Using nitrogen（N

2

） or dry air to replace SF

6

as insulation media in high voltage apparatus is a hot topic in the development of environmental friendly switching equipment. To ensure compact structure and small size of the devices

engineers need to study the insulation characteristics of the utilized gas and the solid-gas compound insulation. The fundamental properties of N

2

as the insulation medium under different uniformities of electric field

pressures and electrodes gaps are studied. Furthermore

the empirical equations of N

2

insulation are obtained for optimizing the design of N

2

insu

lated switchgear. According to the simulation

calculation and analysis of insulation coordination at key points in the N

2

switchgear

and the results of solid-gas insulation coordination experiments

it is concluded that the characteristic of non-uniform electric field is important in designing criterion

and a sufficient gap is required when considering solid-gas compound insulation.