Development of positive streamers along an ice surface: A physical approach.

摘要

On power networks a discharge may end up generally as a failure in the insulation system. Outdoor insulators are the most vulnerable elements of such a system since they are very sensitive to environmental factors. Even if they are designed to operate under the most stringent climatic conditions, their performance may be drastically reduced by ice accumulation on their surfaces. Indeed, under these conditions, corona discharge and partial arcs are initiated at a voltage lower than in absence of ice and can develop to complete flashover in a shorter period. The relative vulnerability of insulators under icing conditions underscores the need of furthering our basic knowledge of ice surface discharge, leading to a better understanding of the physical mechanisms that initiate ice-covered insulator flashover. For this purpose the Industrial Chair NSERC/Hydro-Quebec/UQAC on Atmospheric Icing of Power Networks Equipments (CIGELE) has elaborated a wide research program based on the observation and study of visible discharges propagating along an ice surface.;The investigations undertaken in this project are in the continuum of works already started at CIGELE. They mainly aim at exploring more the characteristics of corona streamers propagating along an ice surface and interpreting the fundamental mechanisms involved in their development. The understanding of these physical processes is the ultimate phase in the elaboration of numerical tools to predict and prevent flashover on ice-covered insulators. This research may be beneficial for the design of new insulators better adapted to the cold climate regions, to improve the reliability of the power transmission lines under atmospheric icing conditions.;Sophisticated techniques of detection and photography including a photomultiplier and an ultra high-speed framing camera have been applied in order to observe the corona streamer development and to determine their characteristics. Since the shape of outdoor insulators is very complex, a simplified physical model of rod-plane configuration has been used, with various gap distances and high-voltage rod radii. The investigations were limited to short gaps, where the corona consists of streamers only, with no tendency to form a leader. In order to analyze the effects of ice purity, various values of freezing water conductivity were used. The variation of the surrounding temperature has made it possible to study the effect of the presence of a quasi-liquid layer on the ice surface. Parameters such as voltage and electric field for streamers inception; the time to breakdown and the streamer propagation velocity were measured. Streamer currents and charge deposited in the gap were also measured to study the effect of surface charge on streamer development. In addition to ice surface discharge, corona discharges were also studied in air alone, in order to provide a baseline for comparison, henceforth referred to as the reference case.;The obtained results expectedly showed that corona streamer characteristics are considerably affected by the presence of ice surface. Investigations have revealed that an increase in freezing water conductivity for a given temperature or in temperature for a given conductivity reduces the corona inception field and enhances the streamer propagation velocity. The existence of a quasi-liquid layer on the ice surface especially at temperatures above -6°C has a severe influence on the development of the streamers. However, for slightly contaminated ice, the characteristics of the streamers were comparable with those in air. In that case, it was found that the discharge could partially or entirely propagate through the free air far from the ice surface. In addition, high speed camera recordings showed that the corona streamers in the presence of an ice surface had smaller extension and were less branched compared to those in air alone. In the presence of ice, discharge propagation could also be initiated midway on the surface, far from the HV rod electrode leading to flashover with a second streamer independently from the main one where propagation was initiated. Charge deposited on the surface prior to streamer propagation explained the existence of such phenomena. By evaluating its influence on the electric field distribution, it was established that this charge deposition contributes more to the enhancement of the streamer propagation velocity than the ice permittivity. The analysis of the physical mechanisms involved in ice surface discharge development made it possible to show that the electron emission due to surface bombardment and intensity of tangential field in the head of the streamer is more effective than the photoionization in air. This conclusion could largely explain the streamer propagation velocities obtained and the observed physical aspects of corona in the presence of an ice surface.