CuCr Contact Research Articles

Molecular dynamics simulation of arc ablation on Mo or W doped CuCr contact materials

To improve the ablation resistance of the contact material in vacuum circuit breakers (VCBs), molecular dynamics (MD) was used to simulate the microscopic arc ablation process. Reinforced CuCr alloys doped with elements Mo or W were designed using ablative mass as the characteristic parameter for ablation resistance. The results demonstrate that Mo or W doping can improve the ablation resistance of the alloys to some extent, and the doped alloys have better ablation resistance than CuCr50. Moreover, Mo or W-doped CuCr alloys with an original Cu content of 70–80 at%, which have less ablative mass than CuCr50, are most suitable for improving the contacts’ ablation resistance.

Microstructure and property evolution of CuCr50 alloy prepared by aluminum thermal reduction-electromagnetic casting during hot forging process

To address the problems of low performance and density in CuCr50 alloys prepared by aluminum thermal reduction-electromagnetic casting, a synergistic process involving hot forging deformation to eliminate micropores in the alloy and heat treatment to modify the alloy was proposed. The effect of hot forging temperature on the microstructure evolution and performance strengthening of CuCr50 alloys during heat treatment was studied. The results show that the properties of CuCr50 alloys forged at different temperatures after heat treatment are better than those after direct heat treatment. After heat treatment, the conductivity of CuCr50 alloys forged at 800 °C reaches 22.41 MS/m, the density reaches 7.94 g/cm3, and the hardness reaches 112 HB, which are 73.59 %, 4.75 % and 37.59 % greater than those of the as-cast alloy, respectively. The microstructure analysis showed that the nano-Cr phase precipitated during the aging process of CuCr50 alloys after hot forging at 750 °C–850 °C had a semi-coherent relationship with the Cu matrix, which played a role in coherent strengthening. After hot forging at 900 °C, the precipitated Cr phase has an incoherent relationship with the Cu matrix, which played a role of dispersion strengthening. The performance test of 40.5 kV simulated vacuum interrupter shows that the breaking and chopping performance of the prepared CuCr50 contact material is obviously better than that of commercial products, which is expected to become a new process for the preparation of high performance CuCr contact materials.

Molecular dynamics simulation of arc ablation for CuCr contact materials and improvement method of ablation resistance

Renewable energy generation is a development trend worldwide due to sustainability and environmental protection. High-voltage direct current (HVDC) transmission is indispensable in the renewable energy system due to its efficient power transmission capability and adaptability to complex grid structures. The DC vacuum circuit breaker (VCB) is essential for system control and fault protection. However, with the development of DC systems towards high voltage and large current, the ablation resistance of CuCr contact materials in VCBs has become the leading technical bottleneck. In order to enhance the ablation resistance of CuCr contact materials, this paper investigates a new molecular dynamics (MD) simulation model for arc ablation of CuCr contacts; therefore, it proposes a design method for contact materials measured by characteristic parameters of ablation resistance. Results are as follows: Firstly, the Cu particles cause a severe ablation on CuCr contacts with the effect of arcs. Subsequently, the model’s accuracy is verified by the influence of Cr content on ablation resistance. Furthermore, Mo and W metal elements were used to dope and strengthen the CuCr10 contact material, and CuCr8Mo2 and CuCr7Mo3 have an apparent improvement in resistance to arc ablation, which are alternatives for reinforced CuCr10 contact materials.

Three-dimensional modeling of multi-component vacuum arc considering anode vapor in actual magnetic field

For vacuum circuit breakers, the anode will change to active mode when the current is high enough. The vapor produced by the anode will become another important source of plasma. In this paper, a three-dimensional (3D) magnetohydrodynamic model considering anode vapor and multiple components is developed to study the vacuum circuit breakers with cup-shaped CuCr contact. The characteristics of arc under the actual magnetic field with different anode temperature distribution and different cathode utilization are simulated. Moreover, the ionization and recombination processes between different kinds of components are considered. The simulation results show that the arc current concentrates toward the center. The current value is high between electrodes slots under the actual magnetic field, which significantly affects the arc characteristics. At the same time, higher anode temperatures will produce more neutral atom vapor area (NAVA). Current is concentrated outside NAVA due to the low conductivity of NAVA. As NAVA expands, the constriction of current on the outside is more severe. The simulation results in this paper are consistent with the experimental results.

Enhancing ablation resistance of CuCr contact materials through metallic element doping: First-principles calculations

In this paper, a first-principles design approach for ablation-resistant contact materials was developed. Take Cu16Cr16 for example, the melting point was used as the criterion to screen the improved doped alloys, and mechanical properties were computed to guarantee that the designed doped alloys satisfy the contacts' operational requirements. This technique revealed that Cu16Cr15W1, Cu16Cr15Os1, and Cu16Cr13Co3 have stronger properties than Cu16Cr16 in terms of melting point and resistance to deformation, which can be utilized to enhance the ablation resistance and mechanical strength of the contacts. Simultaneously, the doped alloys' brittleness guarantees the welding resistance of the contacts. As a result, strengthening contact materials can be made from these doped alloys.

Research on the influence of grain size of CuCr contact on the characteristics of cathode spot and cathode surface erosion in vacuum arc

CuCr alloy is widely used in vacuum interrupters. The characteristics of cathode spot and erosion of cathode surface directly affects the lifetime and breaking capacity of vacuum interrupter. In order to control the cathode spot and crater, two types of contacts with different Cr phase size were used in this paper. The vacuum arc breaking experiments were conducted. The cathode spot images were recorded by ultra high speed camera and craters were measured by Scanning Electron Microscope and Profiler. It was found that the smaller the Cr phase size, the higher the expansion velocity of cathode spots, the larger the number of cathode spot, the smaller the cathode spot size and the lower the current carried by each cathode spot. The reduction of Cr phase size helps to improve the uniform distribution of cathode spots on the contact surface. Besides, the smaller Cr phase size leads to the decrease of radius and depth of crater. For CuCr contacts, thermal diffusion process was analyzed and there is a significant difference in the thermal diffusivity between Cu and Cr. The Cr phase uniformly distributed in Cu prevents the thermal diffusion process which reduces the size of crater and the erosion of cathode surface.

Thermal and arc erosion behavior of CuCr contact material based on large plastic deformation treatment

CuCr alloys are the main contact materials used in medium-voltage and high-current vacuum interrupters. In this paper, the effect of large plastic deformation on microstructure and the transverse/longitudinal thermal conductivity of CuCr30 contact material were studied. The results show that the morphology of Cr particles in CuCr30 changes from dendritic to oriented fibrous after large plastic deformation, and the longitudinal thermal conductivity of CuCr30 contact material is about 1.5 times higher than its transverse thermal conductivity. During the electrical contacting process, the mass loss of the extruded CuCr30 contact material is lower than that of as-cast CuCr30 contact material, which is directly related to the significantly higher longitudinal thermal conductivity of the extruded CuCr30 contact material. The enhancement of longitudinal thermal conductivity is beneficial to reduce the mass loss of the contact material and improve the arc erosion resistance of the contact material. The mass loss of the cathode with higher longitudinal thermal conductivity is significantly lower than that of the anode contact in extruded CuCr30 contact material.

Experimental study on the effect of trace elements in copper-chromium contacts on the current cut-off level of vacuum switches

The current cut-off phenomenon of vacuum switch will generate high overvoltage in the electrical circuit, which threatens the safety and insulation of electrical equipment. Adding trace elements to the contact material is one of the important measures to reduce the cut-off level of vacuum switch. In this paper, based on the research background of adding trace element Te (tellurium) to CuCr (copper-chromium) contacts, the oscillating circuit and detachable vacuum interrupter are used to measure the interception value of CuCrTe contacts with different additions and different processing techniques, and study the trace elements. The effect of element Te on the cut-off value of CuCr contacts preliminarily proves that the improvement of contact material can affect the level of cut-off value. The results show that adding Te element to the CuCr contact can reduce the cutoff value of the material. As the content of Te element in the contact increases, the cut-off value decreases unidirectionally, and the high-frequency oscillation of arc voltage and current near the cut-off moment is more pronounced. Obviously, it indicates the threat of back-arc heavy breakdown. The experimental results can provide a reference for the comprehensive optimization of copper-chromium contact trace element addition and switch performance indicators.

Progress in the Preparation of Large-Size High-Performance CuCr Alloys

CuCr contact material, whose mass fraction of Cr is generally in the range of 25% to 50%, is one of the most desirable contact materials for medium- and high-voltage vacuum switches. The homogeneity and denseness of CuCr alloy are the keys to its application. The infiltration and powder sintering have defects, such as uneven distribution of Cr phase, and great difficulties in preparing high-performance CuCr alloys with low gas content; the equipment investment in the vacuum consumable arc melting method is large and the production cost is high, while mechanical alloying, vacuum induction melting, and laser-selective melting as new technologies are still in the laboratory stage. From the perspective of surface modification treatment and multicomponent alloying, refinement of Cr phase grains can improve the properties of CuCr alloy. The new process of aluminothermic reduction-electromagnetic coupling refining-water and gas composite rapid cooling takes oxides as raw material and obtains a fully miscible CuCr alloy melt by aluminothermic reduction theoretically at 2848K, followed by secondary refining under electromagnetic field coupling and water-gas composite cooling and casting, which can obtain alloy ingots of 50 mm in diameter and 10 mm in height. The density and hardness of CuCr alloy prepared by this process as vacuum contact material are better than the requirements of GB/T 26867-2011, so this process has the potential to become a new-generation preparation method for large-size homogeneous high-performance CuCr alloy.

Vacuum arc characteristics of a new type of curved-surface contact for high-frequency forced interruption in aero applications

Aiming at the arc interruption problem in aviation 270 V DC power systems, this report proposed new contact types of vacuum interrupter contacts with curved surfaces for high-frequency forced interruption. The characteristics of vacuum arc behavior, high-frequency forced interruption capacity and contact surface ablation under curved contact with different structural parameters are investigated. The interruption ability, arc voltage characteristics and contact ablation of CuW80 and CuCr25 contact materials and their differential combinations were obtained. To select the optimal structural parameters of the curved contacts, a method based on the curvature of the electrode surface was proposed for high-frequency forced interruption under a short-gap vacuum arc. Experimental results show that a current of 4.8 kA can be interrupted at a commutation frequency of 20 kHz with a di/dt of 650 A/μs when the curvature of curved contacts is in a certain range.

Simulation and verification of thermal conductivity of CuCr30 contact material based on morphological changes of Cr particles

In order to improve the thermal deposition of CuCr contact material for vacuum interrupter, CuCr30 contact material was treated with large deformation treatment to change the morphology and distribution of Cr particles, and the effect of the morphology change of Cr particle on the thermal conductivity of CuCr30 contact material was studied. At the same time, the finite element simulation of the thermal transfer behavior of the extruded and cast CuCr30 contact materials was carried out by using ANSYS software. The studies show that the large deformation treatment (deformation ratio 16:1) changes the morphology and distribution of Cr particles in CuCr30 contact material, making it change from dendritic to longitudinally arranged quasi-fibrous. Influenced by the morphology and distribution of Cr particles, the thermal conductivity of CuCr30 contact material is anisotropic, and the effective thermal conductivity varies significantly with the direction. Indeed along the deformation direction, the thermal conductivity of the extruded state is obviously higher than that of the cast state, increasing by 5.8 %. However, perpendicular to the extrusion direction, the thermal conductivity has little changes, and obvious deposition phenomenon appears in the process of thermal conduction. The finite element simulation results of the effective thermal conductivity of CuCr30 material by ANSYS are in good agreement with the measured results, and the error range is 0.7 %∼1%.

The effect and dynamic behavior of particles in high-current vacuum arc interruptions

Microparticles produced by high current vacuum arcs significantly reduce the insulation performance of vacuum interrupters. The objective of this paper is to observe the generation and dynamics of microparticles during the arcing and post-arcing period, and to further reveal the possible effects of particles during interruption of a high current vacuum arc. A plasma diagnosis system based on a laser-shadow technique was used to measure the particles in a demountable vacuum chamber. The results show that contact materials affect particle generation patterns. For a CuCr contact with a higher proportion of chromium, the formation of a melting pool is much more difficult and the flow of liquid metal on the surface can be suppressed, which also impedes the formation of particles. Moreover, reignition after arc extinction was found to be triggered by some incandescent particles. These hot particles have two effects on breakdown events during the post-arcing phase: they trigger a micro-discharge between a particle and the electrode, and new weak points are left on the surface that enhance the local field. More crucially, the lifetime of spontaneous particles spans up to hundreds of milliseconds after current zero, which indicates that long-delayed breakdown is correlated with the existence of particles. In addition, some particles could remain suspended near the electrode or oscillate between the contacts. The dynamic behavior of these particles can significantly lower the breakdown threshold and increase the possibility of breakdown.

Modeling of CuCr contact vacuum arc with consideration of its components

This paper involves the simulation of vacuum arc under composite (CuCr) contact. A three dimensional model is developed and we have considered the ionization process as well as recombination in the vacuum arc. The species transport equations, in which the fraction of each component is calculated, provides the distribution of various atoms and ions in vacuum arc. The model describes ions and atoms as two different processes considering that ions are influenced by self-generated magnetic field. Atoms play an inferior role in the process of vacuum arc. They gather in an ‘atom layer’ whose thickness is no more than 1mm and outside the atom layer the density of atoms declines by at most 5 orders of magnitude. The ionization process is dominated by electron temperature. The distribution of ion species is closely related to the degree of ionization. At cathode, singly charged ions are dominant. From cathode to anode, ions are ionized so that doubly charged ions become the major component at anode where singly charged ions reach their minimum density. Finally, the ionization rate is also influenced by the ionization potential of each component, so the fraction of Cr1+ is smaller than Cu1+ while Cr2+ is larger than Cu2+ in the anode center.

Experiment investigation on vacuum arc of AMF contacts under different materials

In this paper, the arc characteristics of cup-shaped AMF contacts in vacuum arc experiments under different materials was studied. The experiment was conducted in a detachable vacuum chamber with a gas pressure of 10-4Pa. The arc voltage and current were measured at the same time by high-voltage probe and Rogowski coil, and the arc pictures were photographed with a high-speed camera. The cup and the rod of the contact are made of pure copper material, and the contact pieces are selected ten common contact materials for comparison (CuCr10, CuCr 20, CuCr 30, CuCr 40, CuCr 50, WCu10, WCu20, WCu30, CuW-WC). Fixed-distance trigger experiments were conducted with currents of 5kA (rms), 10kA (rms), and 15kA (rms) for each contact. The arc appearance, cathode surface morphology, and anode status morphology were observed. The shape of the arc was analyzed in conjunction with the voltage and current after the experiment. The morphology of cathode and anode surface was analyzed by combining the erosion after the experiment. The effect of various material properties was discussed under the AMF. It’s found that the center of the anode erosion area is approximately hexagonal, and the slot is burned more outward than the plane. WCu contact has better erosion resistance than CuCr contact. For CuCr, the erosion of CuCr30 is the slightest with low depth and small area.

Microstructure Formation and Resistivity Change in CuCr during Rapid Solidification

The formation of the surface-near microstructure after a current interruption of CuCr contact materials in a vacuum interrupter is characterized by a fast heating and subsequently rapid solidification process. In the present article, we reveal and analyse the formation of two distinct microstructural regions that result from the heat, which is generated and dissipated during interruption. In the topmost region, local and global texture, as well as the resulting microstructure, indicate that both Cu and Cr were melted during rapid heating and solidification whereas in the region underneath, only Cu was melted and elongated Cu-grains solidified with the <001>-direction perpendicularly aligned to the surface. By analysing the lattice parameter of the Cu solid solution, a supersaturation of the solid solution with about 2.25 at % Cr was found independent if Cu was melted solely or together with the Cr. The according reduction of electrical conductivity in the topmost region subsequent to current interruption and the resulting heat distribution are discussed based on these experimental results.

Dependence of the Chopping Current Level of a Vacuum Interrupter on Parallel Capacitance

The distribution of chopping currents of a vacuum circuit breaker using CuCr25 contact material was measured in a laboratory circuit consisting of a three-phase 20-kV cable system and a dry-type distribution transformer rated 900 kVA. The cable length between the upstream feeding transformer and the circuit breaker was several hundreds of meters. An inductive load connected to the distribution transformer provided a 50-Hz current of 10 A rms. Under these conditions, the mean chopping current level was between 3.2 and 3.5 A and showed no significant dependence on the arcing time, which was varied between 2 and 8 ms. After addition of a surge capacitor of 130 nF between the transformer terminals and ground, which very much reduces the overvoltage during switching, the chopping current level increased to 5.6 to 7.7 A in dependence of the arcing time. In order to explain this, the interaction of the arc with the electric circuit in particular with the cables on both sides of the breaker has to be evaluated. If the cables are long, travelling waves need to be considered as well as all return current paths in a three-phase circuit. The coincidence of excited network oscillations with random arc instabilities may result in either momentary amplification or damping of such oscillations. An additionally installed capacitor serves as a source sustaining a momentary current reduction after an arc instability and allows the excitation of a subsequent arc instability leading to an increase of the chopping current with longer arcing times. With low capacitance on the load side, there is no interaction and therefore no influence from the arcing time.

Preparation and Performance of Cu-Cr Contact Materials for Vacuum Switches with Low Contact Pressure

Insufficient anti-welding properties limit the application of Cu-Cr contact material in vacuum switches with low contact pressure. The CuCr-W-C alloys that are prepared are for decreasing welding tendencies and keeping the voltage withstand by addition of W and C elements. It is found that the average welding force of CuCr-W-C alloys is reduced more than 50% compared with that of the Cu50 Cr50 alloy. Especially for CuCrW3.0C0.3 and CuCrW1.0C0.5, the welding forces reduce to only 10% of Cu50Cr50. Arc erosion areas of CuCr-W-C alloys are enlarged by five times more than that of the Cu50Cr50 alloy in the same arcing conditions. The results of type tests were qualified. The results suggested that the CuCrW2.0C1.0 alloy could be used in vacuum switches with low contact pressure to replace the W-Cu type contacts.

Electrical Properties of Nanocrystalline CuCr25 Contact Material

Four pieces of nanocrystalline CuCr25 contacts with 45-mm diameter and 3-mm thickness were prepared, brazed onto cup-axial magnetic field electrodes, and assembled in two commercial vacuum interrupters to test their chopping current, ac dielectric breakdown voltage, and breaking capacity. For comparison, two microcrystalline CuCr25 contacts were also prepared as above. The test vacuum interrupters were operated by a 24-kV circuit breaker with a spring-operated mechanism. The opening velocity was 1.3 m/s and the arc current frequency was about 50 Hz. The experimental results showed that both the chopping current and the ac dielectric breakdown voltage of nanocrystalline CuCr25 contact material were lower than those of the microcrystalline CuCr25 material. The breaking capacity of nanocrystalline CuCr25 material was significantly lower than that of microcrystalline CuCr25. The cathode and the anode contact surfaces of nanocrystalline CuCr25 were seriously eroded by arc in the local areas, and obvious cracks occurred.

Preparation and Characterization Cu-Cr Contact Material and Study their Hardness and Conductivity

Cu-Cr contact material is used in medium voltage, high current vacuum interrupters. Vacuum interrupter is the heart of Vacuum Circuit breaker. Contact tips are usually made using powder metallurgical techniques in disk shape. These disks are machined to required dimensions and then hermitically sealed by vacuum brazing. The properties of the final contact tips are thus usually rarely studied. In the present study the properties before and after the vacuum sealing are compared in order to establish a correlation between them. The Cu-Cr contact tips were made under different compaction pressures, sintering temperatures and soaking time periods. Cu-Cr contacts were prepared by powder processing route and sintering in vacuum furnace at the temperatures of 1050°C for different soaking time period of 4, 6 and 8 Hrs. The samples were then subjected to simulated sealing cycle at temperatures (870 and 960°C). The electrical conductivity, hardness, microstructure and grain morphology of all the samples were studied. Properties of as-sintered and after sealing were analyzed and compared. The results of quantitative microscopy shows the better result on sintered sample soaked for 4 Hrs. The hardness and conductivity of the sample without brazing, varies between 79.9 - 83.8 HRF and 37.9 - 43.1% IACS respectively. The hardness and conductivity of the brazed sample varies between 52.5 - 63.42 HRF and 48.32 - 51.33 %IACS respectively.

Preparation and Property Study of CuCr Alloy Based on SHS

Copper oxide and pure chromium being as raw materials,CuCr contact composites can be fabricated through the self-propagating high-temperature synthsis technology,then the microstructure of the samples was observed under a metallographic microscope and the effects of different chromium content on hardness and electric conductivity of CuCr alloys were studied.The experiment's results showed that the SHS reaction of CuO-Cr-Al system could react drastcally,give off a lot of heat and CuCr alloy could be obtained simply as well as with low cost. With the increase of the chromium content, the hardness of the CuCr alloy was elevated, but the electric conductivity was decreased.