To investigate the risk of free metallic particles in gas insulated switchgear（GIS） chamber

a test cell is designed for simulation.Ultrasonic signals generated by the free particles with different shape and size are detected to extract the feature parameters such as power frequency phase correlation

amplitude and flight time.Then the harm of the particles is analyzed based on these parameters.The experimental results show that the ultrasonic signals of partial discharges are weak and have strong phase correlation when the particles are static in the cell

and the signals become strong and have weak phase correlation when the particles dance.And the results also show that the flight time varies with the particles’ shape and size

and the flight time of cylindrical particles is more sensitive to the test voltage than that of the spherical particles.This study indicates that different particles impose different harm to the insulation of GIS.Moreover

the signal amplitude and particles flight time are periodical.