Mechanisms of arc propagation over an ice surface /

Abstract

Outdoor insulation design with regard to pollution and icing performance is a critical factor for reliability of overhead transmission lines. The majority of failures occurring on transmission lines are caused by environmental conditions such as lightning, pollution and/or ice. Accumulated ice on insulators may initiate corona discharge; these partial arcs sometimes lead to flashover. This phenomenon is mainly caused by the presence of a highly conductive water film on the surface of the ice and ice-free zones, often called air gaps, along the insulators, and decreased insulator leakage distance caused by ice bridging.The general objective of this research is to investigate the physical mechanisms involved in the propagation of electric arc on the surface of ice. In the present research work, high-speed photography techniques was combined with synchronized electrical measurements to study the various parameters of arc on an ice surface, such as channel radius and propagation velocities. These results, together with studying the propagation patterns and arc foot geometries, provide background information for understanding the complete flashover phenomenon. Arc root structure, current profile, propagation pattern and velocity were investigated under positive and negative polarities. Water film conductivity and thickness, and relative air humidity were also analyzed as influencing parameters of arc propagation. The effects of different mechanisms, e.g. collision and thermal ionization, buoyancy force and electrostatic force, on different stages of arc propagation are also discussed.Repetitive patterns corresponding to arc decays were observed during arc propagation. During arc re-ignitions, an anode-directed streamer was found to be dominant, regardless of the polarity of applied voltage. The arc root under de-positive applied voltage was branched; on the contrary, the negative arc root appeared as a diffused luminous region. In the arc channel section, two distinct regions were distinguished. A linear relationship between the initiation current and the inverse of residual resistance was established, showing how important is the effect of water film thickness on the initiation of discharge. It was found there is a direct relationship between the slope of current increase and the applied water conductivity. Air humidity increases the voltage gradient along the arc channel and reduces the discharge activities at the arc tip. However, it could influence the arc formation and its propagation by changing the thickness of the water film. A change in the dominant mechanism from thermal ionization to collision ionization in the enhanced electric field was found to be responsible for the flashover. Tare. L'effet de différents mécanismes comme les collisions et l'ionisation thermique, la force de flottabilité et la force électrostatique sur les différentes étapes de la propagation de l'arc ont également été discutés. 11 RESUME

Dynamic modeling of AC multiple ARCS of EHV post station insulators covered with ice

“…When the white arc reaches its critical length [20], flashover occurs along the whole insulator [ 23]. In order to study the mechanism of arc propagation over an ice surface [24,25] and estimate the critical flashover voltage [26,27] different static and dynamic models were originally established under AC and DC voltages [20,[28][29][30][31]. Contrary to static models, which calculate the flashover voltage of ice-covered insulators, dynamic models can contribute to a better understanding of the discharge mechanisms involved in the time-dependent evolution of the flashover's main parameters.…”

Two-arc dynamic modeling of AC and DC flashovers of EHV post station insulators covered with ice based on laboratory experiments /

“…In order to study the mechanism of arc propagation over an ice surface [27], [28] and estimate the critical flashover voltage [18], [22], [29], [30], different static and dynamic models were originally established under AC and DC voltages [26], [31], [32], [33]. Contrary to static models which calculate the critical flashover voltage of ice-covered insulators, dynamic models can contribute to a better understanding of the discharge mechanisms involved in the time dependent evolution of the main parameters of the flashover.…”