IEEE 575-2014 pdf download

IEEE 575-2014 pdf download.IEEE Guide for Bonding Shields and Sheaths of Single-Conductor Power Cables Rated 5 kV through 500 kV.
Cable designs typically specify a maximum lcmpcraturc on the shield under this worstca sc condition. Thc rcquircd amount of metal in thc shicld is then specified as a fraction of thc crosssectional area of the core conductor. For example. the shield can be full. [‘2. l/3. 116. or even 1/12 the cross-sectional area of thc core conductor. Designs with large surface areas compared to the solumc of metal alkp. for increased heal 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 resulis in some of the highest heal dissipation tIr the amount of metal included in the shield vhile reducing resistie losses by exhibiting high impedance. The most commonly used shield configuration for distribution class cables is concentric wires.
Incorporating an impersious moisture barrier into the design of a transmission class cable is an important requirement. For this reason, transmission cables ha c historically employed tubular lead sheaths because these could be reathly extruded as a continuous, uninterrupted layer over the cable core while simultancou%ly pros iding the requisite metallic shield More recent moisture-tight designs haie oflen replaced lead with cxtruded 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 arc supplemented wnh moisture-tight alternative polymeric and other designs in order to assure the high degree of operational reliability required of a transmission cable. Halancing the choices between designs. matenals, electrical properties, and economics in the selection of the cable metallic shields sheaths is reined to as .
In many early cable designs. the shield was exposed and in direct contact with the earth, water, mud, and conduit. This resulted in com)slon problems caused by ac electrolysis. leading to shield damage Early eflorts to limit such damage placed restriction on the maximum magnitude of sh,eldshcath voltage. limiting these voltages to the range from about 12 V to 17 V. Nescr 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 ics ck on the shield during faults and other abnormal operating conditions, designs take advantage of the state-of-the-an electrical insulating properties for the jacket to meet needed voltage withstand requirements. A graphite coating or an outer semiconductive layer is usually applied oser the jacket at the factory to allos for testing of the jacket’s electrical integrity.
6. Special bonding techniques
With heavier loads on single conductor cable circuits, shield circulating current losses resulting from mullipoint solidly bonded and grounded systems can be excessive for the intended application. To mitigate these losses. alternative shield grounding methods are available, and these are collectively referred to as xpt’cI hunding wrhniqaw. Hcicausc long circuits and high currents tend to be more common on transmission class circuits, special bonding techniques tend to be more applicable on these types of circuits. However, special bonding techniques can be applied on distribution circuits when operating conditions dictate a reduction m circuit losses.
Shield losses also increase with the spacing between cables, particularly when multiple point grounded. single-conductor cables are installed with wide spacing. such as when cables are placed in separate ducts or when they are direct buried in spaced configurations. When cables arc spaced apart. significantly higher currents flow on the shield of solidly grounded systems, resulting in higher induced shield circulating current losses. Increased spacing decreases the effects of mutual heating but increases the effect of magnetic coupling and therefore, increases shield circulating current losses resulting in lower current ratings. The purpose of special shkld bonding is to eliminate or significantly reduce shield circulating current losscs on single-conductor cables.