Thermal radiation effect on the extinction properties of electric arcs in HV circuit breakers

During the formation of the electric arc at the opening of a high voltage circuit breaker, the generated plasma will be the seat of a very important thermal exchange. Models founded only on conduction and convection thermal transfers don't reproduce the whole thermal exchanges that are governing the extinction process. This paper is devoted to the development of a model of the electric arc extinction in a high voltage circuit breaker taking in account the thermal radiation of the plasma, in addition to the conduction and convection phenomena. The Stefan-Boltzman equation is coupled with the heat equation, and both equations are solved simultaneously in order to follow the evolution of the arc voltage and the conductance of the thermal plasma. The obtained results are found in good agreement with experimental recordings

Comparison of dielectric properties of olive oil, mineral oil, and other natural and synthetic ester liquids under AC and lightning impulse stresses

International audienceThis paper presents experimental results of a comparative study of the characteristics of streamers propagating in olive oil and breakdown voltage with mineral oil, rapeseed oil (natural ester) and tetra-ester and methyl oleate (synthetic ester liquids). The comparison focuses more particularly on the pattern, stopping length, associated current and electrical charge of streamers propagating in a point-plane electrodes geometry under (1.2/50 μs) lightning impulse (LI) voltage as well as breakdown voltages under AC and LI voltages. A statistical analysis of breakdown voltages is also achieved. It is shown that the streamer characteristics (streamers shape, stopping length, associated current and electrical charge) of olive oil in small gaps are comparable to those of mineral oil. Breakdown voltages under AC and LI voltages are found to be higher in olive oil than in mineral oil. Consequently, olive oil can constitute a potential substitute for mineral oil in in high voltage power transformers

Dissolved gases analysis in relation to the energy of electrical discharges in mineral oil

International audienceThis paper deals with the characterization of discharges initiated in mineral oil and the gases that they generate depending on their energy levels. Different types of discharges are identified in function of the electrode geometry and the applied voltage. Each type of discharge is analysed through the generating gases and the processes implicated in oil degradation are discussed according to different standard specifications. By estimating the forms of energy that can be involved in the gas formation, it was evidenced that vaporisation processes are as important for discharges of weak energy as for those of high energy

Creeping discharges propagating on natural ester oils/pressboard interface under AC and lightning impulse voltages

International audienceThis paper presents the results of experimental characterization of creeping discharges propagating over pressboard immersed in insulating oils, under AC and standard lightning impulse voltages. For comparison purposes, synthetic ester, rape seeds oil (natural ester) and mineral oil were also investigated. Two experimental arrangements were used: (1) a point-to-plane electrodes system where the pressboard is inserted between the electrodes such as the point is perpendicular to pressboard allowing a radial propagation of discharges; and (2) a point-to-bar electrodes system allowing a propagation of discharges in one direction, tangentially to pressboard. The characteristics of discharges propagation, their shape, stopping length (the maximum extension of creeping discharge) as well as the associated current and electrical charge versus the thickness of pressboard and the type of oil are analyzed. It is shown that the magnitude and polarity of voltage, the type of oil as well as the thickness of pressboard influence the characteristics of discharges and especially the stopping length and density of branches. The stopping length of creeping discharges is shorter for pressboard/naturel esters' interfaces than with pressboard/mineral oil interface

Comparison of creeping discharges propagating over pressboard immersed in olive oil, mineral oil and other natural and synthetic ester liquids under DC voltage

International audienceThis paper presents an experimental study on the characteristics of creeping discharges propagating over pressboard immersed in olive oil, rapeseed oil, tetra-ester, methyl oleate, and mineral oil, under both negative and positive DC voltages. The investigated characteristics are mainly the shape of discharges and their stopping lengths. Two experimental test cells are used: in the first one, the pressboard is inserted between pointe and plane electrodes so that the tip is perpendicular and at the center of the pressboard sample; and in the second one, the pressboard is placed between a point and bar electrodes in a way that allow a propagation in one direction (tangentially to pressboard). It is observed that the creeping discharges under DC voltage are not radial. Their shapes are different from those observed in previous work under AC and lightning impulse voltages. On the other hand, for given voltage and pressboard thickness, the stopping length L f is shorter when the point is negative than when it is positive indicating that the flashover voltage will be greater with a negative point This also evidences a difference in the processes involved in each polarity especially the space charge resulting of the injected charges and the charge accumulated at the interface (including the charges resulting of double layer) as well as the evolution of the electric field in the vicinity of the point electrode and along the pressboard surface. For a given pressboard thickness, L f increases with the voltage; it decreases when the thickness of pressboard is reduced

Correlation between current, emitted light, electric field and propagation velocity of positive streamers in liquid dielectrics under AC voltage

ISSN : 2153-3725 E-ISBN : 978-1-4244-7354-0 Print ISBN: 978-1-4244-7352-6 INSPEC Accession Number: 12221092International audienceThe paper reports on the search of correlations between the current and emitted light signal, electric field, and propagation velocity associated to positive streamers propagating in mineral oil, tetra-ester and toluene, in a point - plane electrode arrangement, under AC voltage. New correlations, especially between the velocity of positive streamers and the current derivative are established. The frequency spectra of different current pulses and emitted lights are also presented

Comparison of breakdown voltage of vegetable olive with mineral oil, natural and synthetic ester liquids under DC voltage

International audienceThis paper deals with a comparative study of the negative and positive DC breakdown voltages of different insulating liquids namely two natural esters (an extra virgin olive oil (OO) without additive and rapeseed oil), two synthetic ester fluids (a methyleolate obtained by transesterification of olive oil and tetraester), and a naphthenic mineral oil (MO); an oils mixture namely 80% OO + 20% MO is also investigated. Statistical analysis of the experimental data is also performed. The obtained results are also compared with those obtained in our previous work under AC and lightning impulse voltages. It is observed that the negative and positive DC breakdown voltages of natural esters are higher than those of mineral oil. On the other hand, the DC breakdown voltage of (80% OO + 20 % MO) mixture shows comparable behavior to that of mineral oil. The DC negative breakdown voltages are higher than the positive ones in the tested liquids. It is also shown that the DC breakdown voltage values of the investigated liquids generally obey the normal distribution

Electric field assessment for application to live working

This paper presents a numerical survey on electric field distribution in electrode arrangements commonly met while executing a live working on overhead lines. This numerical investigation is achieved using a software package based on charge simulating method. Particularly, the influence of the curvature of the bodies under stress on the maximal value of the electric field and the predominance between the axial and the radial components of the electric field are shown. This knowledge is of a great importance for accurately dimensioning the safety minimum approach distances (MAD) for future works to be performed with nonnormalised tools. First, a rod–plane–wall configuration is addressed and satisfactory results are obtained. This allows bringing up into general use of charge simulating program (CSP) to determine the safe MAD for complex configurations. A method that consists in use of the numerical techniques to adapt the empirical methods for the evaluation of the MAD, when tools of nonconventional dimensions are used, is then developed