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The absence of current-zero points in the direct current (dc) waveform of high voltage dc (HVDC) circuit breakers makes the interruption process more severe than the case of conventional ac networks with sinusoidal currents. In this way, in the current HVDC networks, a parallel precharged capacitor is inserted to inject the reverse current into the interruption chamber of the vacuum circuit breaker and create the artificial current-zero points. In this paper, a novel method for reverse current injection has been proposed. In this method, two separate helical flux compression generators (HFCGs) have been applied to generate the reverse current and an intense axial magnetic field (AMF), respectively. Numerical simulation of high-current vacuum arc (VA) in the presence of very strong AMF has been presented. For this purpose, the magnetohydrodynamic equations describing the behavior of the VA are coupled to a simple circuit analysis and the previously developed multiphysics model of HFCG. The results indicate that the explosively driven current injection set can make current-zero points properly in a typical HVDC network and lead to successful interruption of fault current. This method needs much less volume and cost in comparison to the parallel precharged capacitor sets. Index Terms--DC power transmission, pulse power systems, vacuum arc (VA), vacuum circuit breakers, vacuum interrupters (VIs).

Flux compression generators (FCGs) are widely used to generate extremely high power pulses. When a peak current from a seed source is flowing through the generator circuit, the chemical energy of high explosives is used to increase the amplitude of the output current pulse applied to the load. During the transfer of explosive energy to the output electrical pulse, the critical issue is achieving a change in the inductance in a fast and controlled way. There have been many significant experiments on different kinds of FCGs, and almost all of the theoretical models used to describe generator behavior are based on empirical equations. In this paper, simultaneous detonic, electromagnetic, and circuit simulations are used to study the characteristics of helical FCGs. For this purpose, the magnetic field diffusion and losses due to the induced currents in metallic components are considered in more detail. Based on the results of these theoretical considerations, new modifications for the FCG are proposed. It will be shown that the proposed modified FCG geometries yield output current rise times that are about 60% of those achieved with conventional helical FCGs generating the same current pulse amplitudes. Index Terms--Explosive pulsed power, helical flux compression generator (FCG), magnetic field diffusion.

Streamer branching in liquid dielectrics is driven by stochastic and deterministic factors. The presence of stochastic causes of streamer branching such as inhomogeneities inherited from noisy initial states, impurities, or charge carrier density fluctuations is inevitable in any dielectric. A fully three-dimensional streamer model presented in this paper indicates that deterministic origins of branching are intrinsic attributes of streamers, which in some cases make the branching inevitable depending on shape and velocity of the volume charge at the streamer frontier. Specifically, any given inhomogeneous perturbation can result in streamer branching if the volume charge layer at the original streamer head is relatively thin and slow enough. Furthermore, discrete nature of electrons at the leading edge of an ionization front always guarantees the existence of a non-zero inhomogeneous perturbation ahead of the streamer head propagating even in perfectly homogeneous dielectric. Based on the modeling results for streamers propagating in a liquid dielectric, a gauge on the streamer head geometry is introduced that determines whether the branching occurs under particular inhomogeneous circumstances. Estimated number, diameter, and velocity of the born branches agree qualitatively with experimental images of the streamer branching. [ABSTRACT FROM AUTHOR]

In some plasma applications, such as confinement and pinches, an ultrahigh magnetic field (several teslas) is required. Such magnetic flux densities can be produced by using flux compression generators (FCGs). This paper has proposed a joint thermodynamic and electromagnetic simulation method for a common type of FCGs (i.e., helical type) with respect to the magnetic coupled plasma studies. The images of displacements, pressure, and magnetic flux density distribution in generator and plasma chamber are presented as simulation results. Index Terms--Explosions, flux compression generators (FCGs), magnetic coupled plasmas, thermodynamic and electromagnetic simulation.

Subnanosecond breakdown of low-pressure gaseous spark gaps has been investigated by using a combined modeling of the volume discharge in gas and the electron emission at the electrodes. Simulation results predict minimum breakdown delays of about 400 ps for the geometries considered in this paper. These results are in good accordance with the measurements. Index Terms--Pulsed-power switches, subnanosecond breakdown, thermofield electron emission, volume discharge.

Ultraviolet (UV) radiation decreases the hydrophobicity of silicone rubber (SiR) insulators. Lack of hydrophobicity on the surface of such insulators lowers the flashover voltage and increases the Leakage Current (LC) of insulators in the moist environment. This paper presents the artificial UV radiation effects on three types of 20 kV SiR insulators in a chamber with nine UV-C lamps (50 W/m2) for up to 5000 hours. To simulate the moist environment, solid layer method is applied according to IEC 60507 that includes artificial pollution deposition on the insulator surface. Fast Fourier transform of the LC waveforms indicates that the third and the fifth harmonic components are quite sensitive to any incremental discharge on the insulator surface. This sensitivity makes the frequency spectrum of the LC a good criterion to distinguish three distinct SiR insulator situations under the electric stress: 1) no discharge, 2) dry band arcing/corona discharge and 3) continuous arc. Thermogravimetric analysis (TGA) and scanning electron microscope images depict that the polluted insulators include much larger degraded parts under electric stress after aging. Measurements also show that the flashover voltages and the hydrophobicity of aged insulators decrease as the UV exposure time increases for different levels of moisture and pollution. [ABSTRACT FROM AUTHOR]