In order to understand the breakdown process of air clearance in series

an ‘asymmetric combined clearance’ was designed by taking the ball-sphere clearance as the research object. In other words

under the condition of constant total clearance distance

the distances of two clearance were adjusted separately to make the distances of upper and lower clearance inconsistent. In this paper

the extended Volume-Time theory is used to calculate the U50% of the tandem double clearance and explain the physical breakdown process of the asymmetric combined air clearance under lightning impulse voltage. The experimental results show that the effect of the uneven distribution of the upper and lower clearance distance on the breakdown voltage and the breakdown delay of the series air clearance is studied. When the degree of unevenness is large

the phenomenon of single gap discharge and overall non-breakdown occurs at the lower voltage

and the non-simultaneous breakdown occurs at the higher voltage. When the unevenness is small

the two gaps break down at the same time. In order to explain this phenomenon

the extended Volume-Time theory is adopted in this paper to calculate the breakdown voltage of series double clearance by determining key parameters such as critical volume and critical area. The effective volume and area of the electric field in the air gap exceed the critical value. Therefore

double-gap anchronous breakdown occurs when one gap meets the initial breakdown condition

while the other gap does not.