02-10-2012, 11:56 AM
Transmission lines
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Preliminaries
Generators and loads are connected together through transmission lines transporting electric power from one place to another. Transmission line must, therefore, take power from generators, transmit it to location where it will be used, and then distribute it to individual consumers.
A ROT: the power capability of a transmission line is proportional to the square of the voltage on the line. Therefore, very high voltage levels are used to transmit power over long distances. Once the power reaches the area where it will be used, it is stepped down to a lower voltages in distribution substations, and then delivered to customers through distribution lines.
Resistance
We notice that silver and copper would be among the best conductors. However, aluminum, being much cheaper and lighter, is used to make most of the transmission line conductors. Conductors made out of aluminum should have bigger diameter than copper conductors to offset the higher resistivity of the material and, therefore, support the necessary currents.
AC resistance of a conductor is always higher than its DC resistance due to the skin effect forcing more current flow near the outer surface of the conductor. The higher the frequency of current, the more noticeable skin effect would be.
At frequencies of our interest (50-60 Hz), however, skin effect is not very strong.
Wire manufacturers usually supply tables of resistance per unit length at common frequencies (50 and 60 Hz). Therefore, the resistance can be determined from such tables.
Inductance of a transmission line
Analysis of (9.19.2) shows that:
The greater the spacing between the phases of a transmission line, the greater the inductance of the line. Since the phases of a high-voltage overhead transmission line must be spaced further apart to ensure proper insulation, a high-voltage line will have a higher inductance than a low-voltage line. Since the spacing between lines in buried cables is very small, series inductance of cables is much smaller than the inductance of overhead lines.
The greater the radius of the conductors in a transmission line, the lower the inductance of the line. In practical transmission lines, instead of using heavy and inflexible conductors of large radii, two and more conductors are bundled together to approximate a large diameter conductor. The more conductors included in the bundle, the better the approximation becomes. Bundles are often used in the high-voltage transmission lines.