Change of air gap discharge characteristics caused by factors such as air pressure and electric field is a problem that must be considered in the external insulation of power transmission and transformation equipment. Studying the development characteristics of sphere-plane gap discharge channels at different altitudes can provide a basis for the external insulation design of power transmission and transformation equipment in extreme environments. In this paper

a sphere-plane gap switching impulse test with a diameter of 30~110 cm was carried out in Wuhan (altitude 20 m) and Kunming (altitude 2 100 m)

and the relationship between the leader development speed and development characteristics of the discharge channel and the size and pressure of the spherical electrode was studied. The results show that the leader velocity of the discharge channel increases and becomes saturated with the increase of the sphere diameter

and decreases with the increase of the altitude. When the altitude increases from 20 m to 2100 m

the leader velocity of the spherical electrode with different diameters decreases by about 12.4%~15.4%. The streamer length of the leader head of the discharge channel increases with the increase of sphere diameter and altitude. As for the morphology and development characteristics of the leader channel

with the increase of the diameter of the spherical electrode

the deflection position of the leader channel is gradually closer to the bottom of the electrode. The deflection angle of the leader channel near the bottom of the electrode decreases with the increase of altitude

and the bending frequency of the leader channel decreases obviously with the increase of altitude. The number of leader branches increases with the increase of spherical electrode diameter and decreases with the increase of altitude. The number of leader branches at high altitude is decreased by 26.7%~47.2% compared with that at low altitude. Finally

based on the fractal model analysis of discharge channel

the increase of spherical electrode diameter makes the spatial electric field more uniform and the fractal dimension of discharge channel increases

while the decrease of gas pressure leads to the decrease of gas density and the fractal dimension of discharge channel.