The decline of hydrogen purity is a major hidden danger for safety and economic operation of hydrogencooled generator

which will lead to the reducing efficiency of generating and cooling

the aging and even damage of insulation material. Through test simulating the gas diffusion between hydrogen and sealing oil

it is found that the gas volume fraction of sealing oil should be controlled within 4%

so that the hydrogen purity can be kept above 96%. The research achievement can be used for quality monitoring and control of sealing oil in operation

and is beneficial to stabilize the hydrogen purity of generator. Meanwhile

it also can be used as the basis and data support for the revision of sealing oil quality in operation.