Free metal particle within DC gas insulated line（GIL） greatly threatens the insulation system by distorting the electric field and inducing partial discharge in insulation gas or on insulator surface. To investigate the severity of free particle on insulation and influencing factors

we establish a coaxial cylindrical electrode system filled with SF6 of different pressures

in which line-shape free particles of different lengths are set.Meanwhile

the partial discharge signal is measured with pulse current method and motion behavior is observed with a high speed camera. Then

the average amplitude and time interval of partial discharge are analyzed. The investigation shows that the average p

artial discharge amplitude is proportional to the particle length and the maximum charge attached to the particle. Thus

the partial discharge signal can be used to assess the severity and length of the particle. When the central electrode is of negative voltage

the particle bounces up

as a "firefly"

in the region close to high voltage electrode. Particle bounces from the ground electrode and then falls down to the ground electrode as a circular motion when the central electrode is positive. The average amplitude of partial discharge under negative voltage is roughly 2

5

times that under positive voltage while the time interval is less. The activity of partial discharge decreases with increasing the SF6 pressure and has a saturated tendency. Therefore

the characteristics of partial discharge caused by free metal particles can provide a basis for researching the severity of particle on insulation and fault recognition.