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Chapter 6

Transmission Lines Theory

Testing and Commissioning

6.1 INTRODUCTION
A transmission line is the electrical line that transfers power from one place
to another. It can be over a short distance or long distance. Lines can be
overhead lines or underground cables, and either AC or DC transmission.

6.2 OVERHEAD AC LINES
AC overhead transmission lines consist of conductors, earth wires, insulators
and towers. They are used for voltages in the range of 66 800 kV. This high
level of voltage is to reduce power losses during power transference through
the line.
Power conductors are commonly made from aluminum with a stranded
steel core (aluminum conductor steel reinforced). In higher voltages it is bun-
dled to form one phase of the overhead line to reduce the corona effect (elec-
tric field) and radio interference. The earth wire does not carry any current
and is made from high-strength steel or steel and aluminum. Insulators iso-
late the live phases from the tower body which represent the earth. Towers
are supporting conductors.

6.2.1 Electrical Characteristics of AC Transmission Lines
The characteristics of these transmission lines consist of resistance (R),
inductance (L), and capacitance (C) uniformly distributed over the length of
the line. Fig. 6.1 shows the common representation of transmission line in π
connection.
Overhead transmission has a loading capability limit depending on its
length and system voltage level as shown in Fig. 6.2.
The load carrying capability of cable is also limited due to the charging
current. The longer the cable, the higher capacitance, the higher charging
current, and the higher cost of using compensation equipment to compensate
for the reactive power supplied by this capacitance. The line carrying capa-
bility can be improved by using a series capacitance compensation for long

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