To support risk assessment of phase-to-ground gap short-circuit breakdowns in overhead transmission lines under wildfire conditions

this paper establishes a test platform for long air gap breakdown under simulated wildfires. The platform is used to investigate AC breakdown characteristics across 1.2~9 m gaps

evaluating seven parameters: vegetation type

density

and humidity; smoke concentration; bundle sub-conductor count; gap distance; and altitude. The results show that using fir as a reference

the correction coefficients for the average breakdown voltage gradients of pine

eucalyptus

shrub

and thatch are 1.13

0.96

0.93

and 0.88

respectively; the average breakdown voltage gradient in the flame zone is significantly affected by vegetation humidity

showing a strong positive correlation with a linear correlation coefficient of 0.9998. In the smoke zone

the effect of temperature on gap breakdown is greater than the effect of smoke particles

and the degree of reduction in the level of gap insulation by pure smoke can be taken as 16% when the temperature effect is avoided as much as possible. The average breakdown voltage gradient in the non-flame zone and the length of the non-flame zone have a linear relationship in the 4~9 m gap test at 70% flame bridging ratio. Vegetation fire gap breakdown voltage changes significantly less with altitude than that of pure air gaps

and the influence of altitude on gap breakdown characteristics primarily manifests through two factors: smoke zone properties and vegetation fire combustion behavior.