Voltage Vacuum Interrupter Research Articles

Study on arc characteristics and influencing factors of cup-shaped axial magnetic field contact at different gap distances

The cup-shaped axial magnetic field (AMF) contact, integral to high-voltage vacuum interrupters, undergoes innovation by enhancing the slotting rotation angle of the cup wall. This adjustment significantly amplifies the gap magnetic field, promising breakthroughs in handling long-gap arcs within high-voltage vacuum interrupters. Comprehensive vacuum arc experiments were conducted on cup-shaped AMF contacts with varying slotting rotation angles at different gap distances. The research meticulously analysed the evolution of vacuum arc characteristics under diverse breaking currents. Scanning electron microscope (SEM) images of the cathode surface unveiled three distinctive microscopic morphologies: particle gullies, pore platforms, and particle platforms. Notably, the chromium content corresponding to these morphologies exhibited a progressive increase. Integrating the surface distribution of copper and chromium, the study proposed a plausible cathodic erosion mechanism. Extending the investigation, the research delved into the influence of the iron core, contact diameter, and slot number on vacuum arc characteristics, which expected to provide reference for the application of the cup-shaped AMF contact in high voltage vacuum interrupters.

Arc erosion resistance of WC/CuCr30 composites based on changing of Cr particle morphology and orientation

CuCr30 composite was commonly applied in medium and low voltage vacuum interrupters. The WC/CuCr30 composites based on changing of Cr particle morphology and orientation were successfully prepared by vacuum hot press sintering and hot extrusion, and the arc erosion resistance behavior at different voltages along the deformation direction of Cr particles was investigated on the JF04C electrical contact material testing system. The microstructure characterization of WC/CuCr30 composites shows that the Cr particles in the WC/CuCr30 composite have been elongated and the long axis direction is close to parallel to the extrusion direction after the hot extrusion deformation. Compared with the longitudinal section, the thermal conductivity of CuCr30 and WC/CuCr30 composites in the cross section is increases by 3.75% and 3.63%, respectively. The electrical contact testing results under different voltages shows that the addition of WC particles can extend the service life of the CuCr30 composite, and the operation times of WC/CuCr30 composite (4850 times) increases by 34.72% compared to CuCr30 composite (3600 times) at 30 V DC. The welding force decreases more obviously with the increase of the voltage, and the average welding force of WC/CuCr30 composite decreases by 8.57% and 20.9% at 24 V DC and 30 V DC compared with CuCr30, respectively. The arc erosion mechanism analysis shows that addition of WC particles is beneficial for the decrease of arc duration. And WC particles can effectively inhibit the arc erosion process and delay the formation of bumps and pits.

Effect of Cr@RGO structure on microstructure and properties of RGO/CuCr25 composite

CuCr alloy with high Cr content (20–50 wt.%) is the main contact material for medium voltage and high current vacuum interrupters. However, the influence of network distribution of Cr particles affect the conductivity of CuCr alloy, and its properties strongly depend on the size of Cr particles. In this paper, Cr@RGO (Cr powder coated with reduced GO) was prepared by BSA (bovine serum protein) electrostatic assembly method, and then RGO/CuCr composites with different graphene content were prepared by vacuum hot pressing sintering process. The results show that the size of Cr particles decreases with the increase of graphene content and the network of Cr particles became fractured. When the content of RGO is 3.0 wt.%, the size of Cr phase is 19.9 μm, which is 23.8% less than that of CuCr25 of 26.1 μm. The Brinell hardness and electrical conductivity of 3.0 wt.% RGO/CuCr composite are 102.5 HB and 52.9% IACS, which is increased by 8% and 6% in contrast with that of CuCr25. And the Preparation of RGO/CuCr is simple, feasible, replicable and easy to achieve the goal of the industrial preparation.

Test stand for testing and diagnostics of medium voltage vacuum interrupters

Wydawnictwo SIGMA-NOT wydaje czasopisma fachowe informujące swoich czytelników o najnowszych osiągnięciach naukowych i nowoczesnych rozwiązaniach technicznych w Polsce i na świecie, popularyzuje problemy techniczne oraz poszerza wiedzę i kulturę techniczną.

Experimental study on field emission current in vacuum interrupter at functional limit of vacuum pressure

Significant number of medium voltage Vacuum Interrupters (VI) are currently operating at their lifetime limit of 20–30 years and beyond. Estimation of the pressure inside the VI and of the contact surface condition without demounting of the VI are of importance for Vacuum Circuit Breakers (VCB) operating at medium voltage range and for further development of the VI technology towards higher voltages. This paper reports on the measurements of the field emission current in a commercial vacuum interrupter 12 kV/2500 A/40 kA with the pressure controlled at the VI functional limit of 0.04 Pa. The Fowler-Nordheim equation was applied to indicate the vacuum pressure within the functional limit and to evaluate the influence of the contact surface condition on the microscopic surface projections. A relation between the microscopic processes and the field emission current was studied. The measurement results are presented for a small fixed contact gap of 0.3 mm allowing relatively low DC test voltage level up to 40 kV.

Laser-induced plasma imaging for low-pressure detection.

A novel technique based on laser induced plasma imaging is proposed to measure residual pressure in sealed containers with transparent walls, e.g. high voltage vacuum interrupter in this paper. The images of plasma plumes induced on a copper target at pressure of ambient air between 10-2Pa and 105Pa were acquired at delay times of 200ns, 400ns, 600ns and 800ns. All the plasma images at specific pressures and delay times showed a good repeatability. It was found that ambient gas pressure significantly affects plasma shape, plasma integral intensities and expansion dynamics. A subsection characteristic method was proposed to extract pressure values from plasma images. The method employed three metrics for identification of high, intermediate and low pressures: the distance between the target and plume center, the integral intensity of the plume, and the lateral size of the plume, correspondingly. The accuracy of the method was estimated to be within 15% of nominal values in the entire pressure range between 10-2Pa and 105Pa. The pressure values can be easily extracted from plasma images in the whole pressure range, thus making laser induced plasma imaging a promising technique for gauge-free pressure detection.

Prediction of lightning impulse voltage induced breakdown in vacuum interrupters

A concept to predict the Lightning Impulse Voltage (LIV) breakdown probability of medium voltage Vacuum Interrupter (VI) tubes has been developed and tested for the first time employing the Voltage Holding Prediction Model (VHPM) originally formulated at Consorzio RFX. The VHPM is capable to calculate the Weibull breakdown probability curve of any multi-electrode multi-voltage system insulated in vacuum under dc voltage. Even though the possibility to employ the VHPM in the prediction of voltage breakdown under LIV conditions is not straightforward, the potential benefits of the VHPM usage in VI tube technology with regard to development, design and testing at medium and high voltage levels, strongly recommend the assessment of VHPM under LIV conditions. The paper aims to describe the methodology adopted to identify experimentally the VHPM parameters to be employed in such a novel application to assess its prediction capability. The measurements have been done in the Siemens Berlin VI tube factory test stand for LIV, compliant with the IEC standard. Different vacuum tube types manufactured by Siemens were investigated. The main difficulty was to obtain, with such a test bed, voltage breakdown distributions well fitted by a Weibull Distribution — upon which the VHPM is based — primarily due to the difficulty to reach a well-defined end of the conditioning test sequence preceding the relevant voltage breakdown test sequence suitable to be analyzed by the VHPM. Nevertheless, the VHPM prediction resulted in reasonable agreement with the measured probability curves. Finally, investigating VI tubes with spiral-type Radial Magnetic Field (RMF) contacts, characterized by a geometry strongly deviating from axial 2-D symmetry, the first implementation of a full 3-D version of the VHPM has been tested.

Discharge characteristics for various discharge patterns under negative lightning impulse voltage in vacuum

In order to develop higher voltage vacuum interrupters (VIs), electrical insulation performances in vacuum should be improved. The discharge in VI has various and complex patterns, such as breakdown in vacuum gap and surface flashover on a solid insulator. In this paper, we investigated three discharge patterns: the discharge in vacuum gap, the discharge in vacuum gap via shield, and the surface flashover on solid insulator, respectively. By measuring shield potential and insulator surface potential, we analyzed the discharge characteristics for various patterns, and clarified the relevant discharge development process. We found that the breakdown development process in vacuum gap depends on the gap length and the surface flashover development process on a solid insulator depends on the applied voltage and surface distance.

Impulse surface flashover development associated with transient charging by explosive electron emission in vacuum

Surface discharge on solid insulator in vacuum, which is one of the most significant issues under high electric field, prevents the development of higher voltage vacuum circuit breakers and vacuum interrupters. To clarify the mechanism of the surface discharge in vacuum, charge activities on solid insulators in the development process of the surface discharge should be investigated. By using electrostatic probes behind the alumina dielectrics and measuring fast-framing images of the surface discharge, in the present paper, we clarified that the impulse surface flashover starts in developing with explosive electron emission (EEE) on cathode, which is specific phenomenon on the electron emission site exploding due to extreme field emission current density. Experimental results revealed the characteristics of transient charging by EEE in the surface flash over development as follows: (1) the probe current due to EEE increases with the EEE inception electric field on the cathode, and also depends on the work function of the cathode material, (2) the transient charge distribution on the alumina dielectrics is determined by the EEE spot on the cathode, (3) EEE occurs several times due to the erosion and the regeneration of electron emission sites. (4) the successive EEE makes the transient charge distribution smooth on the alumina dielectrics.

Transient Charging Characteristics in Surface Flashover Development Process due to Explosive Electron Emission in Vacuum

SUMMARYFor the development of higher voltage vacuum circuit breakers and vacuum interrupters, surface discharge in a vacuum is one of the most significant issues. We measured the charge activity on alumina dielectrics in a vacuum through ultrahigh‐speed optical and electrical measurements. We found that surface flashover in a vacuum starts with an explosive electron emission (EEE) with higher current density, which consists of electron emission from the cathode, and results in surface flashover development. The transient charge on the alumina dielectrics formed by electrons emitted by EEE influences the distortion of electric field distribution around the cathode rod and then the subsequent flashover development. In particular, in the case of a shorter gap between the cathode rod and the alumina dielectrics, the electron emission may be suppressed by the relaxation effect of the electric field due to transient charge accumulation.

Mechanism of impulse voltage breakdown in high voltage vacuum interrupters with long contact gap

The objective of this paper is to propose a breakdown mechanism of lightning impulse voltage in high voltage vacuum interrupters (VIs) with long contact gap by complementing the Cranberg "clump" hypothesis with an impulse voltage and a non-uniform field. The Cranberg "clump" hypothesis assumed that the vacuum breakdown was due to the loosely adhering microscopic particles under a DC voltage and a uniform field. The microscopic particles might be torn from an electrode surface at a critical voltage. We considered the microscopic particles located at the area with the maximum electric field had the highest possibility to be accelerated. Therefore, the transit process of the micro-particles to the opposing electrodes was under a non-uniform field distribution driven by an impulse voltage. The rise rate of the impulse voltage applied was also considered. As a result, it was found that the breakdown voltage of a vacuum gap depends upon a 0.76 power of the gap length for the plate-to-plate contacts. The diameter of the contacts was 60 mm. The contact radius of curvature was 6 mm. In the same conditions, our previous experimental results showed that the impulse breakdown voltage was in a range of 0.70 to 0.79 power of the contact gap. So the experimental results supported the Cranberg hypothesis in its extension to the impulse breakdown under non-uniform field in high-voltage VIs. Thereafter, this proposed breakdown mechanism is a complement to the Cranberg "clump" hypothesis in terms of impulse voltage breakdown under a non-uniform field in high voltage VIs.

Transient Charging Characteristics in Surface Flashover Development Process due to Explosive Electron Emission in Vacuum

For the development of higher voltage vacuum circuit breakers (VCB) and vacuum interrupters (VI), surface discharge phenomenon in vacuum is one of the most significant issues. We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed optical and electrical measurements. We found out that the surface flashover in vacuum starts with an explosive electron emission (EEE) with the higher current density, which is the electron emission from the cathode, and results in the surface flashover development. Transient charge on the alumina dielectrics formed by electrons emitted by EEE influences the distortion of electric field distribution around the cathode rod and then the subsequent flashover development. Especially, in the case of the shorter gap between the cathode rod and the alumina dielectrics, the electron emission could be suppressed by the relaxation effect of the electric field due to the transient charge accumulation.

A gravity heat pipe for high voltage vacuum interrupter

To enhance nominal current of high voltage vacuum circuit breakers (VCBs), a gravity heat pipe was proposed to replace stationary conducting rod of a high voltage vacuum interrupter. The heat pipe is composed of two coaxis tubes: the external tube is made of oxygen-free copper and the inner tube is made of stainless steel. The bottom end of the inner stainless steel tube is connected to the external copper tube by holes. Transient and static thermal performance of the heat pipe was measured, and the thermal resistance of it was compared with that of a solid copper rod with the same dimensions. Experimental results showed that thermal resistance of the heat pipe was about 1/3 of that of the copper rod, and it decreased slightly with the rising of the input heat flux. 3D thermal simulation on a 126 kV/2000 A single break VCB was done to compare the thermal performance between the proposed gravity heat pipe and the copper rod serving as the stationary conducting rod of the vacuum interrupter. Simulation results revealed that in the heat pipe case, the maximum temperature between contacts was 67 °C lower than that in the copper rod case.

A utility optimization method of a high voltage vacuum interrupter design

In this paper, a new method of electric field design of high voltage vacuum interrupters is proposed. According to the multi-gap insulation effect and the vulnerable breakdown spots theory, the symmetrical voltage shields are proved to be effective. So the initial model of a high voltage vacuum interrupter is designed. Next with finite element methods and optimization theory, a mathematical model of electric field intensity in interrupters is given. What's more, by optimizing this model with finite element software, a more symmetrical electric field in vacuum interrupters is obtained and its peak value appear as in the second auxiliary gaps (the two gaps between the center floating shield and the symmetrical voltage shields). The result shows that with this method, the vacuum interrupters can withstand higher static breakdown.

Research on Vacuum Arc Characteristics of a New Electrode Structure Applied in High Voltage Vacuum Interrupter

In this paper the vacuum arc behaviors of the axial magnetic field (AMF) electrode with single coil structure were investigated at gap distance 10 mm and 40 mm respectively. And the axial magnetic field distribution was computed at 10 mm and 40 mm. A high-speed charge couple device (CCD) video camera was used as a major diagnostics to investigate the arc behaviors. At gap distance 10 mm, the vacuum arc maintained diffuse. With the gap distance increasing from 10 mm to 40 mm, the vacuum arc experienced constricted arc modes only in short time at beginning. In most of time, the vacuum arc kept diffuse. The test results proved that the AMF electrode with single coil structure was feasible to be applied in high voltage vacuum interrupter.

Influence of Gap Distance on Vacuum Arc Characteristics of Cup Type AMF Electrode in Vacuum Interrupters

The greenhouse effect of SF6 is a great concern today. The development of high voltage vacuum circuit breakers becomes more important. The vacuum circuit breaker has minimum pollution to the environment. The vacuum interrupter is the key part of a vacuum circuit breaker. The interrupting characteristics in vacuum and arc-controlling technique are the main problems to be solved for a longer gap distance in developing high voltage vacuum interrupters. To understand the vacuum arc characteristics and provide effective technique to control vacuum arc in a long gap distance, the arc mode transition of a cup-type axial magnetic field electrode is observed by a high-speed charge coupled device (CCD) video camera under different gap distances while the arc voltage and arc current are recorded. The controlling ability of the axial magnetic field on vacuum arc obviously decreases when the gap distance is longer than 40 mm. The noise components and mean value of the arc voltage significantly increase. The effective method for controlling the vacuum arc characteristics is provided by long gap distances based on the test results. The test results can be used as a reference to develop high voltage and large capacity vacuum interrupters.

Microstructure and properties at elevated temperature of a nano-Al 2O 3 particles dispersion-strengthened copper base composite

High softening resistance, high strength at elevated temperature and high conductivity is important performances for the high voltage breaker and vacuum interrupter contact materials. A novel nano-Al 2O 3–Cu composite was fabricated for them by using a modified internal oxidation method. The microstructure was investigated by means of a scanning electron microscope and a transmission electron microscopy. The softening temperature, tensile strength at elevated temperature and electrical conductivity of the composite were also determined. The grain size of the Cu–0.5 vol.% Al 2O 3 composite is very fine and the annealing temperature greatly affects its microstructure. The electric conductivity reaches 93% IACS and the softening temperature is about 800 °C, while the yield strength reaches 169 MPa and the ultimate tensile strength reaches 172 MPa at the testing temperature of 600 °C. The main contributions of nano-alumina particles is to strengthen the composite, not by Orowan strengthening, but due to the strong pinning effects on the grain boundaries and sub-grain boundaries with high dislocation density to prevent the nuclei formation of recrystallization.

Distribution of cathode current density and breaking capacity of medium voltage vacuum interrupters with axial magnetic field

A high-speed photographic technique incorporating computer-aided reconstruction of the cathode current density was applied to butt solid contacts with an external axial magnetic field (AMF) at relatively low current density j<2 kA/cm/sup 2/. In the present work, we have further developed this technique. This advanced technique is applicable for the analysis of the current density distribution on the complicated electrodes of commercial vacuum interrupters (VIs) carrying currents up to I=50 kA at current density up to j/spl sim/4 kA/cm/sup 2/. The experiments have been carried out for three types of electrode designs generating AMFs with different configurations. The results obtained by this new technique proved a previously derived conclusion that current density tends to distribute evenly across that part of the contact surface where the AMF induction fits the inequality B/sub z//sup (1)/<B/sub z/<B/sub z//sup (2)/, where B/sub z//sup (1)/, B/sub z//sup (2)/ are characteristic induction fields defined in our previous work. Results of the analysis of erosion tracks on the electrodes are also in agreement with this conclusion. Slots on the electrode surface lead to substantial nonuniformity of current density distribution even if the AMF is properly configured. Comparison of the breaking capacity and distribution of current density for different electrode designs proves that interruption failure occurs at peak current density /spl sim/3.5 kA/cm/sup 2/. This value exceeds by at least 50% previous estimates of maximum current density interruptible by a commercial VI. We have also found that changing the configuration of the AMF may substantially improve breaking capacity. In our experiments, a slight modification of the AMF distribution provides a 12% increase of the breaking capacity.

Physics of Electrical Discharges is a Profound and an Extensive World

Three topics related to the field of the technical committee on electrical discharges (TC-ED) are summarized for this special issue. First one deals with the protection against the lightning damages. As the second one, a prototype high voltage vacuum interrupter is introduced. The last one shows the problem on the electromagnetic relays.

Technological progress of axial magnetic field vacuum interrupters

New methods have been developed for investigating vacuum arcs applied to the improvements of vacuum interrupter electrodes. Electrode surface temperature measurements reveal the anode surface melting condition at the current interruption and its influence on interruption capability, and a digital image processing technology gives us a clear understanding of the arcing phenomenon. Measurements and test results have shown the advantages of axial magnetic field electrodes in increasing the interruption capability of vacuum interrupters. It is confirmed that axial magnetic field electrodes are superior to other types of electrodes, and we believe that these new investigation methods promote the optimization of the electrode structure and the realization of higher voltage and current rating vacuum interrupters.