Diffuse Vacuum Arc Research Articles

The Application of Vacuum Interrupters in HVDC Circuit Breakers

It has become necessary to develop high-voltage dc (HVDC) circuit breakers to protect against fault currents that may occur in HVDC grids. One possible HVDC circuit breaker design uses vacuum interrupters as the main dc current path and the dc current interrupter using a counter-pulse system to create an artificial current zero. As the counter-pulse has a relatively high– di / dt, the vacuum arc mode at the current zero is critical when withstanding the resulting transient interruption voltages (TIVs). The fault current in the dc system can also increase rapidly to a very high value. In order to prevent serious damage to the dc grid, it is imperative that the vacuum interrupter open soon after the fault is detected and the counter pulse applied in order to limit its magnitude. This article discusses the vacuum arc modes as the contacts open and the requirements to create a fully developed diffuse vacuum arc at the counter-pulse current zero. The vacuum interrupter also has to develop a large enough contact gap in order to withstand the expected recovery voltages (TIVs). To do this a high-speed opening mechanism is required. Thus, the vacuum interrupter will accelerate faster than it would have experienced in a high-voltage ac (HVAC) vacuum circuit breaker. The article presents the critical design parameters for the vacuum interrupter to operate in a HVDC vacuum circuit breaker and contrast them with vacuum interrupters used in HVAC circuit breakers.

Study of the Ion Composition of the Diffuse Vacuum Arc on a Hot Cathode by the Time-of-Flight Method

The study of a diffuse vacuum arc (DVA) is of interest in connection with the developing methods of plasma processing of the spent nuclear fuel (SNF). This type of discharge can provide a high degree of ionization of the vapors of the cathode material in the absence of droplets and multiply charged ions. This work is devoted to the study of the ion composition of the arc discharge plasma at a consumable hot gadolinium cathode by analyzing the optical and mass spectra of ions. The plasma ion composition is determined at a discharge current of 50 A, arc voltage ranging from 4 to 14 V, and cathode temperature ranging from 1.9 to 1.95 kK. The obtained data, together with the data recorded using the condensation probe method, showed that at an arc voltage of 5 to 6 V a plasma is generated with an ionization degree close to 100%, which contains more than 95% of singly charged ions among all the charged states.

Diffuse vacuum arc with heated cathode made of ceramic (CeO2) and metal (Cr) mixture

The paper presents a diffuse vacuum arc with heated cathode made of ceramic (CeO2) and metal (Cr) mixture, initiated and studied for the first time. Plasma sources of multicomponent mixtures are demanded for plasma based methods of rare-earths recycling and spent nuclear fuel reprocessing, which are currently under development. The discharge current–voltage characteristic was measured for different cathode temperatures (1950–2250 K); in the range of the arc current 30–100 A the voltage varied from 6 to 11 V. A cooling effect of the mixed cathode due to the thermionic emission at presence of the arc plasma was found; the effect reached about 150 W at the arc current of 30 A. The cooling effect and the current–voltage characteristic distinguish the discharge from the diffuse vacuum arcs with single-component cathodes made of cerium dioxide and chromium. The plasma parameters and optical emission spectra were analyzed for various cathode temperatures and arc currents (electron temperature was 0.4–0.6 eV, local particle densities in plasma near the cathode were in range 1011–1013 cm−3). The study is of interest for plasma sources development and increase of their efficiency as well as for the tasks aimed at ionization of multicomponent mixtures.

Control of Diffuse Vacuum Arc Using Axial Magnetic Fields in Commercial High Voltage Switchgear

During the development of a commercial vacuum interrupter for application in HV (high voltage) switchgear at a rated voltage of 145kV, we investigated the behavior of vacuum arcs controlled by axial magnetic fields (AMF). AMF arc control is already extensively used in medium voltage (1-52kV) applications, the key difference is the 2-3 times larger contact gap and the corresponding reduction of the AMF strength for HV applications. We conducted several stress tests with short circuit currents up to 40kA, thus not only testing the interrupting capability, but also the electrical endurance of such a contact system. We also investigated the dielectric behavior of the vacuum interrupter by testing the capacitive switching duty. Overall, the contacts were used in about 40 operations at high currents. Despite this large number of operations, they showed a minimal amount of contact erosion and damage and demonstrated behavior very similar to the extensive experience with MV vacuum interrupters. In line with simulation results, we conclude that even at high contact gaps and currents, a diffuse vacuum arc was maintained which distributed the arc energy evenly over the contacts.

Numerical Study of the Current Constriction in a Vacuum Arc at Large Contact Gap

A classical two-temperature magnetohydrodynamic modeling approach is used to study the influence of a large contact gap (up to 40 mm) on the behavior of a diffuse vacuum arc controlled by a 5-mT/kA uniform external axial magnetic field between two copper electrodes with a 20-mm radius. The current constriction and the energy flux density at the anode surface are more particularly analyzed, considering both supersonic and subsonic flow conditions. In the case of a supersonic arc, simulations show that the constriction of the current develops in the whole interelectrode region, but the constriction level at the anode surface does not evolve monotonically with the contact gap. The constriction is partly correlated with the radial compression of the plasma. In the case of a subsonic arc, the current constriction is related to the presence of the anode sheath. It occurs only close to the anode (from a constant distance from the anode of around 15 mm). Whereas the current constriction at the anode surface increases when the gap length goes from 10 to 20 mm, it no longer evolves when increasing the gap length from 20 to 40 mm. For both flow conditions, the evolution with the gap length of the radial profile of the energy flux density transferred by the plasma to the anode is similar as that followed by the current constriction at the anode.

Diffuse vacuum arc on cerium dioxide hot cathode

A diffuse (spotless) vacuum arc was investigated on a hot cathode made from cerium dioxide. The discharge is obtained in the following range of current, voltage, and cathode temperature of I = 15–150 A, Va = 9–14 V, and Tc = 2.1–2.4 kK. The main characteristics of the plasma flow in space behind the anode with a hole were determined: it was found that the electron temperature at the working parameters lies in the range of 0.4–1 eV, the ions are predominantly singly charged, the average charge of the outgoing heavy particles reaches 0.9 e (elementary charge), and the most probable kinetic energy of the ions does not exceed 9 eV. Potentially found regimes of vacuum arc operation are promising for use in the work on implementation of the plasma method for spent nuclear fuel and/or radioactive waste reprocessing.

Diffuse Vacuum Arc on the Nonthermionic Lead Cathode

Experimental study of the diffuse vacuum arc discharge on the nonthermionic lead cathode is presented. At the working cathode temperatures of 1.2–1.6-kK current–voltage characteristic of the discharge, cathode heat operation regime and its erosion rate were measured. Using probe methods, electron temperature (0.3–1.2 eV) and heavy particles (atoms and ions) mean charge (0.17–0.28 e) were determined in after-anode plasma. Evaluated current densities on the cathode are in agreement with a hypothesis on the ion mechanism of charge transfer on its surface. Parameters of the obtained discharge have been compared with the characteristics of the earlier investigated diffuse vacuum arcs on nonthermionic chromium and on thermionic gadolinium cathodes. The obtained data might be useful when creating plasma sources for verifying the plasma separation method of the spent nuclear fuel using the nonradioactive substances.

Diffuse vacuum arc with cerium oxide hot cathode

Diffuse vacuum arc with hot cathode is one of the perspective plasma sources for the development of spent nuclear fuel plasma reprocessing technology. Experimental data is known for such type of discharges on metal cathodes. In this work discharge with cerium dioxide hot cathode was studied. Cerium dioxide properties are similar to uranium dioxide. Its feature as dielectric is that it becomes conductive in oxygen-free atmosphere. Vacuum arc was studied at following parameters: cathode temperatures were between 2.0 and 2.2 kK, discharge currents was between 30 and 65 A and voltages was in range from 15 to 25 V. Power flows from plasma to cathode were estimated in achieved regimes. Analysis of generated plasma component composition was made by radiation spectrum diagnostics. These results were compared with calculations of equilibrium gaseous phase above solid sample of cerium dioxide in close to experimental conditions. Cerium dioxide vacuum evaporation rate and evaporation rate in arc were measured.

Effect of an axial magnetic field and arc current on the anode current density in diffuse vacuum arcs

The objective of this paper is to measure the effect of an axial magnetic field (AMF) BAMF and arc current on the anode current density in diffuse vacuum arcs. The experimental geometry included a split anode and a butt-type cathode, both with a diameter of 60 mm. The anode surface was divided into a central area and three symmetrically disposed peripheral annular areas. The central area of the split anode had a diameter of 20 mm. The contact material was CuCr25 (25% Cr). The arc current IARC ranged from 4 to 14 kA (rms) at 50 Hz. The opening velocity was 2.4 m/s. The currents of the four areas on the anode contact were measured using four Rogowski coils situated outside the vacuum chamber. An external uniform AMF BAMF ranging from 0 to 110 mT was applied during the experiment. The observed arc modes were recorded by a high-speed charge-coupled device video camera. The experimental results quantitatively reveal that the current density distribution on the anode surface in the diffuse arc mode was not uniform but concentrated in the central area. The current density in the central anode area at the current peak JPeakArea I decreased with increasing BAMF following a power law. For BAMF of 0–110 mT and IARC of 4–14 kA, JPeakArea I = (2.2 IARC + 0.069 IARC2) BAMF−0.22, where JPeakArea I is in A/mm2, BAMF is in mT, and IARC is in kA. Moreover, the current distribution was uneven in the three peripheral areas.

Temperature Analysis on Contact Surface after High-Current Diffuse Vacuum Arc

Temperature Analysis on Contact Surface after High-Current Diffuse Vacuum Arc

Contact temperature and erosion in high-current diffuse vacuum arcs on axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I/sub P/, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I/sub P/. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion.

Correction to "Visualization and characterization of high-current diffuse vacuum arcs on axial magnetic field contacts"

Correction to "Visualization and characterization of high-current diffuse vacuum arcs on axial magnetic field contacts"

Visualization and characterization of high-current diffuse vacuum arcs on axial magnetic field contacts

We have investigated the behavior of drawn vacuum arcs for several designs of axial magnetic field (AMF) contacts using high-speed digital photography and arc voltage measurements, As the peak current was increased, a gradual transition occurred in the arc appearance from a multiple cathode-spot arc to the high-current diffuse mode, and then to a high-current diffuse columnar mode. Two relatively simple models based on the literature are used to explain the results. The first is an empirical criterion for using the arc voltage behavior to determine the maximum arc current for which an AMF geometry can produce a high-current diffuse mode from the initial bridge column arc. The second model predicts the highest arc current that can be forced into a fully diffuse mode for given values of the AMF and the contact arcing radius. The predictions of these models are compared to our experimental and analytical results.

Transition from constricted to diffuse vacuum arc modes during high AC current interruption

Modes of high AC current vacuum arcs between butt contacts at a fixed gap of 10 mm during the decrease of current to zero, in the current range from 8-25 kA peak, were characterized. Three groups of arc modes were featured: (1) constricted without or with a small number of autonomous cathode spots, (2) constricted with a considerable number of cathode spots beyond the concentrated cathode root, and (3) diffuse modes. It was shown that the most typical high-current vacuum arc mode during a decreasing current is the local constricted discharge situated on the contact edge, which coexists with a dissipate discharge created by groups of autonomous cathode spots. The threshold current for the transition from the constricted to diffuse arc was measured.

Measurement of ion parameters during a forced switching off of current in vacuum

This paper presents the results of ion parameters measured during a forced current commutation of a diffuse vacuum arc, aimed at evaluating the role of ions in discharges occurring rarely after the counterpulse commutation process, in a contact arrangement corresponding to a low-voltage vacuum switch. A sinusoidal half-wave current was interrupted near its amplitude of 400 A with a counterpulse of rise velocities 2.4 and 32 A//spl mu/s. In this way, a forced switching off of dc current was modeled. A retarding field analyzer was used for ion parameter measurements at the forced commutation process. Ion potentials up to 100 V were measured at the current zero moment, and the average decay times of ions after current zero were in the range from a few microseconds up to 30 /spl mu/s as a function of observed ion potential. The measured values of decay times show the rather negligible role of ions in the discharge process after the counterpulse commutation in the examined contact arrangement. The rise time of the countercurrent is proposed as a factor in the successful forced current commutation under these conditions, and values up to a few amperes per microsecond of countercurrent rise velocity can be considered as the limit value.

Plasma-activated electron beam deposition with diffuse cathodic vacuum arc discharge (SAD): a technique for coating strip steel

The diffuse cathodic vacuum arc discharge is an efficient plasma source suitable to active the process of high-rate electron beam deposition on large areas. With condensation rates of some microns per second, ion current densities of, say, 100 mA cm −2 and more can be obtained on the substrate. The layout of equipment and process control offers many opportunities to match this coating process, termed SAD, to the specific requirement of the various layer materials and applications. Presented in this paper are the results of process development. The deposited layers are characterized especially by their high density, ductility and smooth surface. Exemplified by the coating of strip steel with titanium, the resulting effects on the properties of the coated product are shown in detail. Of particular significance is the high corrosion protecting action that is maintained also after deformation of the coated substrates. Finally, an outlook is given on practicable possibilities of use.

Effect of an axial magnetic field upon the development of the vacuum arc between opening electric currents (currents read contacts)

In one series of experiments, a constant axial magnetic field B/sub ax/ was produced in the gap between opening Bruce-profile, chromium-copper contacts. Peak arc currents up to 40 kA at 50 Hz were studied with B/sub ax/ u to 112 mT. For each peak current, a critical field B/sub crit/ was determined above which anode spots did not form. For B/sub ax/ greater than B/sub crit/, the time from contact parting to the formation of a diffuse vacuum arc was linearly dependent on separation current. In a second series, the contacts were designed to generate a self B/sub ax/ when AC current up to 123 kA peak flowed. This field was approximately proportional to the current waveform through the contacts, and it was always higher than the B/sub crit/ predicted for the instantaneous current. The time from contact parting to the formation of the diffuse arc was consistent with that observed in the first experimental series. Once the diffuse arc had formed, the contacts showed no distributed melting for currents up to 81 kA, but gross melting occurred at 102-123 kA. The formation of diffuse arcs is discussed, and the anode melting is explained. >

Erosion rate and voltage distribution in contracted (with cathode spot) and diffuse (spotless) low-current vacuum arcs

The influence of ambient gas pressure and emission current on the potential distribution in a diffuse arc has been studied. The conditions determining the formation of the potential 'hump' near the cathode have been determined. The erosion rate Upsilon approximately=10-3 g C-1 for an individual crater (local erosion rate) has been measured for a contracted vacuum arc. In both types of arc the local erosion rate is well in excess of the erosion rate measured by the weighing method. The conclusion is drawn that most of the evaporated atoms ( approximately= 90%) have to return to the cathode. The diffuse and the contracted vacuum arcs reveal common properties despite a great difference in the current density. The idea of the potential hump can unite both types of arc and explain their characteristics.

Integrated ion flux emitted from the cathode spot region of a diffuse vacuum arc

This paper reviews the properties of the cathode ion flux generated in the vacuum arc, concentrating on the characteristics of the ion energy distributions of the cathode ions. The cathode ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply-charged ions. The authors' calculations are based mainly upon the energy distribution data of the different ion fluxes, the data available for various cathode materials are summarized. They assume that the integrated energy distribution (IED) of the ion flux for a particular material may be treated as the sum of all the fractional ion energy distributions and present plots of such IEDS. They further assume that a shifted Maxwellian type velocity distribution gives the best fit to the IED, and calculate the characteristic parameters (shifted velocity v0, thermal velocity beta and normalization constant C1) for each material. IEDS calculated using these parameters agree reasonable well with the experimental data. The ratios vp/v0 (vp is the most probable velocity) and v0/ beta are evaluated and used in discussing theoretical ion acceleration mechanisms. Their analysis suggests that the mechanism making the greatest contribution to the energy of the ion flux is the electro-ion collision mechanism, but that the contributions of the electric field force and flow interaction (pressure gradient) mechanisms are also significant. Their results should be applicable to any vacuum arc where the cathode spots may be treated as individual emitting sites, i.e. where collective interactions between cathode spots may be neglected.

Collisional transfer and neutral copper vapour density during a diffuse vacuum arc

Measurements of the number density of neutral metal vapour during vacuum arcs by laser-induced fluorescence (LIF) are disturbed by electronic collisions which depopulate the upper fluorescence level. To overcome this difficulty, the sinusoidal arc current was rapidly forced to zero which resulted in a rapid reduction of the collisional interaction. This is shown by measurement of the collisional transfer from the optically pumped 42P3/2 level to the neighbouring 42P1/2 level of copper. Measurements of the laser-induced fluorescence were performed shortly before and after the artificial current zero. Due to the reduction of the collisional interaction, the LIF signal after current zero rises by an order of magnitude above its value before the artificial current zero, reaches a maximum two microseconds after current zero and decays as the vapour disperses. The vapour density inferred from the maximum LIF signal is considered as an approximate value for the density before the artificial current zero. By forcing current zeros at preselected times during a 50 Hz sinusoidal half-cycle with 200 A peak current, a lower limit for the vapour density during arcing was obtained. The copper vapour density rises to a maximum value of 3.6*1018 m-3 near current crest and drops to 5*1016 m-3 at the natural current zero of the half-cycle. Afterwards it decays by about two orders of magnitude within 1 ms.