| Compared with overhead transmission lines, power cables are widely used in urbandistribution network due to their no interference with city view and higher reliability. Withincreasing demand for electricity in cities, the requirement of power cable current carryingcapacity is increasing. And the structure of cable conductor and cable sheath has a greatinfluence on cable current carrying capacity. IEC-60287has the formulas of ac resistance andsheath losses about solid conductor, but not related formulas and evaluation methods underother structures and conditions. This paper will study the AC resistance calculation forsingle-core power cables without segmental conductors and with segmental conductors andthe eddy current losses calculation in cable composite screens.In this paper, the analysis of the magnetic properties of cable conductors withoutsegment is made by reference to Morgan’s analytical methods on aluminum conductor steelreinforced (ACSR) used in the overhead transmission lines and making some modification.The magnetic flux generated by the current in each layer of wires consists of longitudinalmagnetic flux and circular magnetic flux. Then the voltage equation based on circuit theory tocalculate AC resistance of single-core power cables is established. In the voltage equation,some parameters are included to consider their effects on the AC resistance, such as thedirection of lay, the lay length, etc. The calculation results show that the AC resistance ofbidirectional stranded conductor is a few percentage points higher than that of unidirectionalstranded conductor, and the larger the lay length is, the smaller the AC resistance becomes. Insummary, the difference of the AC resistance between unidirectional stranding andbidirectional stranding is not very large. But, such difference will be significant in theoptimization of cable design as even slight decrease of ac resistance can save considerablecopper in large-scale industrial application.In this paper, the research method of segmental conductors by CIGRE working group isadopted. In their analysis, the losses of each segment are the total effects of two parts, i.e. thecurrent in the segment itself and the current in other segments. This paper considers theparticular cases of solid conductor and segmental conductors with insulated stranded wires.The AC resistance calculation of solid conductor points out that the applicability of the theory.And the basic principle that the AC resistance can be reduced when using segmentalconductors with insulated stranded wires is analyzed in the paper. Then the theoreticalcalculation values of the AC resistance are compared with the existing historical experiment data to verify the correctness of the theory.The composite screens are composed of screen wires and metallic laminated foil. Thereare some formulas of eddy current losses about metallic laminated foil in IEC–60287, but notformulas of eddy current losses about screen wires. In this paper, the eddy current losses incable screen wires and the losses between wires when the wires are bonded at both endsrespectively and at least there is one end not being connected to ground are calculated byusing numerical simulation software. The calculation results are compared to that of themodels built by CIGRE working group to verify the correctness of the models. Thecalculation results show that the eddy current losses of screen wires are much less than that ofthe sheath with a same cross-section. When the screen wires of cables are laid straightly, thelosses due to circulating current between wires and the eddy current losses of the sheath witha same cross-section are the same. However, if the screen wires are stranded and insulated,there will be not circulating current between wires and circulating current losses would beeliminated. |
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