Using dry air replace sulfur hexafluoride（SF

6

） as insulation medium for ring main unit meets the requirement of environmental protection development of switch equipment. However

the much lower dielectric strength of dry air than SF

6

make the further study on the solid-gas composite insulation become necessary

on which area the systematical research work is limited. In this work

different types of fundamental electrode are used to research the influences of electrical field uniformity

gas pressure and gap distance on the power frequency characteristic. The data of maximum breakdown field strength on different field unifo

rmity is obtained. Simulation and experimental researches are carried out on the insulation bush position of the actual product

on this basis

the influence of composite insulation structure on the position and value of maximum electric field strength is systematically analyzed. The results show that

the relationship between electric field uniformity and maximum field strength under gaps can be the criterion for the characteristics of composite insulation. Also

the voltage withstanding properties of composite insulation can be improved effectively by using solid insulation material covering

increasing distance and optimizing electric field uniformity.