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This paper deals with the numerical simulation of the constricted high-current vacuum arc (> 15 kA), driven by a transverse magnetic field (TMF), as found in vacuum circuit breakers applying the TMF arc control. The magnetohydrodynamic approach, together with the detailed heat transfer and evaporation equations for the electrodes, is used to describe the arc behavior self-consistently, restricted to 2-D. A newly developed model describes the cathode attachment of the constricted arc, as a large laterally extended foot points, instead of regular cathode spots. The arc maintains itself when the electrode temperatures are higher than 3400 K on the cathode and 2900 K on the anode. This model leads to the characterization of the physical quantities of the arc plasma and describes the arc motion. A stepwise movement of the arc results due to different instantaneous velocities of the current attachment areas at the cathode and anode. Index Terms--Cathode connection, heat transfer, magnetohydrodynamic (MHD), moving arc, numerical simulations, transverse magnetic field (TMF) vacuum interrupter, vacuum arc.

We study, by optical spectroscopy, the properties of high-current electric arcs that form between opening Cu contacts. The presence of a current that increases linearly with a rate of about 14 kA/ms leads to the separation of the contacts and to the formation of a bridge of molten metal that eventually ruptures. We show that after the molten bridge rupture, it takes about 40 $\mu \text{s}$ for the initial arc to be at local thermal equilibrium. This long thermalization time suggests that the plasma density during that time range is low, which we confirm using a simple modeling of the energy transfer between electrons and heavy particles. We explain these low densities by the fact that the molten bridge rupture leads to the formation of a shock wave. The initial dense plasma rapidly expands and a region with a pressure below the background pressure forms between the two contacts, leading to low plasma densities and long thermalization times. [ABSTRACT FROM AUTHOR]

We investigate by time-resolved optical emission spectroscopy the properties of electric arcs forming at the opening of Cu contacts and with a current of a few kA. The systematic observation of well-resolved transitions demonstrates that the dense Cu vapor that results from the rupture of the molten metal bridge between the contacts expands rapidly. Solving the radiative transfer equation through the line-of-sight to fit the lineshape of the detected spectra, we obtain that both the pressure and the temperature of the Cu-dominated plasma increase during the first 100 µs of arcing. At the same time, the peak wavelength and broadening of Cu I transitions exhibit a non-monotonic time-dependence. In particular, both the blueshift and narrowing of these transitions by Stark effect during the first 40 µs of arcing demonstrate a decrease in electron temperature. This result leads us to the conclusion that it takes at least 40 µs for the arc to be at local thermal equilibrium after the rupture of the molten metal bridge. [ABSTRACT FROM AUTHOR]

The electrode surface temperatures of Cu–Cr butt electrodes exposed to vacuum arcs with sinusoidal currents of 10–20 kA and under external axial magnetic field were determined. Different experimental techniques were applied that can be distinguished by the used spectral wavelength range, their temporal, and spatial resolution. Near infrared spectroscopy was carried out by means of a fiber optic spectrometer working in the wavelength range from 900 to 1670 nm with a temporal resolution of 1–2 ms. Electrode surface temperatures after current zero were obtained from the relative shape of the spectrum using the Planck curve fitting procedure. Furthermore, electrode emissivities were derived after performing absolute calibration of the spectra. Pyrometric measurements were performed in the spectral range around 2 \mu \textm with high temporal but without spatial resolution. 2-D distributions of the electrode temperature were obtained from high-speed video thermography using a camera with a metal interference filter at 890 nm. [ABSTRACT FROM PUBLISHER]

The resistance of vacuum interrupters can be calculated from the geometry and resistivity of current carrying parts and the additional resistance of the contact points between movable and fixed contacts. Since vacuum interrupter contacts are designed as flat contacts facing each other, the resistance is mainly determined by contact force, hardness and resistivity of the contact material. It is known that the contact material changes consistency and structure during short-circuit interruptions within melting depth. Indeed, the overall resistance of a vacuum interrupter has been observed to increase by up to 60% after short-circuit making and breaking tests. Since the resistance increase across the switching device is considered by IEC and IEEE standards as one of the acceptance criteria for the integrity of the interrupter after tests, it is essential to understand the origin of this increase. Different causes are discussed, among them the change of grain structure, resistivity and hardness of the molten top layer of the contact, and flatness of the contact surface. An increase of hardness and a different position of contact points seem to be the main factors responsible for the increased resistance. [ABSTRACT FROM PUBLISHER]

This paper is devoted to numerical simulation of the cathode attachment zone of constricted high-current vacuum arcs driven by a transverse magnetic field. With help of particle-in-cell and direct simulation Monte Carlo methods, a 1-D model of the attachment zone was developed, and the stationary-cathode-spot mode was investigated. The states of the charged particles and atoms in the Knudsen layer, the ionization layer, and the plasma sheath are determined in detail. The results support the adaptation of simplified cathode attachment models to be able to reduce the computational effort needed for self-consistent numerical simulations of magnetically driven constricted high-current vacuum arcs. [ABSTRACT FROM PUBLISHER]