Improved gap factor of large sphere-plane and its application in calculating air gap clearance in UHVDC converter station

The choice of air gap clearance in valve hall has great impact on the costs and safe operation of UHVDC projects. In former designs, the air gap clearance was normally calculated using the method in IEC standards, which only simply took the gap factor K into consideration. Thus, on the basis of U50 experiments of typical electrodes, we proposed a novel gap factor for calculating air gap clearance, in which the influence of electrode dimension, gap distance, and the proximity of wall were taken into consideration. The results reveal that, with the increase of gap distance L(within 4~9 m), the gap factor decreases. The gap factor also decreases as the diameter D of a sphere electrode decreases. At the proximity of a wall, the gap factor is reduced by 0.06 times, which requires a larger air gap clearance. The modified air gap clearance calculation method is of great significance to structure optimizing and size reducing of valve hall. Taking a ±800 kV project for example, by the modified method, the air gap clearance of some gaps can be reduced by 0.6~0.9 m, i.e. 8%~34% of the original values.