S. Rowe

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.

Influence of absorbed water on the dielectric properties and glass-transition temperature of silica-filled epoxy nanocomposites

Work on dielectric spectroscopy of epoxy resin filled with nano-SiO2 at different relative humidities and temperatures is reported. Above the glass-transition temperature (Tg), dc-like imperfect charge transport (QDC or LFD) dominates the low frequency dielectric spectrum. Another mid-frequency relaxation process was found in the non-dried composites. Water also induces glass-transition temperature decreases, which can be measured both by dielectric spectroscopy and DSC. Both theory and experiment demonstrated that a higher water content could exist in nanocomposites than unfilled epoxy suggesting a bigger free volume when nanostructured. In our system, the hydrophilic surface of silica is likely to cause water to surround and lead to delamination of the epoxy from SiO2. This is a potential mechanical and dielectric weakness in the nanocomposites, which may lead to an ageing phenomenon. Hydrophobic surface group may reduce the water adsorption in nanocomposites.

Space charge behaviour in an epoxy resin: the influence of fillers, temperature and electrode material

This study aims to characterize the behaviour of the space charge in an epoxy resin used as electrical insulation in systems such as transformers and bus bars. Temperature, field level, filler content and nature of the electrodes are the parameters that were considered. Space charge measurements were performed using the pulsed electro acoustic technique, in a range of field and temperature up to 40 kV mm−1 and 72°C, respectively, on gold-coated and un-coated samples. We discuss the possibility of performing space charge measurement on filled epoxy resin despite the piezoelectricity of quartz fillers. Under dc field we observed a quasi-symmetrical build-up of homocharges at both electrodes, followed by a substitution of the homocharges by heterocharges, mainly close to the cathode. In addition, we recorded the space charge behaviour just before breakdown on a filled sample at 72°C under 12 kV mm−1.

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.

Space charge distribution measurement methods and particle loaded insulating materials

In this paper the authors discuss the effects of particles (fillers) mixed in a composite polymer on the space charge measurement techniques. The origin of particle-induced spurious signals is determined and silica filled epoxy resin is analysed by using the laser-induced-pressure-pulse (LIPP), the pulsed-electroacoustic (PEA) and the laser-induced-thermal-pulse (LITP) methods. A spurious signal identified as the consequence of a piezoelectric effect of some silica particles is visible for all the methods. Moreover, space charges are clearly detected at the epoxy/silica interface after a 10-kV/mm poling at room temperature for 2 hours.

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.

Electrically active defects in silica-filled epoxy as revealed by light emission analysis

Epoxy resins have long been used as the insulation of electrical systems. They are generally formulated with a dispersion of micro-fillers to improve thermal and mechanical properties. However, there are concerns about the possible influence of these fillers on the electric behaviour, especially on the long term ageing under functional stresses. At the loose interface between matrix and fillers, macro- and micro-voids in the resin can provide weak points that are difficult to detect using conventional spectroscopy. Light emission analysis from the material under electrical stress is an efficient way to reveal such electrically active defects since internal ionizing events would give rise to photon emission. A detailed analysis of the light emitted by silica-filled and unfilled epoxy samples is presented. The photon counting technique, spectral analysis and imaging give a firm basis to discuss the contributing emission processes to the detected signal. They reveal the existence of ionizing events into internal defects. The sensitivity of the optical method is order of magnitudes higher than the sensitivity of conventional partial discharge detection.

Transient and steady-state currents in epoxy resin

Charging and discharging currents have been measured in a diglycidyl ether of bisphenol-A epoxy resin with and without silica fillers, below and above its glass transition temperature Tg = 65 °C. Both transient and steady-state current densities have been analysed. The average applied fields ranged from 3 to 35 kV mm−1 with a sample thickness of 0.5 mm. Above Tg, transient currents suggested a phenomenon of charge injection forming trapped space charges even at low fields. Steady-state currents confirmed that the behaviour was not Ohmic and suggested Schottky-type injection. Below Tg, the current is not controlled by the metal–dielectric interface but by the conduction in the volume: the current is Ohmic at low fields and both transient and steady-state currents suggest a phenomenon of space-charge limited currents at high fields. The field threshold is similar in the filler-free and the filled resin. Values in the range 12–17 kV mm−1 have been measured.

The influence of filler particles on space charge measurements

The effects of filler particles on the signal measured using the pressure-wave-propagation method to determine the space charge distribution are analysed. A special test sample geometry is used to classify the influence of particles, and in particular, the role of piezoelectricity is discussed. Applications with Epoxy resin samples loaded either by silica or alumina particles are presented. Silica particles are shown to exhibit piezoelectricity which strongly modifies the measured signals.

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...