Triple point surface discharge photography in atmospheric gases using Intensified high-speed camera system

In this paper investigations of surface discharges using an ultraviolet (UV) intensified high-speed camera system are presented, accompanied with high frequency response and resolution current recordings. A needle-plane electrode configuration is employed for the generation of a strongly non-uniform electric field on the surface of disk-shaped insulator samples made of polytetrafluoroethylene (PTFE) or epoxy resin. The electrode arrangement is further insulated by a gaseous medium of either technical air (21% O2/79% N2), nitrogen (N2) or carbon dioxide (CO2). An alternating (AC) voltage waveform at power frequency (50Hz) is maintained at levels sufficiently below the flashover voltage corresponding to each presented case. Detailed descriptions of the technical specifications of the utilised equipment are provided for both optical and electrical measurements in the experimental set-up. The obtained results demonstrate the discharge propagation during the AC-cycle and its dependence on the insulator type and gaseous insulating medium. Individual surface discharges are captured in the microsecond range to describe the discharge morphology based on the generated current pulse and instantaneous applied voltage level. Back-discharges on the insulating disc are also discussed, and a relevant image capture is presente

Electrical Network-Based Time-Dependent Model of Electrical Breakdown in Water

A time-dependent, two-dimensional, percolative approach to model dielectric breakdown based on a network of parallel resistor–capacitor elements having random values, has been developed. The breakdown criteria rely on a threshold electric field and on energy dissipation exceeding the heat of vaporization. By carrying out this time-dependent analysis, the development and propagation of streamers and prebreakdown dynamical evolution have been obtained directly. These model simulations also provide the streamer shape, characteristics such as streamer velocity, the prebreakdown delay time, time-dependent current, and relationship between breakdown times, and applied electric fields for a given geometry. The results agree well with experimental data and reports in literature. The time to breakdown (tbr) for a 100 μm water gap has been shown to be strong function of the applied bias, with a 15–185 ns range. It is also shown that the current is fashioned not only by dynamic changes in local resistance, but that capacitive modifications arising from vaporization and streamer development also affect the transient behavior

Detecteur optique de phase liquide dans un disjoncteur au SF6_6 sous pression

In this paper, we present a device allowing us to detect gas-liquid or liquid-gas transition in an pressurized cell. This phase detector can be implemented in high voltage system.Dans cet article, on présente un dispositif permettant de détecter localement la transition liquide-gaz ou gaz-liquide dans une enceinte sous pression. Ce détecteur de phase peut etre installé dans des systèmes portés à potentiel élevé

Dielectric properties of periodic heterostructures: A computational electrostatics approach

The dielectric properties of heterogeneous materials for various condensed-matter systems are important for several technologies, e.g. impregnated polymers for high-density capacitors, polymer carbon black mixtures for automotive tires and current limiters in circuit protection. These multiscale systems lead to challenging problems of connecting microstructural features (shape, spatial arrangement and size distribution of inclusions) to macroscopic materials response (permittivity, conductivity). In this paper, we briefly discuss an ab initio computational electrostatics approach, based either on the use of the field calculation package FLUX3D (or FLUX2D) and a conventional finite elements method, or the use of the field calculation package PHI3D and the resolution of boundary integral equations, for calculating the effective permittivity of two-component dielectric heterostructures. Numerical results concerning inclusions of permittivity ε1 with various geometrical shapes periodically arranged in a host matrix of permittivity ε2 are provided. Next we discuss these results in terms of phenomenological mixing laws, analytical theory and connectedness. During the pursuit of these activities, several interesting phenomena were discovered that will stimulate further investigation

Propagation et génération des streamers dans les diélectriques liquides

This study concerns the initiation and propagation of streamers in point-plane geometry. Our experimental set-up has permitted us to show the existence of a close correlation between the shape of the streamer, its propagation velocity, the current shape and the light it emits, whatever are the polarity of the point and the liquid nature. The more filamentary the streamer, the higher its velocity. The propagation velocity has a minimum for a large variety of insulating liquids wheter pure or containing different additives. The amplitude of the velocity depends strongly on the electronic properties of the liquid and additives. The effects of hydrostatic pressure on the streamer appearance for both polarities suggest the existence of a gas phase. It appears that electrohydrodynamically induced cavitation phenomena, resulting from unipolar charge injection, are not responsible for the gas phase.Cette étude concerne la génération et la propagation des streamers en géométrie pointe-plan sous créneau de tension. Un nouveau dispositif expérimental a permis de montrer qu'il existe une relation étroite entre la forme du streamer, sa vitesse de propagation, la forme du courant et la lumière qu'il émet qu'elles que soient la polarité de la pointe et la nature du liquide. Plus le streamer est filamentaire, plus il est rapide et réciproquement. La distribution de vitesse admet un minimum pour un très grand nombre de liquides isolants pur ou renfermant divers additifs. L'amplitude des vitesses dépend d'une manière importante des propriétés électroniques du liquide et des additifs. L'effet de la pression hydrostatique sur l'apparition des streamers dans les deux polarités, indique l'existence d'une phase gazeuse. Il semble que le mécanisme de cavitation induite par des mouvements électrohydrodynamiques résultant d'une injection unipolaire de charges ne soit pas à l'origine de cette phase gazeuse

Modelling of dielectric strength in long air gaps: application to a complex geometry

International audienceThis paper presents new contributions to the study of the positive discharge in large air gaps. A model enabling us to determine the voltage U50 and the k-factor in non-standard geometry using the circuit model developed by Beroual's group for long positive and negative discharges is presented. This model is based on an equivalent circuit diagram, the parameters of which vary with time according to the leader channel characteristics and the geometry/morphology of the discharge. The propagation of the leader is based on a criterion related to the calculation of the electric field at its head and that takes into account the randomness of the discharge path. As with most of models found in the literature, this model applies only to the point-plane type electrodes gap. This paper consists of two parts: (1) the first part, presents how to extend the proposed model to complex geometries such as rod–rod electrode gaps; and (2) the second part presents the experimental tests we achieved for rod-plane and rod–rod geometries. The experimental results especially the values of U50 and k-factor are compared with the simulated ones deduced from our model, for two types of voltage waveforms: the lightning impulse shape and the switching impulse shape (precisely, the critical impulse shape)

Influence of thin layers on static electrification phenomena of materials for power transformers

The static electrification phenomenon is investigated, using a test cell with a rotating metallic disk coated on both sides with thin layers (0,5 and 2 pm) of conducting, semiconducting and insulating materials, obtained by magnetron sputtering. The current characterising the static electrification is studied as a function of the nature of the coating material and the oil, the temperature and the rotating velocity of the disk. Modification of the surface properties has a decisive influence on the static electrification of insulating oils, the sign and the amplitude of the currents are strongly affected