IEEE 575-2014 pdf free download – IEEE Guide for Bonding Shields and Sheaths of Single-Conductor Power Cables Rated 5 kV through 500 kV..
5. Shield optimization
For distribution class cables, shield losses can sometimes be reduced by increasing the shield impedance through a reduction of the metal content of the shield. However, this approach is generally limited by fault current magnitude and duration requirements for the shield. The fault duration requirement will need to consider any delay introduced by failure of the primary circuit protection and dependence on a backup protection scheme. Cable designs typically specify a maximum temperature on the shield under this worst- case condition. The required amount of metal in the shield is then specified as a fraction of the cross- sectional area of the core conductor. For example, the shield can be full, 1/2, 1/3, 1/6, or even 1/12 the cross-sectional area of the core conductor. Designs with large surface areas compared to the volume of metal allow for increased heat dissipation into surrounding materials through thermal conduction. Thin corrugated shields tend to incorporate the highest shield to core conductor ratios because of the very large surface area presented for the minimum amount of metal present. The design results in some of the highest heat dissipation for the amount of metal included in the shield while reducing resistive losses by exhibiting high impedance. The most commonly used shield configuration for distribution class cables is concentric wires.
Incorporating an impervious moisture barrier into the design of a transmission class cable is an important requirement. For this reason, transmission cables have historically employed tubular lead sheaths because these could be readily extruded as a continuous, uninterrupted layer over the cable core while simultaneously providing the requisite metallic shield. More recent moisture-tight designs have often replaced lead with extruded corrugated aluminum sheaths and various combinations of corrugated and flat copper tapes in conjunction with copper wires. Where extruded metallic sheaths are not part of the design, metallic shields are supplemented with moisture-tight alternative polymeric and other designs in order to assure the high degree of operational reliability required of a transmission cable. Balancing the choices between designs, materials, electrical properties, and economics in the selection of the cable metallic shields/sheaths is referred to as shield optimization.
In many early cable designs, the shield was exposed and in direct contact with the earth, water, mud, and conduit. This resulted in corrosion problems caused by ac electrolysis, leading to shield damage. Early efforts to limit such damage placed restriction on the maximum magnitude of shield/sheath voltage, limiting these voltages to the range from about 12 V to 17 V. Newer cable designs generally include an outer jacket that is insulating and the likelihood of corrosion is thus effectively eliminated as long as the jacket remains intact. Since application of special bonding results in the build-up of significant voltage levels on the shield during faults and other abnormal operating conditions, designs take advantage of the state-of-the-art electrical insulating properties for the jacket to meet needed voltage withstand requirements. A graphite coating or an outer semiconductive layer is usually applied over the jacket at the factory to allow for testing of the jacket’s electrical integrity.IEEE 575 pdf download