An approach to the spacer design of HVDC SF6 gas insulated equipment

Some of design and technological measures employed to increase SF ₆ insulation reliability are discussed. It is pointed out that the insulation characteristics of SF₆ HVDC equipment markedly differ from those of analogous HVAC equipment. The minimum discharge electric stress values measured with long-term voltage application and reduced gas pressure are approximately equal to 20 MV/m×MPa, which is substantially lower than values measured with smooth voltage rise; a decreased electric strength on the insulator surface is directly associated with transient processes in the insulation, caused by free surface charge distortion. The generalization of the calculated and experimental data concerning the electric characteristics of CEI/SF₆ DC insulation has enabled the formulation of a general approach to choosing proper HVDC SF₆ equipment spacer configurations. This approach is based on the criterion of spacer-accumulated surface charge minimization. As a technological measure the abrasive treatment of the insulation surface as well as using external shielding electrodes with dielectric coating is recommended