In order to provide support for the differentiated prevention and control of air gap breakdown risks in DC overhead transmission lines under wildfire conditions

this paper establishes a conductor-plane long air gap breakdown test platform for transmission lines under crown fire and crib fire conditions

and compares the effects of tree species

fire types

gap distance and other factors on the DC breakdown characteristics of conductor-plane air gap. The results show that the living state crown fire leads to a significant decrease in the insulation strength of the air gap. At the maximum burning phase

the average breakdown voltage gradient of pine crown fire and cypress crown fire at a 5 m air gap are 84.7 kV/m and 43.2 kV/m

respectively

which is 31.6% and 16.1% of the average breakdown voltage gradient of a pure air gap at the same altitude. The influence of tree species difference on the breakdown characteristics is mainly reflected in the combustion parameters in the flame zone and the combustion particle size in the non-flame zone. Regardless of gap length

the average proportion of crown fire flame bridging is close

and the average breakdown voltage gradient is also close. However

for wood crib fires

the flame bridging proportion under a 9 m gap is lower than that under a 5 m gap

while the insulation strength of the former is also lower. The reason is that the temperature rise and smoke particle concentration of non-flame section under the 9 m gap are higher than those of the 5 m gap. The influence of the living state crown fire on gap insulation strength is greater than that of crib fire. The influence of fire type difference on breakdown characteristics is mainly dominated by smoke zone. With the increase of the length of air gap

the difference of air gap DC breakdown characteristics between crown fire and crib fire decreases.