Capillary absorption as an audible noise reduction scheme for UHV transmission lines

Audible noise generated from water drops collected on the transmission line conductor surface during foul weather is a major concern in UHV (ultrahigh voltage) transmission line design. Although the noise can be kept to an acceptable level by maintaining a surface electric gradient below 15 to 17 kV/cm, this alternative is quite costly since it would mean having larger conductor bundles and hence larger support towers as well. Several noise-reduction methods have previously been tried but none gave sufficiently satisfactory results. A novel noise-reduction method is proposed in which capillary action is used to absorb the surface water drops into the field-free conductor interior. A high degree of surface wettability is a crucial ingredient in the absorption approach. Saturation places a theoretical limit to the effectiveness of this method during rain, but gravity-induced flow of the absorbed water helps to keep the conductor at an unsaturated state. Electrically shielded wicks can be placed at various points on the transmission line span to siphon off the draining water. Fog and after-rain tests on several absorbent conductor designs demonstrated the effectiveness of the absorption scheme in reducing foul weather corona activity. Significant reduction in audible noise was achieved even at a surface electric gradient of 22 kV/cm. Degradation of surface wettability was found in the laboratory. A quasi-one-dimensional model of vscosity-dominated,free surface flow is studied and laboratory experiments show that it is a useful representation of flow in an unsaturated conductor. It predicts that at the upper parts of the transmission line span, the flow tends to be uniform with a velocity linearly dependent on the conductor flow permeability and angle of incline. At the horizontal midsection, the flow is shown to be diffusional in nature and must be promoted via the use of wicks.