The influence of free moving particles on the breakdown voltage of GIS under different electrical stresses

Although Gas Insulated Switchgears (GIS) have proven to be reliable for more than forty years, some failures have been reported. In particular, failures due to free particles have been a serious problem for years. The presence of these particles results in a local field concentration which influences the insulation medium and can result in a breakdown. In this paper, the influence of free moving particles on the breakdown strength in GIS under AC voltage, AC + LI (Lightning Impulse) overvoltage and AC + VFTO (Very Fast Transient Overvoltage) has been investigated with the maximum amplitudes of 1.2 p.u., 3.5 p.u. and 2.1 p.u. respectively. Elongated metallic particles with the lengths ranging between 2 to 30 mm have been investigated. The results show that particle with a length of 10 mm and longer were dangerous to the insulation system under continuous operating AC voltage. A breakdown area has been distinguished based on the particles length under AC + LI and AC + VFTO. Particles of 15 mm long were found to be critical under AC + LI, while under AC + VFTO, the 30 mm particle was critical. However, due to limitations in the test setup, the voltage level for LI was 1.4 p.u. higher than for VFTO. The particles detectability have also been investigated by means of partial discharge (PD) measurements of different methods: the conventional method (IEC 60270) :the UHF method : the acoustic method. The PD results have shown that some defects can cause a PD level higher than 10pC. In general, such PD levels are detectable by all methods.