A. Beroual

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.

Effective dielectric constant of periodic composite materials

We presentcomputer simulation data for the effective permittivity ~in the quasistatic limit ! of a system composed of discrete inhomogeneities of permittivity e1, embedded in a three-dimensional homogeneous matrix of permittivity e2. The primary purpose of this paper is to study the related issue of the effect of the geometric shape of the components on the dielectric properties of the medium. The secondary purpose is to analyse how the spatial arrangement in these two-phase materials affects the effective permittivity. The structures considered are periodic lattices of inhomogeneities. The numerical method proceeds by an algorithm based upon the resolution of boundary integral equations. Finally, we compare the prediction of our numerical simulation with the effective medium approach and with results of previous analytical works and numerical experiments.

Dielectric and microstructure properties of polymer carbon black composites

Dielectric and physicochemical properties of a composite material prepared by incorporating carbon black particles into a polymer matrix were investigated. Two types of carbon blacks, having very different structures of aggregates, were used. The volume fraction of the carbon blacks ranged from 0.2% to 7%, i.e. below and above the percolation threshold concentration observed from the measurements of dc conductivity. The composite samples were characterized in terms of: swelling by a compatible solvent, electron paramagnetic resonance (EPR) response, and frequency variation of permittivity. First, the article attempts to evaluate the diffusion coefficient of an appropriate solvent in these materials. Sorption kinetics experiments with toluene indicate that the initial uptake of solvent exhibits a square root dependence in time as a consequence of Fick’s law and permit to evaluate the effective diffusion coefficient in the range 10−11–10−12 m2 s−1 depending on the volume fraction of the carbon black in the ...

Electronic and gaseous processes in the prebreakdown phenomena of dielectric liquids

The article shows experimentally the simultaneous influence of electronic scavenger additives and the hydrostatic pressure on the current and emitted light by streamers, especially in cyclohexane under negative polarity. This suggests that both electronic and gaseous mechanisms do not act singly in the prebreakdown phase of dielectric liquids. A relationship between the streamer current, the electrical charge, the velocity, and the propagation mode of streamers is discussed. Based on energy considerations, we establish a mathematical expression allowing us to evaluate the velocity of streamers and to explain the propagation modes. The streamer the current of which consists of discrete pulses moves by steps. The higher the number and/or the amplitude of the current pulses, the shorter the durations between these steps and the higher the average velocity of the streamers; its propagation tends to become continuous. That is the case of the slow streamers in the presence of electronic scavenger additives. On the other hand, the streamer becomes rapid then more energetic and the hydrostatic pressure necessary for its disappearance increases. That is why it is thought that both gaseous and electronic processes simultaneously act in the prebreakdown phase even if one of them is dominant in one case and the second in another.

Experimental investigations on insulating liquids for power transformers: Mineral, ester, and silicone oils

In this paper the dielectric strength, electrostatic charging tendency (ECT), ageing stability, gassing tendency, heat transfer efficiency and stray gassing issue of mineral and silicone oils, synthetic and natural esters are analyzed and discussed.

How do shape anisotropy and spatial orientation of the constituents affect the permittivity of dielectric heterostructures

The study of dielectric heterostructures has been advancing at a rapid pace. Much of the interest in these materials stems from the fact that their physical properties can be systematically tuned by variation of the size and shape of the constituents. Here we report on extensive computer simulations of the effective permittivity of dielectric periodic (deterministic) heterostructures, having monosized hard core inclusions of anisotropic shape (rod, ellipsoid) embedded in an otherwise homogeneous and isotropic matrix. The real and imaginary parts of the permittivity, in the quasistatic limit, are rigorously evaluated with the use of the PHI3D field calculation package and the resolution of boundary integral equations. In this article, we show that the effective permittivity has critical properties near a conduction threshold. The conduction threshold concentration can be significantly modified by the size, shape, and spatial arrangement of the constituents. More specifically, it obeys a square law dependen...

Complex effective permittivity of a lossy composite material

In recent work, boundary integral equations and finite elements were used to study the (real) effective permittivity for two‐component dense composite materials in the quasistatic limit. In the present work, this approach is extended to investigate in detail the role of losses. We consider the special but important case of the axisymmetric configuration consisting of infinite circular cylinders (assumed to be parallel to the z axis and of permittivity e1) organized into a crystalline arrangement (simple square lattice) within a homogeneous background medium of permittivity e2=1. The intersections of the cylinders with the x–y plane form a periodic two‐dimensional structure. We carried out simulations taking e1=3−0.03i or e1=30−0.3i and e2=1. The concentration dependence of the loss tangent of the composite material can be fitted very well, at low and intermediate concentrations of inhomogeneities, with a power law. In the case at hand, it is found that the exponent parameter depends significantly on the r...

A new proposal model for flashover of polluted insulators

In this paper, the principal pollution flashover models are reviewed and then an impedance criterion for arc propagation is proposed. This criterion differentiates the case in which the arc elongates until total flashover occurs from the case in which the arc stops before it reaches the end of the insulator. It is shown that the Hampton criterion is not a sufficient condition for initiation of the arc. Indeed, the latter can expand, under certain conditions, even if the Hampton criterion is not satisfied. An analytical model allowing one to determine the critical voltage that insulators can withstand is also established. This is based on an energetic balance, an equivalent electrical circuit and the physical characteristics of the arc. The critical voltage, the critical current and the critical arc length for polluted insulators are calculated using the elaborated model. The results so obtained are found to be in accordance with the experimental ones represented by known empirical relations.

A predictive model of the positive discharge in long air gaps under pure and oscillating impulse shapes

This paper presents a general model allowing the prediction of the behaviour of given gap submitted to a given voltage. The only input data are the electrode geometry, the applied voltage waveform and the atmospheric pressure and temperature. The model also allows for various conditions for arrest or instability of the discharge. By assuming the discharge channel to be a long conductor and using a typical single LCR conductor line (L, C and R being respectively the line inductance, capacitance and resistance), we derive a predictive model for a positive discharge in long air gaps. This allows one to describe the evolution of the entire discharge, with the parameters L, C and R varying with time according to the channel characteristics and discharge geometry. The model allows us to determine the time histories of the current both during the leader and during the return stroke, the charge, the potential gradient in the leader channel, the power and energy injected into the gap and the channels thermal radius. It also permits the simulation of an image converter working in streak or frame mode and the leader propagation velocities. Furthermore, it allows the trajectory of the discharge, which is obtained from a probability distribution, to be plotted in real time. Good agreement between computed and experimental results was obtained for various test configurations.

Dielectric spectroscopy techniques as quality control tool: a feasibility study

In this article, the feasibility of using time- and frequency-domain dielectric spectroscopy measurements to monitor the condition of oil-impregnated paper (OIP) condenser bushings is discussed.