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An Analysis of the Instability Phenomena of a Low current Vacuum Arc
Abstract
In order to clarify the instability phenomena of a low current vacuum arc, characterized by noise on the current trace prior to the actual current chopping, an analysis of copper cathode spot has been made. For the analysis, cathode spot region is recognized as the collisionless space charge sheath connected with singly ionizad collisional plasmas.
It is proposed that a current level below that no real solution exists is unstable current and this current corresponds to the point at which a vacuum arc turns into unstable.
It is found that there is no real solution when the current is below 25A. The physical explanation is considered that the electrons returning to the sheath region from the plasma one dominate over positive ions and, consequently, the electric field at the cathode surface is F0²≤0 below 25A.
... The prior studies revealed that the unstable arc phenomena occurred at current I o (I C ) before the chopping current (I c) is triggered, and this phenomena triggered the current chopping [2], [3]. The chopping current I c largely depends on the circuit parameters that determine recovery voltage after cut off, particularly on the capacitance C connected between electrodes. ...
... Cylindrical electrodes with a diameter of 39 mm and a length of 26 mm were used as electrodes. The material with low thermal conductivity needed to be used as a supporting rod for electrodes so that the heat inside the electrodes will not dissipate [2], [3]. As the material for this supporting rod, we used a rod made with SUS304 (diameter of 20 mm, and length of 80 mm with a thickness of 0.5 mm). ...
... Integrating the energy conservation equation for plasma gives this expression. Solving the equation of motion will be as follows[3,4]. ...
... Integrating the energy conservation equation for plasma gives this expression. Solving the equation of motion will be as follows [3,4]. ...
In vacuum breakers, the instability phenomenon occurs in the arc current at a current value before current cutting is generated. This arc phenomenon sometimes causes a current cutting due to this arc phenomenon. Therefore, it is the purpose of this report to capture the instability phenomenon due to vacuum arc discharge in the small-current arc region when cupper electrodes are used. A cathode spot model where a collision less sheath and collision plasma was connected to each other was constructed. A unique solution to the arc current (independent variable of discharge phenomenon) was obtained by simulation of dichotomy. As a result, it was supposed that the current that gives a real solution has a lower limit, the electrons that back-diffused from the collision plasma in cathode ion sheath exceed the ion under 19 A and the cathode spot that requires the existence of the ion sheath as a necessary antecedent or precondition does not exist anymore. It was concluded the non-existence of the cathode spot is connected the current instability phenomenon.
... The qualitative discussion of physical mechanism of the stability phenomena of a low-current metal vacuum arc as shown inFig. 1-2 remains unclear[4][8]. These investigations are very important for the development of cathode materials for low-surge switching vacuum circuit breakers. ...
... obtained using the calorimetric method [4] is shown inFig. 3 Arc current, A Arc Voltage、Varc Cathode Input、Veff Anode Input、Veff・a WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS Narong Mungkung, Tanes Tanitteerapan, Somchai Arunrungrusmi, Weerachai Chaokumnerd, Toshifumi Yuji ...
The purpose of this research was to investigate instability phenomena in low-current metal vacuum arc using the calculation data comparison with the experimental data. The instability phenomena are characterized by noise on the current trace prior to the actual current chopping. The instability current was investigated for various electrode materials. To study the parameters affecting the stability arc factors, the parameter scan of cathode materials and ion current fraction by numerical analysis, it was found that the critical current of the stable current is highly dependent on the thermal conductivity of the cathode material. However, the thermal conductivity effect on the instability phenomena of low-current metal vacuum arc was important for low surge switching electrode material development. Therefore, the experiment of major commercial switching electrode such as Ag-Pd is performed in this study. The arc during time and the maximum arc current were fixed to 8 ms and 100 A, respectively. The vacuum was maintained to about 1*10-6 Pa. The observed waveforms, instability current, chopping current are measured by the oscilloscope. As a result, the critical current of the stable current is inversely dependent on the thermal conductivity of the cathode material. This is very important result for the development of cathode materials for low-surge vacuum interrupters.
Secondary arc on solar array in space is defined as an arc discharge short-circuiting adjacent solar cells. As operational voltage of spacecraft increases to 100V, there is more risk of secondary arc that can destroy solar array circuit. Secondary arc is categorized as vacuum arc, although its current is very low, even less than 1A. One-dimensional modeling is carried out to derive sets of parameters that satisfy the governing equations. Genetic algorithm is used to find the optimal set of solutions. Even at a current less than 1A, the solutions exist though the arc voltage is higher than the experimental values by several ten volts
The purpose of this research was to analyses and compare the instability of a contact surface between copper and nickel as an alloy cathode in vacuum, the different ratio of copper and nickel were conducted at 1%, 4% and 6% by using the cathode spot model which is recognized as the collisionless space charge sheath connected with singly ionized collisional plasma. It was found that the minimum stable arc current is highly dependent on the thermal conductivity of the cathode material due to amount of percent nickel. The chopping current is proportional to the level of minimum stable arc current. As results, the smaller recovery voltage which is induced by chopping current. This is a very important result for the development of cathode materials for low-surge vacuum interrupters. The results obtained by this study clearly demonstrate the physical mechanism of current instability occurrence, the level of chopping current and the recovery voltage in power system
In order to clarify the instability phenomena of a low current vacuum arc, characterized by noise on the current trace prior to the actual current chopping, an analysis of silver cathode spot has been made. For the analysis, cathode spot region is recognized as the collisionless space charge sheath connected with singly ionized collisional plasmas.It is proposed that a current level below that no real solution exists is unstable current and this current corresponds to the point at which a vacuum arc turns into unstable. It is found that there is no real solution when the current is below 19.4A. The physical explanation is considered that the electrons returning to the sheath region from the plasma one dominate over positive ions and, consequently, the electric field at the cathode surface is F02
A cathode spot model is used to analyze the instability phenomena of a vacuum arc. The current below which no real solution exists is believed to be an unstable region. When the current decreases below 19 A, the electrons returning to the sheath region from the plasma region were found to dominate over the positive ions and, thus, the cathode electric field has an imaginary solution. An electrostatic probe was used to confirm these findings, and the analytical results were similar to measurement results. The present cathode spot model may be valid for volatile materials.
The arc current for initiating the appearance of noise on a current trace, the instability initiation current I0, and the arc voltage V0 corresponding to I0, have been measured using Cu-Ni alloy and Ag-Pd alloy electrode metals. The average values of I0 and V0 were plotted as a function of the thermal conductivity lambda, showing a decrease with decreasing lambda under a constant vapor pressure Ph.
It is shown that in the cathode drop region of a metal vapor arc, there are four equations and two limiting conditions relating five dependent variables. The five dependent variables treated are temperature of the cathode spot, electric field at the cathode, total current density, current density carried by electrons, and the radius of the spot. When these equations are combined, a current level is found below which no solution exists. It is proposed that this current corresponds to the point at which a vacuum arc extinguishes in an alternating‐current circit. Experimental measurements of the current level at which this event occurs have been made, and the results are compared with the theoretical calculations.
Existing models of the cathode processes in the Hg vacuum arc were
analyzed and found to lead to contradictions. Specifically, if the
electron emission is based on field emission, a considerable ion density
is required near the cathode surface, leading to ion bombardment
overheating of the surface. An alternative mechanism is proposed and
analyzed. The near-cathode region of the discharge consists of three
regions, which in order of their distance from the cathode surface are
called the first plasma region, the double sheath, and the second plasma
region. The surface of the first plasma region serves as a virtual
plasma cathode for the discharge. Current at the cathode surface is
conducted primarily by ion current from this plasma region. Electrons
emitted from the surface of the first plasma region are accelerated
through the double sheath region into the second plasma region. Likewise
ions flowing from the second ion region are accelerated through the
double sheath into the first plasma region where they serve as a
significant heat source. A system of equations describing current
continuity and energy conservation was formulated and solved for the
above model. Two types of time-dependent solutions were found, with
characteristics times of 0.1-1 and 100 μs, respectively. These times
correspond to the experimentally observed spot lifetimes for the
transitional and fundamental discharge forms, respectively. Excited atom
flow into the first plasma region plays an important role in its energy
balance, as does the outflow of neutral atoms. The potential drop in the
plasma regions, typically <0.5 V, is small in comparison to the
double sheath potential drop U<sub>sh</sub> (10-15 V). U<sub>sh</sub>
depends only weakly on the spot current, if the cathode erosion
coefficient is constant. The length of the first plasma region is
calculated to be 10<sup>-5</sup> m, which is approximately equal to the
experimentally observed dark space in the Hg vacuum arc. Acceleration of
electrons from the first plasma region through U<sub>sh</sub> explains
the experimental observation of fast electrons