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The current rating of an underground cable system is defined as the current level that will result in the maximum defined cable conductor temperature if the cable is installed as intended in the specified location. The current rating can be calculated for static (steady state) and/or cyclical loading conditions, but in each case the cable construction, the type of installation, soil characteristics and other environmental conditions must be considered in order to obtain an accurate value for the rating. To determine the optimum rating for the specific installation and loading situations, KEMA developed an optical fiber technology.
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Static current rating
Calculation of the static current rating is based on equations contained in IEC 60287. Conductor current losses, metallic sheath current losses, influence from neighboring circuits, thermal resistances inside the cable and the thermal soil properties are taken into account. Reference operating conditions, selection of cable types and economic optimization of power cable size are also considered. The calculations do not consider cyclic loading or the actual local environment of the installed cable. This steady-state model is applied to medium voltage and high voltage cables, irrespective of the type of cable insulation, and the results of the calculations are slightly conservative. Although local conditions are not taken into account, the calculation of the static current rating is important and is made for most applications.
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Example of static current rating.
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Cyclic and emergency current ratings
Improved results over the static rating calculations can be obtained by considering the effects of cyclic loading. Calculations are based on the equations contained in IEC 60853. Significantly improved rating calculations can be made by considering the thermal capacities of the installed cable and the time delay between the actual conductor temperature and the conductor current. Both assumed and known values are utilized for soil temperature and soil thermal properties. The cyclic model is used for higher current medium voltage and high voltage circuits and produces results that are slightly conservative.
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Dynamic current rating
Current rating optimization can be realized by considering the actual temperature values of the cable sheath and soil and the dynamic thermal behavior of the cable and the environment.
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Thermally permissible short-circuit currents and short-circuit temperature operating limits
Short-circuit calculations can be performed to determine the
adequacy of the cable and construction for the application. The
methods of IEC 60949 and IEC 61443 are used for this purpose.
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