Research on the Calculation Method of Arc Sag in Overhead Transmission Lines under Wildfire Conditions

When wildfires occur under overhead power lines, the high temperatures caused by the fire increase the sag of the lines, decreasing the safe distance between the lines and the ground, and increasing the likelihood of power line trips due to wildfires. Existing methods for calculating line sag under wildfire conditions consider too few factors, making it difficult to accurately assess the degree of line sag under wildfire conditions. Based on the theory of fire burning zone division, this paper establishes a multi-physical field heat transfer model for wildfires, calculates and studies the effects of wildfire burning area, burning temperature, height above the ground, and horizontal distance from the center of wildfire burning on the spatial temperature distribution, and constructs a wildfire temperature distribution function model. Finally, combining the wildfire temperature distribution function with the principles of mass conservation and isothermal expansion, a method for calculating line sag is proposed. Taking a span of 200 meters as an example, the influence of the line's distance from the center of wildfire burning, height above the ground, temperature at the base of the fire, and burning area on the maximum sag of the line is analyzed. The results show that when wildfires occur directly under transmission lines and burn intensely, the higher the burning temperature and the larger the burning area, the lower the line height, the corresponding high temperatures will significantly increase the maximum sag of the line, reducing the ground clearance of the line. Even at distances more than 100 meters from the center of the wildfire, the high temperatures can also slightly increase the line sag.