Risk Assessment of Free Metal Particles in DC GIL

Abstract

Free metal particle within DC gas insulated line (GIL) imposes great threat on 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, a coaxial cylindrical electrode system filled with SF 6 of different pressures is established, in which line-shape free particles of different lengths are set, 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 partial 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 are roughly 2~5 times of that under positive voltage while the time interval is less. Therefore, the characteristics of partial discharge caused by free metal particles can provide basis for research on the severity of particle on insulation and fault recognition.