Using nitrogen（N

2

） or dry air to replace SF

6

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

engineers need to fully understand the insulation characteristics of a new gas and study the solid-gas compound insulation. Based on the basic insulation characteristics of N

2

the breakdown characteristics at power frequency and with lightning pulse in the sphere-plate electrodes has been studied and the effect of humidity on breakdown voltage analysed. Results shows that the breakdown voltage of N

2

first increases with the gap distance

and approaches saturation eventually. It also reveals that the breakdown voltage of N

2

is not affected by water concentration under a humid condition of 1 000 μL/L water content.