Surface charge accumulation and pre‐flashover characteristics induced by metal particles on the insulator surfaces of 1100 kV GILs under AC voltage

With the increase in the voltage and capacitance of gas‐insulated transmission lines (GILs), the insulation failure of GIL has attracted more and more attention. Further understanding of the partial discharge (PD) and flashover characteristics of metal particles on the insulator surface, as well as the flashover mechanism, is necessary to reduce the failure rate and improve the reliability of the equipment. In this study, an ultra‐high voltage (UHV) AC insulator PD test and measurement system based on the cross‐reference pulse current (PC) and ultra‐high frequency (UHF) methods were established. The PD development and flashover characteristics of 5 mm‐long metal particles at different positions on the surface of a UHV AC insulator, as well as the surface charge accumulation, were studied. The results show that the discharge of millimetre‐scale metal particles on the insulator surface under PD test conditions is relatively low (generally lower than 2PC), and it is difficult to be detected by the conventional PC and UHF methods due to their insufficient sensitivity. Moreover, it is found that the weak PDs of the millimetre‐scale metal particles will result in a charge accumulation on the insulator surface under AC voltage, which eventually will lead to the insulator flashover. The PDs of the UHV AC insulator before flashover are generally small, which will make it difficult for online monitoring systems to give an effective alarm before flashover.