In recent years

flashover faults frequently occur in GIS insulators

and micro-metal particles on the surface of the insulator are generally considered to be flashover inducement. However

the partial discharge level of the micro-metal particles of different lengths on the surface of GIS insulator under the actual operating voltage is not known yet. Consequently

based on the pulse current mutual reference method

we established a measurement system for highly sensitive GIS pulse current partial discharge which had a PD resolution of up to 0.02 pC. Near the actual operating voltage level

the partial discharge phenomenon of 0.5 mm

2 mm

5 mm

8 mm and 10 mm micro-metal particles on the surface of 126 kV GIS spacer were observed. It is found that the partial discharge of metal particles below 5 mm on the surface of the insulator is less than 1 p C

which is difficult to detect by the UHF method. The influences of the length and position of micro-metal particles on the surface on the discharge characteristics can be revealed. The shorter the particle length is

the lower the field strength of the insulator surface and the lower the discharge amount will be. It is proven that there exist the shortcomings of the current PD test and the UHF method for the detection of mi-cro-metal particles on the surface of GIS insulator.