Thursday, November 24, 2011

Boundled Conductors

      The Fig. 1 shows the arrangement if the conductors are bundled one
Fig. 1

       The conductors of any one bundle are in parallel and charge per bundle is assumed to divide equally between the conductors of bundle.
       The composite or stranded conductors touch each other while the bundled conductors are away from each other. The typical distance is about 30 cm and more. The conductors of each phase are connected by using connecting wires at particular length.
       Due excessive corona loss, the round conductors are not feasible for use for voltage level more than 230 kV. It is preferable to use hollow conductor in substations while bundled conductors in transmission lines.
       Following are advantages of bundled conductors.
1. Low radio interference and corona loss.
2. Reduced voltage gradient at conductor surface.
3. Increase in capacitance.
4. Low reactance due to increase in self GMD.
5. Increase in surge impedance loading.
       If the charge on phase a is qa then charge on each of the conductors a and 'a' will be qa/2. Same is the case with remaining two phases.
       This equation is similar to the expression we have written for 3 phase line with unsymmetrical spacing. Combining the terms we get
       Thus, for a two strand bundle
       For a three strand bundle
1.1 Stranded Conductor
       The stranded conductor usually has a central wire which is surrounded by the layers of wires. These layers consists of 6, 12, 18, ......... wires successively. Thus the total strands are 7, 13, 19  .............. .
       Such a stranded conductor with 37 strands is shown in the Fig. 2.
Fig. 2

       Let             d = diameter of each strand
       Then the total diameter of a stranded conductor (cable) is given by,
                         dc = (2 n + 1) d
       where n = number of layers in which the strands are arranged around central strand.
       The stranded conductor is specified as number of strands and diameter of strand. 
       For example 7/0.295 mm which indicates 7 strands with 0.295 mm diameter of each strand.
       If at all the number of layers are not specified then the number of layers can be calculated as number of strands and layers are related to each other by the equation,
                             x = 3n2 + 3n + 1
       where            x = number of strands
       and                n = number of layers
       The stranded number of strands in each successive layer from inner to outer is 6, 12, 18, 24 .......

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