Numerical simulations of long spark gaps – lightning attachment and its application

Sammanfattning: The Licenciate Thesis presented here is a research work on numerical simulations of two different electrical phenomena: Long gap discharges under switching impulses and the lightning attachment process of positive upward leaders. The development of a positive upward leader and the process of progress of a discharge in long gaps are composed of two intertwined physical phenomena, namely the leader channel and the corona zone. The physical description and the proposed calculations of above mentioned phenomena are based on experimental tests on long gaps. In the methodology proposed here, a geometrical approximation for the representation of the corona zone is used. Furthermore, two different approaches are applied and compared to represent the leader channel. The used methodologies for the computation of the leader channel are an engineering approximation and a physics equation that takes into account the thermoequilibrium process. In order to introduce a more realistic behavior of the discharge, statistical delays for the inception and for the tortuous characteristic of the channel were brought in. A comparison between a model with or without tortuous channel was implemented. A very good agreement was found between the physical model and the test laboratory results. In addition, based on previous works related to the physics of lightning and the lightning attachment process, a new methodology is developed and tested here. The new approach refines previous calculations; the background electric field and the ionized region considered for the advance of the leader segment are computed within an alternative approach. The proposed methodology was employed to test two engineering methods that are accepted international standards, the mesh method and the electro – geometrical method. The results demonstrated that the engineering approximations are consistent with the physical approach. Besides the electrical phenomena mentioned above, one should keep in mind that there are real effects of the lightning attachment process that are not included or are avoided to simplify the calculation. In fact, when a structure is subjected to a strong electric field, it is possible to generate multiple upward leaders from the structure. This effect has not been taken into account in the different numerical models available until now. The published models consider every upward leader as an individual case. And therefore; a first approximation to the process of generation of multiple upward leaders incepted over a structure is presented here. The preliminary results have shown that it is possible to observe an influence on the background electric field when one leader develops simultaneously with other leaders.

  Denna avhandling är EVENTUELLT nedladdningsbar som PDF. Kolla denna länk för att se om den går att ladda ner.