Effect of an Electric Field on an Optical Breakdown in the Air Stream

Experimental data on the effect of an electric field on the plasma of an optical discharge in an air flow have been obtained. Two configurations of an external electric field under the action of an optical discharge on the plasma are considered. To create an electric field, flat (the field along the beam and across the flow) and ring electrodes (the field across the laser beam and along the flow) were used. It was found that there are two modes of combined discharge (optical and electrical). When the field was created symmetrically with respect to the flow axis, an electrical breakdown was observed from the nozzle exit (positively charged electrode) to the focusing point of the laser beam, while no streamers were observed in the optical discharge wake. In another case, an electric discharge is realized between flat electrodes simultaneously with optical breakdown. In a field of constant strength above 3 kV/cm, the presence of an optical discharge plasma promoted electrical breakdown of the medium. In this case, the parameters of the electrical breakdown depended on the shape of the electrodes, the polarity of the applied voltage, and the air flow rate.

On the slow ionization waves forming the breakdown in a long capillary tube with helium at low pressure

The results of studies of an electrical breakdown leading to the glow discharge ignition in a long capillary quartz tube are presented. Under such conditions, the breakdown completion is preceded by the development of direct, backward, and counter slow ionization waves traveling in the tube. The initiation of the waves was created in helium at low pressure (P=10 Torr) by the high-voltage pulses of positive and negative polarity with amplitude of several kilovolts. In the beginning, the regime without the breakdown completion in the tube was studied. In this regime, the propagation of only direct positive and direct negative ionization waves happens. The research on dynamics of the direct, backward, and counter positive and negative waves followed by a complete breakdown was done as well. The influence of the pre-existing plasma on the ionization waves propagation was also studied. The plasma was created in advance by low-current glow discharge being formed in the tube. The instant images of ionization waves were correlated with the electrical currents formed by the waves, that is, with the displacement current through the dielectric wall and the conductive current through the plasma column. In the experiments, the fine-sectioned electrode wrapped around the lateral tube surface was used. The usage of such electrode allowed one to study the dynamics of the surface charge deposition and deletion happening during the direct and backward wave propagation, respectively. Finally, a strong difference in the spatial structure and velocity of positive and negative direct waves traveling through non-ionized gas was revealed. Contrary, both the positive and negative backward waves traveling through the plasma formed by previous direct waves have the parameters close to each other.

Dielectric Strength of Polymeric Solid–Solid Interfaces under Dry-Mate and Wet-Mate Conditions

One of the most important causes of insulation system failure is the breakdown of the interface between two solid dielectrics; understanding the mechanisms governing this breakdown phenomenon is therefore critical. To that end, investigating and reviewing the practical limitations of the electrical breakdown strength of solid–solid interfaces present in insulating components is the primary objective of this work. The published literature from experimental and theoretical studies carried out in order to scrutinize the effects of the presence of solid–solid interfaces is investigated and discussed, considering macro, micro, and nano-scale characteristics. The reviewed literature suggests that solid–solid interfaces in accessories have non-uniform distributions of electrical fields within them in comparison to cables, where the distribution is mostly radial and symmetrical. Many agree that the elastic modulus (elasticity), radial/tangential pressure, surface smoothness/roughness, and dielectric strength of the ambient environment are the main parameters determining the tangential AC breakdown strength of solid–solid interfaces.

Preparation and electrostrictive properties of polyurethane filled with polypyrrole-carbon black for the energy harvesting application

The polymer composites based on electrostrictive materials with high polarization are attracting scientists because of the prospect of application in energy conversion and electromechanical performance. Polyurethane is one of a good flexible dielectric polymers which is more interesting in electromechanical applications due to its easy fabrication, high dielectric constant, and high electrostrictive coefficient. This work focuses on the dielectric properties and electrostrictive properties of Polyurethane (PU) filled with Polypyrrole-Carbon Black (PPy-CB) various the different concentrations. All polymer composites were prepared by using a film-casting process in solution with DMF solvent. The dielectric properties and conductivity of thin films were investigated by using LCR meter at 1-105 Hz. The results showed that the dielectric constant of composites significantly increases when the concentration of PPy-CB was increased. Moreover, the electrostrictive coefficient of composites also increases when the concentration of PPy-CB was increased. Furthermore, the electrical breakdown stretches were analyzed by using the Weibull model, this result will discuss the relationship when the PPy-CB fillers were used.

Effect of repeated electrical breakdowns on mineral and natural ester insulating oils

Transformer insulating oils are exposed to repeated electrical discharge or breakdowns inside power transformers. Durability tests are conducted to analyze the ability of oil to resist decomposition due to such high electrical stresses. With the increasing demand for alternative insulating oils for oil-immersed transformers, it is worthy to compare the performance of different types of insulating oils (conventional mineral-based insulating oil and natural ester-based insulating oil) under repeated electrical breakdown. In this paper, the AC breakdown voltage of different mineral-based and natural ester-based insulating oils is reported. Durability tests were conducted based on the AC breakdown voltage behavior of insulating oils after 50 electrical breakdown shots. The AC breakdown voltage of each insulating oil sample was assessed according to the ASTM D1816 standard test method. Based on the results, it can be concluded that the dissimilarity in chemical composition of the insulating oils has a significant effect on the AC breakdown voltage behavior of these oils under repeated electrical breakdowns.

Analysis of light emission and Schlieren from short gap high voltage streamers representing lightning impulses

Light emissions and Schlieren structures were simultaneously observed from streamers produced by tens of kilovolts 1.2/50 μs impulses, representing the high voltage component of lightning, applied across a 4 cm air gap between a variety of electrode geometries and a ground plane in an unconfined environment. The results demonstrated that the light emissions and Schlieren structures coincide along the same streamer filaments but on different timescales; the light existing only during the microsecond timeframe impulse whereas the Schlieren continued to develop into the millisecond timeframe, moving towards the centre of the air gap whilst diffusing into the surrounding air within 100 ms. If an electrical breakdown did occur, the Schlieren structures outside the arc remained visible. Streamer formation theory for high voltage impulses is subsequently refined to include the observed Schlieren mechanism.