Free conductive particles are one of the main causes of gas-insulated switchgear（GIS） failure

and it is difficult to detect these particles by on-site tests

so it is necessary to propose new detecting methods to supplement the on-site tests. Free conductive particles can easily take off and move under DC voltage

thus causing local electric field distortion.Impulse voltage is effective in detecting local field concentration. By combining the advantages of the two kinds of voltages

the effectiveness of detecting free conductive particles in GIS using impulse voltage with DC superimposition was studied in this paper;meanwhile

by establishing a superimposed voltage test circuit

the effectiveness of the method was studied on a 500 kV GIS. The results show that

under DC voltage

the movement of linear particles is mainly bounce or firefly. With DC voltage increasing

the probability of firefly motion increases. And the firefly occurs on the enclosure under positive DC voltage and on the bus bar under negative DC voltage. The probability of firefly motion is much greater under negative DC voltage than that under positive DC voltage. Negative DC voltage is firstly applied to the GIS

then impulse voltage is applied and superimposed on the DC voltage. The superimposed voltage can effectively detect the free conductive particles in GIS. As the particle length increases

the discharge voltage decreases rapidly. With the increase of particle radius

the discharge voltage tends to decrease. Under positive impulse voltage

the discharge voltage of GIS is much lower than that under negative impulse voltage. Therefore

positive impulse voltage with negative DC voltage superimposed is more favorable for the detection of free conductive particles.