of 15. The voltage-holding capability of the cathode edge of a sample with a cathode metallized on an alumina surface was evaluated with and without a vanadate glass coating. The wettability of the vanadate glass to the metallized and alumina surfaces was excellent, and smooth surfaces were formed without defects such as peeling and cracking. Theoretical calculations showed that the effect of the field concentration mitigation was approximately three-fold greater than that of vanadate glass. The field emission electron emitter is used in the electron gun of the SEM, and the vanadate glass must be stable in an ultra-high vacuum environment without gas desorption. Therefore, the voltage-holding capability was evaluated using voltage-holding experimental tools incorporated in an ultra-high vacuum (around 10$^{-7}$ Pa) chamber. Consequently, a voltage-holding property of approximately 3-fold the electric field mitigation effect without outgassing was obtained, and a favorable field relaxation effect was confirmed.
We have to elucidate the partial discharge phenomenon in a vacuum for the vacuum interrupter development, which occurs with regional light emission and electron charging on ceramic surfaces. They are caused by electric field electrons emitted from high electric field points, e.g., cathode triple junctions, and secondary electrons emitted from ceramics when the electric field electron hits on ceramic surfaces. Therefore, the suppression of these electron emissions leads to the reduction of partial discharge occurrence. There are some researches about the suppression of field electron and secondary electron emissions. We focused on chromium oxide coating on ceramics surface that suppress these electron emissions effectively. The secondary electron emission coefficient of chromium oxide is lower than that of alumina ceramics, so the secondary electrons are hard to be emitted from chromium oxide coating regions. Some researchers investigate the effects of coating chromium oxide on breakdown performance, but few researchers do the effects on partial discharge in a vacuum. In this article, we investigated the effects on partial discharge in a vacuum with chromium oxide coating and measured Partial Discharge Extinction Voltage (PDEV), which is one of the parameters to evaluate the characteristics of partial discharge in a vacuum. We used the evaluation samples that represent the internal structure of a vacuum interrupter and have different chromium oxide coating regions in order to investigate the effect. As a result, it was confirmed that PDEV in the sample with coating was about 1.5 times higher than that of without, and the larger the coating region was, the higher PDEV was. From the above results, it turned out that coating chromium oxide to ceramic surface was the effective way to suppress partial discharge in a vacuum.
In this study, we attempted to detect partial discharge occurring between motor winding using the pulsed electroacoustic method. In recent years, our research group has been conducting space charge distribution measurements using a voltage application method via a spatial gap for the measurement samples. To verify the detection of discharge using this method, we conducted space charge distribution measurements under atmospheric pressure and vacuum environments. As a result, charge drift due to discharge was not detected under vacuum environment, whereas it occurred under atmospheric pressure environment, and the discharge initial voltage roughly corresponded to Paschen’s law. Therefore, this study has revealed the possibility of detecting discharge using the PEA method.
This paper presents a first characterization of direct current Switch-On effect, for high voltage, high vacuum electrodes. Considerations are exposed to explain this Switch-On effect as a consequence of the accumulation of electric charge at the MI cathode interface.
Two heating Neutral Beam Injectors (NBIs), required for plasma heating and current drive, are foreseen for ITER operation. Each beam will be generated by a 40A current of Deuterium negative ions, accelerated up to the specific energy of 1MeV and then neutralized, delivering to the plasma a power up to 16.5 MW each. The beam source (BS) will be constituted by an RF-driven negative ion source at –1 MV potential and by an electrostatic accelerator (consisting of 5 stages at intermediate potentials). All components will be installed in a vacuum vessel, together with a high-capacity cryo-pumping system that controls the background gas pressure.In order to validate the ITER NBI design and address all the outstanding issues related to these demanding requirements, a full-scale prototype called MITICA (Megavolt ITER Injector & Concept Advancement) is under construction in Padova at Consorzio RFX. Voltage insulation in vacuum and/or very low-pressure gas on a single gap is indeed one of the expected issues that MITICA will have to deal with. An effective solution for increasing the voltage holding capability of the system consists in the use of an additional intermediate electrostatic shield, biased at an intermediate potential, placed between the ion source and the vacuum vessel. In this paper, the electrostatic design of the shield is presented, by considering the voltage holding capability. A novel 3D version of a numerical tool, called Voltage Holding Prediction Model (VHPM), is applied to the additional intermediate shield design to assess the expected voltage holding capability of the experiment in high vacuum.
We discuss the electrode material adhesion between cathode and anode during spark conditioning process in vacuum, and its effect on dielectric strength in terms of breakdown (BD) charge. We focus on the conditioning effects under rod-plane electrodes with different combination of electrode materials. The material of the rod (cathode) is Cu and that of the plane (anode) is stainless steel (SUS), and their gap length is 1 mm. We observe the electrode surface before and after conditioning experiments. As a result, when BD charge is smaller than 2.0 mC, the anode material (SUS) is found on the cathode (Cu) surface and the dielectric strength becomes high. With increasing BD charge, the adhesion ratio of anode material on the cathode surface decreases, and the dielectric strength also tends to decrease. In the case where BD charge is larger than 2.0 mC, on the contrary, the cathode material (Cu) adheres to the anode (SUS) surface and the dielectric strength is low, which is consistent with the conditioning characteristics when both the cathode and anode are made of Cu. These results indicate that up to a certain BD charge, the anode material adheres to the cathode surface, and above this BD charge the cathode material adheres to the anode surface. When the cathode (Cu) surface is excessively melted, i.e., damaged, the cathode material Cu can adhere to the anode SUS surface. In other words, too large BD charge can not only fails to improve the dielectric strength due to the adhesion of the anode material SUS to the cathode Cu surface, but also causes a critical decrease in the dielectric strength, or deconditioning.
This article presents the basic physical and numerical principles of a fluid model implemented into the numerical code fluid solver for SPIDER in 2D (FSFS2D) used for simulations of the source for the production of ions of deuterium extracted from RF plasma (SPIDER) negative ion source. It gives self-consistent 2-D description of the source, including the neutral gas flow, plasma chemistry, radio frequency (RF) coupling in the source driver, and plasma transport through the magnetic filter (MF). In order to capture the neutral depletion and its impact on the plasma dynamics the continuity equation for molecules and the corresponding surface chemistry have been included in the model. An important element in the development of the numerical framework is the validation of the code results against the experimental data. For this aim, a series of numerical simulations have been performed and compared with the experimental data from the SPIDER experimental campaigns. This article presents critical assessment of the validation results and outlines the necessary code/model enhancements required to improve the predictive capability of the FSFS2D code.
The paper describes the development of a diagnostic system to localize the sources of X-ray in a High Voltage Direct Current (HVDC) device insulated by large vacuum gaps. Electrons extracted from cathodic surfaces by Field Emission (FE) processes are accelerated to energies below 1MeV producing bremsstrahlung radiation from the impact areas on anodic surfaces. This radiation is one of the main experimental evidences observable during the high voltage conditioning in systems insulated by large vacuum gaps with voltages beyond $50 \div 100$ kV. The technological challenge associated to the achievement of a stable accelerating voltage in MITICA, the prototype of the Neutral Beam Injector for ITER, will require diagnostic systems able to monitor the evolution of the high voltage conditioning of the electrostatic accelerator by detectors located outside the beam source vessel. The measurement of the spatial distribution of the X-ray sources could be essential to understand the weak points in term of voltage holding for the MITICA electrostatic accelerator. The adoption of a X-ray collimator embedding a Cerium doped Lutetium-based scintillation crystal has been proposed to cope this target. A prototype of a Single Pixel Collimator with variable geometry shield by lead has been manufactured and tested at the High Voltage Padova Test Facility (HVPTF). High voltage tests with acceleration voltages up to 780kVdc have been carried out to improve the collimator design.
A vacuum arc discharge occurs between the electrodes during current interruption process in the vacuum circuit breaker (VCB). This vacuum arc is maintained by metal vapor supplied from the metal electrodes. The vacuum arc behavior is extremely complex and therefore difficult to elucidate solely from experimental observations. Therefore, it is essential to investigate the vacuum arc behavior from numerical simulation to improve the current interruption performance of VCBs. We have been developing a numerical model to simulate the behavior of low-current vacuum arc by our original model, a hybrid model of the Moving Particle Semi-implicit (MPS) method and the Finite Volume Method (FVM). In this paper, the low-current decaying vacuum arc was simulated from 50 A to 0 A at different current decay rates using our developing MPS+FVM model. As results, during the current decay process, the metal evaporation rate from the cathode spot decayed with time due to a decrease in Joule heat, leading to a decrease in the amount of metal vapor supplied to a space between the electrodes. The heavy-particle temperature $(T_{\mathrm{h}})$ near the cathode spot was found to remain high, whereas the averaged $T_{\mathrm{h}}$ between the electrodes decreased due to the smaller power injected between the electrodes.
Two-dimensional spectroscopic observation was carried out in the experiments to obtain the radiation intensity distribution of specified spectral lines for a study of vapor dynamics in vacuum arcs at low currents. Two ICCDs were used to intensify the two-dimensional spectral radiation intensity from the copper neutral atoms and ions between the copper electrodes. In addition, we have developed a hybrid model using unique particle and fluid methods to obtain the dynamic behavior of metal vapor in a vacuum arc in low-current phase. The developed simulation model was used to calculate temporal evolutions in radiation intensity distribution of the specified emission coefficients from copper atoms. The calculated emission coefficient distributions were compared to the experimentally obtained two-dimensional radiation from copper atoms to verify the developed model. Results showed a rough agreement between them.
Surface flash triggered vacuum switch (STVS) is widely used in pulse power closure systems due to its excellent working performance. The overall size of STVS is also an important factor affecting its application. This paper demonstrates a high-performance miniaturized STVS with an operating voltage up to 35 kV. Based on Comsol electric field simulation optimize the internal shield and main electrode structure of STVS to make the package shell diameter less than 45mm. A special trigger electrode structure is designed and tungsten metal is selected as the electrode material to optimize the trigger threshold and working life of the miniaturized STVS. We fabricated a miniaturized STVS prototype, built a test circuit, and measured the miniaturized STVS working performance. The experimental results show that the designed miniaturized STVS has a minimum trigger delay time less than 120 ns, a jitter less than 30 ns, a trigger voltage threshold less than 3 kV, a trigger current threshold less than 5 A, and a working life more than 800 times.
The Electrothermal pulsed plasma thruster (EPPT) is widely used in microsatellite propulsion systems because of its simple structure, high impulse bit, and precise control over thrust. The EPPT consists of an ignitor, electrode, and propellant, and the cavity is located in the center of the propellant. The ignitor generates the initial plasma that, under the control of the electric field, makes the propellant melt, detach and ionize. This process leads to the formation of a plasma jet, which is the source of thrust. Ignition stability is crucial for EPPT since it affects the formation process of the plasma jet. In this paper, an EPPT has been designed and the effects of various factors associated with the ignitor on the ignition stability were investigated, including the structure parameters, discharge parameters, and installation position. The results showed that, within a reasonable range, increasing the ignitor diameter and ignition voltage improves discharge stability. In addition, the discharge surface of the ignitor should not be too far from the cavity and should be tangential to the inner wall of the cathode to improve ignition stability.
Tungsten surfaces with Nano-tendril bundles (NTBs), a collective group of fibrous nanostructures which forms isolated and grow up to tens of microns, are exposed to high density hydrogen plasmas. Under the conditions in which sheath thickness are comparable or smaller than the size of NTBs’, with negative biasing voltage, the NTBs presented bright emission due to concentrated heating by incident ions, leading to the formation of hot spots. At the same time, arc traces are observed on the surface, but far from the NTBs area, suggesting that the concentrated heat load precedingly degrades NTB’s tip, lowering the local field emission and hence arc ignition probability.
Low current vacuum arcs have the natural tendency to interrupt currents before the natural current zero of a system, which is known as current chopping. This paper presents results of investigations comparing the chopping current performance of isotropic and anisotropic WCCu contacts for two compositions (WCCu 60 wt.% and WCCu 79 wt.%). The goal is to show the influence of the contact microstructure, and the role of the associated increase in the electrical and thermal conductivities of the anisotropic material on the chopping current. The results indicate an improved median chopping current value for the anisotropic compositions compared to the isotropic ones. It is lower of about 1A for WCCu 60 wt.% (5.1A vs 6.0 A), while is it is only slightly lower for WCCu 79 wt.% (9.0 A vs 9.4 A). The SEM investigations of the top surface structure do not show significant changes after a few arcing operations at 75A peak current between the two types of microstructures. It is assumed that the current flow localized below the contact surface is affected by the microstructure in a way that favors local heating at the last cathode spots and help to sustain the arc stability.
The vacuum interrupters (VI) are an environmentally friendly alternative to circuit breakers based on sulphur hexafluoride (SF6). The aim is to replace the SF6 interrupters. During the interruption process of a VI a plasma consisting of metal vapour occurs. This plasma must be extinguished as fast as possible to recover the dielectric strength. Therefore, it is necessary to understand the plasma behaviour. The plasma can be described through the voltages and currents during the breaking process. In this paper an analysis of the post-arc current is carried out.The aim is to describe the phenomenon during the current zero crossing (CZC) like chopping current. Limits and appearances will be described by the current and voltage curves at CZC. Thereby, the circuit elements, which generate a circuit response will consider and with a circuit model will verify the plasma behaviour by the current curves. The measurements were taken in a synthetic test field, a Weil-Dobke circuit. High short-circuit current up to 15 kA rms, 50 Hz and transient recovery voltages (TRV) of 60 kV peak were carried out.
The paper is dedicated to study of external breakdowns during voltage conditioning process of medium voltage vacuum interrupters (VI). Impact of internal design on external dielectric withstand was investigated. It was shown that due to effective shielding of triple junctions and optimization of shape of ceramics shields level of external withstand can be significantly increased. This permits to apply high voltages for conditioning of compact VI’ s leading to more rapid and efficient process.
Magnetic field generated by the contacts of Vacuum Interrupter (VI) support the interruption of fault current. Therefore, the interruption performance of VI is mainly influenced by the contact configuration. To develop high voltage VI, it is important to design and optimize the contact configuration applicable in high voltage system. Various contact configurations have been suggested for high voltage VI in the literature, whereas the optimization technique of suggested designs has not been performed yet. Therefore, in this paper, the optimization technique of the contact configuration for high voltage VI was proposed based on response surface method (RSM) and multi objective genetic algorithm (MOGA). The four design factors and three objectives were considered for optimization. The four design factors are coil height (h), coil width (w), slit length (l) and mounted angle (a). The three objectives are maximum axial magnetic flux density at peak current (B z ), effective area ($\lambda$), and maximum axial magnetic flux density at current zero (B$_{zo}$). The optimization was performed by applying MOGA to the regression equation of the objectives derived by RSM with Central Composite Design (CCD). As a result of optimization, B z and effective area increased by 6% and 9%, while B$_{zo}$ decreased by 10%. Consequently, it was confirmed that the interruption performance of high voltage VI could be improved by optimization.
Switching opening operations in vacuum are characterized by an arc, which occurs during mechanical contact separation and is created from vaporized contact material. At low currents it is unstable and can chop before the natural current zero crossing. The result of this transient process is a high current gradient, which can lead to high overvoltage’s e.g. at inductive loads due to resonance phenomena. Damages to the equipment and electromagnetic compatibility (EMC) issues can occur. Contact materials for contactor applications like tungsten carbide silver (WCAg) show a chopping current in the range of 0.7… 2.5 A. Circuit breakers must interrupt high short circuit currents without damaging the contact surface by material erosion. Therefore, zero chopping is not the focus. In circuit breakers the chopping current is between 4 and 6 A with contacts made of copper chromium (CuCr). Different approaches can be used to improve the chopping behavior. The choice of the used contact materials can reduce current chopping or shift it closer to the zero crossing. Lower chopping values with the reduction of transient overvoltage’s at inductive loads are important especially for contactor applications. Therefore, an emission of the contact material at low temperatures is necessary. Another approach is the superposition of an externally applied axial magnetic field during the switching operation. Some publications indicate an influence on the chopping behavior. Thirdly, the switching speed for the selected materials is investigated in more detail. This paper determines both, different contact materials and superimposed axial magnetic fields regarding their chopping behavior. Contact materials comparable to WCAg show a 52% reduction of the chopping current without a superimposed magnetic field. Other materials show a behavior like CuCr. The superimposed magnetic flux density (up to B=200mT) provides higher values for the chopping current with a minimum at approx. B=80mT.
