Numerical Simulation Of Lightning Attachment To UHV Transmission Lines

Nowadays, more and more electricity supply is needed to meet the demand of the rapid development of society and economy, and a large number of UHV transmission projects are put into the electric power construction field in China. The transmission line fault caused by the lightning strike is a huge threat to the safety and stability of the power grid, therefore, the assessment of lightning shielding performance is attached great importance in designing the UHV transmission lines. The large scale and high operating voltage of the UHV transmission lines increase the probability of being struck by lightning. There are several models which can be used to analyze the lightning shielding performance of transmission lines. However, the reliability of assessment results for UHV transmission lines by these existing models is difficult to be guaranteed. Hence, how to reasonably describe the physical process of lightning attachment to the transmission line, and then improve the validity of the shielding performance assessment becomes a significant problem urgent to be studied. This paper is focused on the physical mechanism of lightning interaction with the transmission line, and resolves some key issues of lightning attachment process via theoretical analysis, experimental research and numerical simulation, which provides the theoretical basis and more effective analysis method for the study of lightning shielding performance of the UHV transmission lines.As lightning discharge usually presents significant branched or tortuous channels, the charge distribution along the branched channels and the stochastic feature of downward leader propagation were investigated in this paper. The charge density along the leader channel and the charge in the leader tip for each lightning branch were approximated by introducing branch correlation coefficients. In combination with geometric characteristics of natural lightning discharge, a stochastic downward leader propagation model was presented based on the fractal theory. The fractal dimension and the variation of electric field on the ground were applied to evaluate the validity of this proposed model. Results of the proposed model agree well with the observations and measurements of natural lightning discharge, which indicates the validity of the model. Moreover, the lightning striking distance to flat ground was analyzed.Space charge effect in the long air gap discharge was studied through the experimental investigation. Based on the leader discharge theory, the potential distortion around the conductor caused by the space charge was related with the leader advancement process, and then a numerical simulation model for analyzing the inception of stable upward connecting leader issued from the conductor was presented, in which some key parameters of the leader discharge were set according to the observations and measurements. By analyzing the electric field distribution around the conductor during the upward leader inception process, a new inception criterion for the upward connecting leader—average background electric field criterion was proposed. The observations of long air gap discharge for the horizontal conductors under laboratory conditions were used to verify the proposed criterion. Also, comparative analysis indicates that the existing criteria may somewhat overestimate the upward leader inception from transmission lines.Combining with the observations of the triggered lightning experiments, the expression for the propagation speed of upward connecting leader was deduced by analyzing the temporal and spatial distribution characteristics of the electric field around the conductor during the upward leader propagation process, and it was verified by the observation data of natural lightning discharge. The simulation model for the dynamic propagation of the upward connecting leader was built, and then the interaction of multiple upward connecting leaders and its impact on the inception and propagation of upward leaders were analyzed. Taking the typical UHVDC transmission line as an example, the influence of the interaction of multiple upward leaders, operating voltage and protective angle on the competition of upward connecting leaders was analyzed.A physical model for the simulation of lightning attachment to the transmission lines was established based on the research results in the aspects of lightning downward leader model, the inception and propagation characteristic of upward connecting leaders and so on. The proposed model was used to reproduce the shielding failure performance of typical EHV/UHV transmission lines. The model-based calculation results agree well with the field data, which show the effectiveness and validity of the simulation model. Moreover, the simulation model was used to evaluate the lightning shielding failure performance of AC/DC hybrid transmission lines on the same tower under different operating conditions, and also the possible improvement measures of the lightning protection design was put forward.The work of this paper involves many common issues about lightning interaction with transmission lines, and its related theoretical achievements and technological innovations enrich the basic theory and analysis method of lightning shielding design of transmission lines, which is of great significance to guarantee the safe and reliable operation of UHV power transmission projects in China.