AC TRANSMISSION LINES.

AC TRANSMISSION LINES
Introduction to ac transmission lines.
Transmission line evacuates generated electrical power from the generating
substations such as hydro, solar power or wind power to the distribution network.
The higher the transmission voltage, the lower the transmission current in the
network hence reduces transmission losses (since PLoss=R I 2). Most transmission
lines are designed for 3-phase 3 wire aerial lines since it is cheaper than
underground lines. The line conductor is made of aluminum alloy strengthened with
steel. Active power must be kept high while inevitable reactive power must be kept
minimum.
Voltage Regulation Characteristics.
Equivalent circuit of an ac transmission
Basic transmission line has 3 basic characteristics namely; resistance, inductive
reactance and capacitive reactance as shown below.
For analysis purpose, the transmission line is simplified into its equivalent circuit by
finding equivalent resistance R, series equivalent inductive value L and shunt
capacitance equivalent value C as shown below for a single phase.
The active power loss, reactive power loss of the inductor and capacitor of the
transmission line are given by
PLOSS=RI L
2 , QL= XL I L
2 , and QC= XC I C
2 respectively.
Reactive capacitive power losses at the both ends of transmission lines are given by
QC= ES
2
X C
= ER
2
XC