Electrical Power Systems

124 Electrical Power Systems
Characteristics and Performance of
Transmission Lines
6.1 INTRODUCTION
6
This chapter deals with the characteristics and performance of transmission lines. It is convenient
to represent a transmission line by the two-port network, wherein the sending-end voltage VS and current IS are related to the receiving-end voltage VR and current IR through A, B, C and
D parameters as
VS = A VR + B IR Volts ...(6.1)
IS = CVR + DIR Amp ...(6.2)
or, in matrix form,
V
I S L O LA
BOV
= NM SQP
NM C DQP
I
L RO
...(6.3)
NM RQP
A, B, C and D are the parameters that depend on the transmission-line constants R, L, C
and G. The ABCD parameters are, in general, complex numbers. A and D are dimensionless. B
has units of ohms and C has units of siemens.
Also the following identify holds for ABCD constants.
AD – BC = 1 ...(6.4)
To avoid confusion between total series impedance and series impedance per unit length,
the following notation is used.
z = g + j wL W/m, series impedance per unit length
y = G + j wC S/m, shunt admittance per unit length
Z = zl W, total series impedance
Y = yl S, total shunt admittance
l = line length, m.
Note that the shunt conductance G is usually neglected for overhead transmission system.
6.2 SHORT TRANSMISSION LINE
Capacitance may be ignored without much error if the lines are less than 80 km long or if the
voltage is not over 66 kV. The short line model on a per-phase basis is shown in ig. 6.1.