Effect of structural modification on the electrophysical properties of impregnated organic dielectrics

It is shown that the electric strength and resistance to thermal ageing of an electric insulating paper may be improved by modifying the cellulose base with a chitosan biopolymer. The effects of the morphological features of the polymer foil on the electrophysical properties of the impregnated dielectrics and the interaction between an impregnating liquid and a polymer are presented.     

Propagation and structure of streamers in liquid dielectrics

Our purpose is to present a critical review of the current understanding of streamer propagation in dielectric liquids in order to help define the direction of future research. We show that the molecular structure has a significant effect on streamer propagation. The main parameter affecting propagation is the electronic affinity of the liquid molecules     

The effect of the electrode gap on breakdown in liquid dielectrics

It is a well known fact that the breakdown voltage of an insulating liquid increases nonlinearly with increasing electrode gap. Under nonuniform field dc conditions two breakdown voltages are determined depending on the electrode polarity. Using point-plane geometries, with gaps of 5 mm or larger, one finds that typical transformer oils have higher breakdown voltages when the point is negative than when it is positive. Recently, perfluorinated polyethers have been found to produce opposite results when using average gap sizes of 5 to 10 mm. To elucidate this situation a study of the effect of gap size on breakdown voltage has been undertaken. The results show that for the polyethers at larger gaps the sequence is reversed. These observations are explained in terms of the respective streamer developments in these liquids and the relative stability of their negative ions. In the case of the conventional hydrocarbon type oils, the positive streamer usually grows slightly faster than the negative one, but the streamer inception voltages of both are rather similar. In the case of perfluorinated polyethers the positive streamers move at least 10× faster than the negative ones, but they require much higher inception voltages than negative ones. At gaps ≳12 mm and the associated higher applied voltages this breakdown at negative polarity occurs at higher voltages than at the positive one. The negative ions of the perfluoro compounds are much more stable than those of conventional hydrocarbons. The discharge of the negatively charged perfluoro ions requires much higher fields than do those of conventional hydrocarbons. The implications of these observations are discussed in some detail     

The sequence of events in mid-gap laser-triggered electricalbreakdown in liquid dielectrics

Recent experiments on mid-gap laser-triggered electrical breakdown in liquid dielectrics have been scrutinised. The sequence of events that leads to the disruptive discharge is described and accompanied by electric field calculations     

Method for repetitive measurement of the electrostatic chargingtendency of liquid dielectrics

The construction of an apparatus for the measurement of the electrostatic charging tendency (ECT) of a dielectric liquid is described. A sealed reciprocating piston arrangement permits a liquid to be repetitively passed through a charge separating element. The use of a gold element is shown to minimize the effects of moisture and control temporal effects. The system is evaluated by testing samples of transformer oil drawn from the field. In general, the new system ranks the oils in the same way as a conventional assessment against a paper filter, although anomalies have been found which are an impediment to adoption     

Engineering with nonlinear dielectrics

All dielectrics are nonlinear if subjected to sufficiently high electric fields. A high degree of dielectric nonlinearity can bring simplicity to many problems, especially those involving highly inhomogeneous field geometries. Given the simplifying assumptions that result from the dielectric nonlinearity, the field distribution often can be computed analytically to good accuracy, which is highly desirable given the alternative of a nonlinear numerical computation. This article discusses three examples in which nonlinear conditions affect dielectric performance in a manner that is easily predictable if the fundamentals of nonlinear systems are understood.     

Dielectric breakdown characteristics of cryogenic nitrogen gasabove liquid nitrogen

We measured the DC dielectric breakdown characteristics of cryogenic nitrogen gas above a liquid nitrogen surface for needle-to-plane and sphere-to-plane electrode configurations. Experimental results revealed that the DC breakdown voltage of cryogenic nitrogen gas increased, as the distance from the gap axis to the liquid nitrogen surface diminished, i.e. as the liquid nitrogen surface got closer to the gap. The breakdown voltage proved to be enhanced not only by the temperature drop of nitrogen gas due to the existence of liquid nitrogen, but also by an effect of vapor mist arising from vaporization of liquid nitrogen. For the quasi-uniform electrode configuration, the relation between breakdown voltage and the gas density times the gap spacing agreed well with the Paschen curve for nitrogen with both the temperature falling and the vapor-mist density considered. For the nonuniform needle electrode, positive breakdown voltage was higher than negative one; the polarity effect was interpreted in terms of the electric field relaxation at the tip of the positive needle resulting from partial discharges observed only for the positive needle     

A 2-dimensional Kerr-effect technique for electric fielddistribution in liquid dielectrics

Based on the electro-optical Kerr effect, an attempt is made to develop a measurement technique for the two-dimensional distribution of the electrical field in dielectric liquids. For rapid, two-dimensional measurements, it is convenient to introduce a computer-controlled electronic camera for optical detection. However, because of the two-dimensional non-uniformity resulting from the system components, the measurement principle needs a two-dimensional analysis. To simplify the mathematical analysis, square pulsed modulations are employed. The influence of the system non-uniformity is eliminated by use of both optical and electrical modulations. To improve the system sensitivity, an optimum system setup is analyzed. To decrease the effect of the noise component resulting from the EHD (electrohydrodynamic) motion of the dielectric liquid, a diagnostic image lock-in technique is proposed. The validity of the two-dimensional measurement principle and the effectiveness of the diagnostic image lock-in processing are examined by the measurement of the electrical field distribution in a nitrobenzene specimen between two parallel plate electrodes     

Self-trapping of electrons in liquid nitrogen

A possibility of electron self trapping in nitrogen is discussed. It is found that electrons are free when injected in liquid N₂ near the triple point and localized inside bubbles and in the gaseous phase, far enough from the critical point. On the saturation line near the critical point a transition of electrons from bound to free states takes place     

Dielectric behavior of insulating materials under liquid nitrogen

Measurements were made to assess the AC breakdown voltages in liquid nitrogen (LN/sub 2/) with different electrode configurations such as sphere-sphere, needle-needle, hemisphere-hemisphere, plane-plane, sphere-needle, etc. Experimental results reveal that the breakdown voltage is a function of electrode geometry and gap length. This study also addresses the effect on the breakdown strength of solid insulating materials under LN/sub 2/ environment with a sphere-sphere electrode configuration. In this paper, special emphasis has been attributed to the effect on loss index of a variety of dielectrics dipped in LN/sub 2/. The measured values of breakdown strength and loss index have been compared with those obtained under atmospheric condition. The study reveals that the breakdown strength of cellulosic materials like paper or pressboard increases manifold while the loss index decreases significantly when dipped in LN/sub 2/ with variations of the order of 50% to 90%. However, for impervious noncellulosic materials like Perspex (acrylic glass) or presspahn, the increase in breakdown strength is not that pronounced and the decrease in loss index is of the order of 2 to 30%.     

Radiation protection properties of dielectrics with space charge

The main radiative danger for outside elements of orbiting satellites (for example, solar power elements) is caused by fast electrons. It is known that the presence of electric space charge in the radiation-protection cover can reduce significantly the total dose of radiation and thus increase the durability of such elements. In order to research this phenomenon, a mathematical interaction model between electron beams and dielectric materials was made for numerical investigation     

Prebreakdown phenomena at high voltage in liquid nitrogen and comparison with mineral oil

This paper presents a study of prebreakdown phenomena (usually called streamers) occurring at high voltage in liquid nitrogen. A special cryostat has been constructed to allow dielectric measurements in liquid nitrogen up to 200 kV, and 0.5 MPa hydrostatic pressure. Streamers are detected and studied with various techniques: high speed visualization, streak camera, and recording of transient currents and emitted light. The propagation of positive and negative streamers is investigated at gap distances up to 80 mm. Streamers are then compared to similar phenomena observed in mineral oil. In liquid nitrogen, streamers are different compared to mineral oil. A transition from slow to fast positive streamers (velocity about 10 km/s) is observed at a threshold voltage below the breakdown voltage. The hydrostatic pressure has almost no influence on this phenomenon. Some experiments carried out with a solid barrier perpendicular to the gap axis are also presented. The practical consequences for the design of HV insulation in superconducting systems are then discussed.     

Charge buildup in lossy dielectrics with induced inhomogeneities

In addition to their normal inhomogeneities, insulation may also become inhomogeneous under stress, for instance if their temperature is not uniform. Then, a charge density builds up wherever material properties such as conductivity and permittivity are not uniform. This is the case of HVDC cables, in which the core is heated by the Joule effect, while the sheath remains at near ambient temperature, so that an average temperature gradient of ⩽50 K/cm may occur in the insulation, under a field which can reach 100 kV/cm. If the conductivity of the insulation is assumed to depend only on temperature, with an activation energy of 1 eV, the steady charge density predicted by electrostatics may reach levels easily measurable by the techniques now available. This charge density is somehow lowered in non-ohmic insulation. An early analysis of the transient charge build-up is discussed here, and extended to the case of a non-ohmic insulation. This allows calculation of the transient current generated in the external circuit by the accumulating charge, in the cases of ohmic and non-ohmic insulation. Finally, other manifestations of the space charges in question are mentioned. Among these, unexpected results on electroconvection in slightly ionic liquids in the presence of a temperature gradient are analyzed in terms of these charges, and suggest practical applications     

Initial images of complete breakdown of liquid nitrogen/polymerfilm

This paper describes an experimental study,on the prebreakdown phenomena in a liquid-nitrogen/polymer-film composite subjected to dc ramp and step voltages under point-plane electrode system. In order to clarify the formative process of the complete breakdown of the composite, photographic observation was carried out using a long image guide scope and an image intensifier with high speed gate in the nanosecond region. When a negative-plane electrode was covered by a polymer film (N-cover arrangement), a luminous region on the initial image of the complete breakdown was located near the negative-plane under both the ramp and the step voltages. These results suggested that the complete breakdown of the composite in the N-cover arrangement was caused by the film breakdown due to positive charges deposition and subsequent electron emission from the film breakdown spot. When a positive-plane electrode was covered by the film (P-cover arrangement), film breakdown could not trigger the complete breakdown under the ramp voltage because the film breakdown occurred at a low applied voltage. However, intense light near the positive-plane was observed at the initial stage of the complete breakdown under the relatively high step voltage. This was because the film breakdown under the high applied voltage induced the positive streamer propagation from the film breakdown spot     

Thermal bubble breakdown in liquid nitrogen under nonuniform fields

The electrical forces acting on thermally induced bubbles in nonuniform field gaps with/without fins and the correlation between bubble behavior and dielectric characteristics in liquid nitrogen are studied. The results show that the gradient force and Maxwell stress strongly affect the bubble dynamics and bubble shape in the gap. Especially the pronounced gradient force near the fin tip reduces the growth of bubble there. These electrical forces acting on the bubble result in a lesser effect of thermal bubble on the breakdown voltage if the fins are formed to guide a bubble trapped in the groove from a higher field region to a lower one