G. Sandolache

Expanding sheath in a bounded plasma in the context of the post-arc phase of a vacuum arc

A numerical model of sheath expansion and plasma decay in a bounded plasma subjected to a linearly increasing voltage has been developed. Numerical results obtained with a hybrid-MB model (Maxwell–Boltzmann electrons, particle ions and Poissons equations) are compared with analytical theory and results from particle-in-cell (PIC) simulations. The hybrid-MB model is similar to models used for plasma immersion ion implantation except that plasma decay due to particle losses to the electrodes is taken into account. The comparisons with more accurate and much more time consuming PIC models show that the hybrid-MB model provides a very satisfactory description of the sheath expansion and plasma decay even for conditions where the grid spacing is much larger than the Debye length. The model is used for high plasma density conditions, corresponding to the post-arc phase of a vacuum arc circuit breaker where a vacuum gap is subject to a transient recovery voltage (TRV) after it has ceased to sustain a vacuum arc. The results show that the plasma sheath expansion is subsonic under these conditions, and that the plasma starts to decay exponentially after two rarefaction waves from the cathode and anode merge in the centre of the gap. A parametric study also shows the strong influence of the TRV rise rate and initial plasma density on the plasma decay time and on the ion current collected by each electrode. The effect of collisions between charged particles and metal atoms resulting for the electrode evaporation is also discussed.

Observation of the Plasma Plume at the Anode of High-Current Vacuum Arc

Stable and bright plumes with a distinct and bright shell are observed at the anode of the high-current vacuum arc burning during 10 ms at a current of up to 15 kA in peak. A plume is attached to a hot spot at the anode. The substance inside a plume is almost dark. Light from a plume shell is emitted mostly by neutral atoms. A distinct bright shell is surrounded by substances emitting light in ion lines. Appearance of plumes looks to be a result of the interaction between cathode and anode jets. Plume dimensions depend inversely on both arc current and arc voltage, which makes plumes small and inconvenient for observations except as near current zero. Anode plumes are recognized on copper-chromium electrodes to be a stable object. However, similar objects on pure copper are much less stable and appear only at heat-insulated liquid protrusions and droplets flying in a gap. This fact indicates the evaporation rate to play a key role in appearance of plumes. In addition, the evaporation rate of a plane copper-chromium surface under high-current vacuum arc is supposed to be as high as that of heat-insulated liquid copper protrusions and droplets.

Field electron emission current in vacuum interrupters after large inrush current

Field electron emission (FEE) current in the open(ing) gap of nine different designs of 36 kV vacuum interrupters under standard recovery voltage conditions was measured during full-power back-to-back capacitive switching. During closing operation, 20 kA inrush current (@ 4250 Hz) was applied. The measurement system is described, able to measure FEE current with a threshold of 30 μA but that also has to withstand a full capacitor bank discharge current of up to 40 kA after re-strike. It was concluded that in spite of the wide variety of measured FEE current magnitude (up to several mA) there is no direct relationship between magnitude of FEE current and probability of breakdown. Longer arc duration of the normal load current reduces the FEE current level after load current interruption.

Diagnostics of the Cathode Sheath Expansion After Current Zero in a Vacuum Circuit Breaker

Langmuir probes operating in the electron saturation current mode were used to detect the time of the arrival of the cathode sheath at the probe position. The probe current was measured just before and after current zero (CZ) followed the transient recovery voltage in a gap with copper electrodes. Probe measurements were accompanied with the detection of the postarc currents. Basic plasma parameters such as electron temperature and density were measured in the vicinity of the arcing gap. The data set allowed us to reconstruct the sheath dynamics after CZ.

Sheath expansion and plasma dynamics in the presence of electrode evaporation: Application to a vacuum circuit breaker

During the postarc dielectric recovery phase in a vacuum circuit breaker, a cathode sheath forms and expels the plasma from the electrode gap. The success or failure of current breaking depends on how efficiently the plasma is expelled from the electrode gap. The sheath expansion in the postarc phase can be compared to sheath expansion in plasma immersion ion implantation except that collisions between charged particles and atoms generated by electrode evaporation may become important in a vacuum circuit breaker. In this paper, we show that electrode evaporation plays a significant role in the dynamics of the sheath expansion in this context not only because charged particle transport is no longer collisionless but also because the neutral flow due to evaporation and temperature gradients may push the plasma toward one of the electrodes. Using a hybrid model of the nonequilibrium postarc plasma and cathode sheath coupled with a direct simulation Monte Carlo method to describe collisions between heavy spec...

A new optical technique for investigations of low-Voltage circuit breakers

The technique of broad-band optical absorption spectroscopy has been successfully used for investigations of transient media in low-voltage circuit breakers, thanks to an intense radiation source developed at our laboratory. It enabled the determination of the concentration of copper atoms and C/sub 2/ molecules in hot gas behind a moving arc. The temperature of this gas was estimated using the molecular absorption spectrum of C/sub 2/ Swan bands. Thanks to the high-spectral intensity of the auxiliary source, measurements were also performed in an electrical arc. The measurements allowed the determination of the population of excited levels for several metallic atoms in the arc. Assuming local thermodynamic equilibrium, the electron temperature of the arc and the total concentration of these metallic atoms, coming from contacts and splitters, were deduced. This temperature is in good agreement with the one deduced from optical emission spectroscopy.

Anode Temperature and Plasma Sheath Dynamics of High Current Vacuum Arc After Current Zero

This paper addresses the behavior of a high-current vacuum arc after current zero (CZ). Langmuir probes operating in the electron saturation current mode are used to detect the time of arrival of the cathode sheath at the probe position. The probe current is measured just before and after the CZ followed by transient recovery voltage both with and without external axial magnetic field (AMF). For an AMF-stabilized arc, the plasma sheath expanded radially with respect to the electrode axis, whereas for an arc with no AMF, the sheath boundary expanded rather spherically away from the anode. The anode temperature immediately after the CZ is estimated using the optical pyrometry method. Thermal radiation of a hot sample is recorded using a HSFC four-channel high-speed camera. Each camera channel is equipped with an interference filter to record radiation at a particular wavelength (600, 700, 775, and 825 nm). The camera channels are preliminarily calibrated. The calibration sample is a molybdenum crucible with a known spectral emissivity.

Two-Dimensional Simulation of the Post-Arc Phase of a Vacuum Circuit Breaker

The post-arc phase of a vacuum circuit breaker is a critical phase for dielectric recovery in the context of medium voltage current interruption. A 2-D hybrid model aimed at understanding this phase and at evaluating the risk of circuit-breaking failure during this phase has been developed. In this paper, we show how the recovery voltage expels the plasma from the gap and illustrate the influence of the shield polarity.

Observation of an anode spot shell at the high-current vacuum arc

Stable and bright shells were observed at the anode of a high-current vacuum arc burning for 10 ms at current of up to 15 kA in peak. A shell covers a hot spot at the anode. Substance inside a shell is dark. Light from a shell is emitted mostly by neutral atoms. A distinct bright shell is surrounded by substance emitting light in ion lines. Appearance of shells looks to be a result of interaction between cathode and anode jets. Shell dimensions depend inversely on the arc current. Anode spot shells were recognized on copper-chromium electrodes to be a stable object. However, similar shells on pure copper are much less stable and appear only at heat-insulated liquid protrusions and droplets flying in a gap. This fact indicates that the evaporation rate plays a key role in appearance of shells. In addition, evaporation rate of a plane copper-chromium surface under high-current vacuum arc is supposed to be as high as that of heat-insulated liquid copper protrusions and droplets.

Dielectric strength of the metal vapour

In vacuum circuit breakers (VCB) the presence of the metal vapour in the switching gap, following the interruption of high currents, is a source of limitation of dielectric performance during the recovery period. The temperature of the contacts is still high after current extinction, and metal vapour continues to evaporate from the contacts. Breakdown can initiate for metal vapour density at the condition below the Paschen breakdown limit. The Paschen curve of copper is only known from calculation for a small region around the minimum; because experimental data for copper vapour is not available. This paper deals with the dielectric strength corresponding to copper metal vapour. A special setup was devised for this purpose. This experimental device has allowed us to separate the pure dielectric effect of the metal vapour from that of the post arc plasma sheath and its dynamic movement, under the influence of the transient recovery voltage, TRV. The results allow an improved understanding of the impact of the metal vapour density on the breakdown process relative to the failure of vacuum circuit breaker.