Jun 21st, 2015
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Abraham Lincoln University
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Notes 9: Impedance of Overhead Lines 9.0 IntroductionThe series impedance of any conductor has two parts to it resistance and reactance. We will discuss resistance briefly first and then spend significant effort in considering the reactance.9.1 ResistanceThe DC resistance is computed according to (1)where is the resistivity, l is the conductor length, and A is the cross-sectional area of the conductor.There are three effects which cause the value used in power system studies to be differ from RDC. There are1. Temperature: The resistivity is affected by the temperature and this in turn affects the DC resistance. The influence of temperature on resistivity is captured by the following equation: (2)where all values of temperature T are given in degrees centigrade. The parameter ranges from 0.0034-0.004 for copper and from 0.0032-0.0056 for aluminum.2. Stranding: The effective length is affected by stranding because of spiraling, which tends to increase the DC resistance. This influence is captured by the following equation:(3)where is a function of the number of strands and the relative pitch. The relative pitch is given by d/2c, where d and c are as illustrated in Fig. 1 for a single cycle of a strand.Fig. 13. Skin effect: This influence is from the alternating magnetic field set up by the alternating current in the conductor. The field interacts with the current flow to cause more flow at the periphery of the conductor. This resu

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