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This paper deals with the experimental characterization of discharges propagating over insulators of epoxy and glass, immersed in a gas or a gaseous mixture, under lightning impulse voltages (1.2/50 µs), using a point-plane electrode arrangement. The gases and mixtures we considered are SF 6 , N 2 , CO 2 , SF 6-N 2 and SF 6-CO 2. The morphology of creeping discharges and their final lengths are investigated versus the kind of insulator material, the amplitude and polarity of the voltage, the type of the gas (resp. mixture) and its pressure. It's shown that the shape of discharges and their final (stopping) lengths L f depend significantly on the solid insulator and the type of gas. For given solid and gas, L f increases quasi-linearly with the voltage and decreases when the gas pressure increases. The discharges don't always present a radial structure as reported in literature. For given voltage and pressure, L f is longer when the point electrode is positive than when it's negative while the initiation voltage of discharges is higher with a negative point than with a positive one; and L f is longer with glass than with epoxy. L f is shorter in SF 6 than in CO 2 or N 2. On the other hand, the increase of SF 6 content in SF 6-CO 2 mixture leads to a significant decrease of L f. Therefore the addition of small concentration of SF 6 in a given gas mixture improve the dielectric strength of insulating structure.

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The paper, prepared by CIGRE WG D1.03 (TF 09), presents the guidelines for risk assessment procedure on defects in GIS based on PD diagnostics. The procedure, described in detail in CIGRE Technical Brochure 525, starts with sensitive PD measurement to detect the critical defects and follows with identification of the type of the defect and its location inside the GIS. This information taken together with other essential data from laboratory measurements, manufacturer's experience, design aspects and trend analysis of the PD activity, are the base for the estimation of the criticality of the defects. Finally, the risk assessment is performed based on the estimated dielectric failure probability and failure consequences that can be different in case of on-site testing or in service activity.Index Terms -Gas insulated substation, defects, partial discharge measurements, diagnostics and risk assessment.

This study is devoted to the optical and electrical characterisation of discharges propagating over insulators made of polytetrafluoroethylene (PTFE) filled with different kinds of micro-mineral fillers immersed in gas or gaseous mixture, under lightning impulse voltage (1.2/50 ms), using a point-plane electrode arrangement. The fillers the authors investigated are MoS 2 , Al 2 CoO 4 , SiO 2 and CaF 2 . The gases and mixture the authors considered are SF 6 , CO 2 and SF 6 -CO 2 . It is shown that the stopping length of discharges L f increases quasi-linearly with the voltage; L f is shorter in SF 6 than in CO 2 and it is higher when the point electrode is positive than when it is negative while the initiation voltage of discharges is higher with a negative point than with a positive one. The discharges do not always present a radial structure as reported in the literature. Also, the type of filler greatly influences the characteristics of creeping discharges. In a given gas or mixture, the shortest L f is obtained with PTFE filled with MoS 2 or Al 2 CoO 4 . Thus, insulators with these fillers appear as the best insulators.

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