Processing of upward leader on the wind turbine blade and critical length inception criterion

As a key physical process of the research about lightning protection of wind turbine blades, the inception mechanism and criterion of the upward leader need further investigation. A model of line charge with point charge at the head is used to simulate charge distribution of lightning downward leader. A curve about density of surface charges on blade is presented after considering the movement, diffusion and neutralization of the space charges nearby blade. Under the influence of space charge in both lightning downward leader channel and that on surface of blade, the potential nearby the blade is distorted obviously. Based on a simplified model of the development of upward leader, the paper proposes a critical length inception criterion for upward leader. The kernel of criterion is `there exists a certain zone above the tip of blade where the background potential is larger than that in streamer, and if the length of this zone is longer than a background potential critical length, a stable upward leader will incept'. Lightning parameters and blade parameters such as amplitude of lightning current, lateral distance, length of blade and radium of receptor have no effect on the critical length. The critical length decreases with the increase of the relative density and humidity of air, and it increases with the increase of altitude. The feasibility of the proposed criterion is verified by the observation results of long air gap discharge for the vertical conductors under laboratory conditions.