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Inverted polarity effect was studied at the breakdown of point-to-plane discharge gap filled with air at a pressure range of 0.013–0.3 MPa. High-voltage nanosecond pulses were applied to the electrode with a small radius of curvature. Pulser was operated at both negative and positive polarities in the single-pulse mode. Waveforms of voltage pulse, discharge current, and intensity of the discharge plasma radiation near the electrode with a small radius of curvature were registered. It was obtained that at the breakdown of point-to-plane discharge gap filled with air, the breakdown voltage at positive polarity of the pointed electrode was higher than that at negative one. The inverted polarity effect was shown to come to naught with an increasing of air pressure up to 0.3 MPa. It is suggested that the inverted polarity effect is related to the impeded formation of the explosive emission centers on the flat electrode (cathode) after arrival of the ionization wave front to the one. In addition, both the inverted and the ordinary polarity effects were not observed at the breakdown of nitrogen at pressures of 0.4 and 0.7 MPa. [ABSTRACT FROM PUBLISHER]

Abstract: Space charge is a key insulation problem of HVDC equipment. In this paper, the influence of voltage polarity and electrode material were studied by the pulse electroacoustic (PEA) method. It had been found that the type of space charge accumulating at the interface was the same with the voltage polarity. With different electrode materials, the space charge injection was consequently different, but not evident. The interface made the drift velocity slower in two-layer samples. It suggested that the interface had a significant impact on space charge distribution and the contact state was complex, so a modified Maxwell–Wagner model was proposed. [Copyright &y& Elsevier]

The focus of this work has been on the pre-breakdown phenomena and the breakdown characteristics of N gas in a sphere-plane gap under various impulse voltages. Both electrical and optical experimental investigation methods were used. Following parameters were considered: gas pressure range from 0.2 to 0.6 MPa, electric field utilization factor of the electrode configuration 71%, positive and negative impulse waveforms with the rise time of 500 ns, 1.2 μs and 180 μs. The observed discharge processes before the breakdown through the light emission images by the ICCD camera are in good agreement with the streamer mechanism. Under both polarity stresses, discharges are initially concentrated around the tip of the sphere and later pointing towards the earth electrode. However, negative streamers are thinner and more diffuse. As expected, the breakdown voltages for negative polarity are lower than those for positive polarity regardless of the gas pressure and shape of the applied impulse voltage. The breakdown voltage is increased with shortening the rise time of pulse waveforms. As a substitute for SF, N gas under pressures above 0.3 MPa can reach the standard rated withstand voltage for 24 kV C-GIS. [ABSTRACT FROM AUTHOR]

An experimental parametric study is made to investigate how the electrical corona discharge characteristics are influenced by the geometrical configuration and dimensions of the reactor and the electrode polarity of the applied voltage. Furthermore, features of the corona discharge plasma formed around the stressed electrode in some different gases are recorded photographically to provide more information on the physical mechanisms of the corona discharge in the investigated gases. The obtained results have been discussed in the light of gas discharge physics and its applications. [ABSTRACT FROM AUTHOR]

The nanosecond single-pulse voltage was employed here to generate a large-volume discharge between the wire-plane electrodes in nitrogen. In order to find out the effect of pulse polarity on the propagation of a large-volume discharge, both the positive and negative pulses were applied to the wire-plane electrodes, respectively. Images presented here reveal that the discharge channels are more homogeneous under the negative pulse, and the propagation speed of the discharge channels is faster under the positive pulse. [ABSTRACT FROM AUTHOR]

A volatile switching of conductive filament (CF) in a Cu/TaOx/δ-Cu/Pt device has been observed. The device differs from a conventional Cu/TaOx/Pt device by the insertion of a thin Cu-layer (δ-Cu) between the electrolyte and the inert electrode. The Cu CF is formed the same way as in the conventional nonvolatile devices. However, when applied voltage becomes zero, CF ruptures spontaneously. The dynamic balance between Cu+ field-supported hopping transport and the Cu self-diffusion explains the effect of CF volatility. The device can operate reliably in volatile and nonvolatile modes. [ABSTRACT FROM AUTHOR]

Very fast transient overvoltage (VFTO) in gas insulated switchgear (GIS) is of great concern at the higher rated voltages because the difference between the rated lightning impulse (LI) and the VFTO withstand voltage decreases. In order to clarify VFTO withstand level of 1100 kV GIS, a simulation system was designed for generation of VFTO. The breakdown characteristics of sphere-plane gap and rod-plane gap in SF6 under VFTO and LI were investigated for different gas pressures. Experimental results show that the breakdown voltages in slightly inhomogeneous field gaps under VFTO are higher than that under LI. However, for the rod-plane gaps, the breakdown voltages and voltage-time curves under VFTO fall below that of LI at higher pressure in the case of negative polarity. The polarity effect of rod-plane gaps under LI is reversed at a certain pressure. Based on the analyses, it is concluded that the different breakdown characteristics of SF6 gaps may be attribute to the difference between space charge behaviors during the discharge development under the impulse with different wave front and oscillation tail of VFTO and LI. [ABSTRACT FROM PUBLISHER]

Prebreakdown phenomena in 0.1 M N,N-dimethylaniline (DMA) and 0.1 M trichloroethene (TCE) in n-tridecane have been investigated experimentally in liquid and solid phases in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Results have been compared with results obtained in neat n-tridecane and in cyclohexane with the same additives. Adding TCE to n-tridecane increases the pre-inception current and decreases the inception voltage for negative and positive polarity. The increase in pre-inception current is probably caused by trap assisted conduction, and may be responsible for the decrease in inception voltage. TCE had no effect on propagation of streamers in n-tridecane. The main effect of DMA in n-tridecane is to enhance the propagation of streamers and electrical trees with positive polarity. This is in line with what has been reported for cyclohexane, and is explained by the low ionization potential of DMA. Charge injection for negative streamers is reduced by the addition of DMA in n-tridecane, while the emitted light is increased. This indicates that some of the energy otherwise used for propagation is emitted as light. A low electronic excitation energy for DMA at 3.8 eV supports this hypothesis. The additives have the same effect on prebreakdown phenomena in solid and liquid phases with positive polarity, and thus the same mechanism is suggested to be responsible for electrical treeing and streamer inception and propagation. The scatter is generally found to increase when going from liquid to solid phase, which is explained by the inhomogeneity in the solid phase. The transition to solid phase with negative polarity typically results in an even larger scatter than for positive polarity. The effect of additives seems to be secondary to the morphology at the point electrode with negative polarity. [ABSTRACT FROM PUBLISHER]

In this paper, particle velocity fields inside a plane-to-plane dielectric barrier discharge (DBD) precipitator are investigated using time-resolved particle image velocimetry. The main objective is to analyse the effect of a filamentary discharge on the particle trajectory. A sine wave high voltage (24 kV, 30 Hz) is applied to create a DBD inside a planar gap (6.4 mm) filled with particles with a mean size of about 0.28 um. The time-averaged velocity of the flow in the centre of the channel is about 1 m s[?]1.After the establishment of the discharge several filaments cross the gap, and induce a strong effect on the particle trajectory. During a complete period of the voltage, successive phenomena are observed. Before the first filament propagation, the shape of the velocity profiles is typical of a laminar flow. At the early stage following the filament propagation across the gap, the grey-level images show a sudden disappearance of the particles at the same location where the filament takes place. This is due to the fast precipitation of particles. During the positive half-cycle, the particles migrate mainly towards the grounded electrode due to their positive net charge. At the end of a half-cycle, the polarity of the electric field is reversed then the particles initially charged return towards the channel centre. Consequently, the particles oscillate delaying their collection. [ABSTRACT FROM AUTHOR]