Surface charge accumulation on insulators in SF₆ and its effects on the flashover characteristics of HVDC GIL

As the process of UHVDC power transmission projects accelerates in China, the demand for DC gas insulated transmission lines (GIL) is increasing. Thus it is very necessary to investigate the key issues of GIL under UHVDC voltage. Focusing on the surface charge accumulation phenomenon of GIL insulator under DC voltage, we established a surface charge measurement system based on an electrostatic voltmeter. Moreover, we studied the surface charge accumulation patterns in SF₆ under different voltage amplitudes and under polarity reversal conditions. Meanwhile, we experimentally studied the flashover in a UHVDC GIL test unit in order to study the influence of surface charge accumulation on the flashover characteristics of the GIL insulator. The results show that, in SF₆ at 0.5 MPa, the most accumulated charges on the insulator surface are those with the opposite polarity of the applied voltage, which can enhance the electric field between the electrode and insulator surface and even leads to flashover on the insulator. The flashover experiments show that the maximum withstand voltage of GIL insulator under DC voltage is just 64% of that under AC voltage. The study can give a possible explanation for the reduction of DC flashover voltage of GIL insulator.