1. Transmission Line Parameters
Transmission Lines
• Used for the transmission of electrical power from generating
substation to the various distribution units.
• Transmits the wave of voltage and current from one end to another.
• Transmission line is made up of a conductor having a uniform cross-
section along the line.
• Electrical tower is used for supporting the conductors of the
transmission line.
• For transmitting high voltage, over long-distance high voltage direct
current is used in the transmission line.
2. TYPES OF TRANSMISSION LINE
Overhead Transmission Line
• Usually power transmission line is an overhead line
• The conductor is not insulated and supported by the transmission
tower through a chain of insulators.
3. Underground Transmission Line
• Usually used when the lines cross river or sea and also with in the
urban area
• High cost and high maintenance
• Not suitable for high voltage transmission
• Heat is not well absorbed
4. HVAC Transmission Line
• Practically used in transmit power
• Using 3 phase system to transmit to the distribution
• AC voltage is raised using transformer to the transmission voltage
• 3 levels : 132kV, 275kV and 500kV
HVDC Transmission Line
• Usually used in 2 situations:
a) long power transfer
b) To connect two different asynchronous power system or different
frequency.
5. Advantages:
a) No effect on magnetic field
b) No effect on electrical field
c) Easy in power flow control in two different power system frequency.
Disadvantages:
a) High in cost (DC-AC ect)
b) More dangerous
9. Earth Wire : The earth wire is usually grounded (earthed) at the top of
the supporting structure, to minimize the likelihood of direct lightning
strikes to the phase conductors.
10. Insulators: It is used to support the conductors and withstand both the
normal operating voltage and surges due to switching and lightning.
11. Spacer : to establish the distance between the partial conductors of a
bundle line in order to prevent the conductors from knocking together
and thus avoid damage done to conductors.
12. Dampers : to reduce the vibration and oscillation due to wind that can
be damage to the overhead transmission line.
13. Conductors
• Recently aluminium replaced copper
• Lower cost and lighter weight . Supply is abundant.
• Four types of conductor that are usually used:
a) All aluminium conductor – AAC
b) All aluminium alloy conductor – AAAC
c) Aluminium conductor steel reinforced – ACSR (most common)
d) Aluminium conductor alloy reinforced – ACAR
14. Benefit of ACSR:
a) Easier to manufactured
b) Easier to handle
c) More flexible
d) High strength-to-weight ratio (no insulating cover)
15. • Benefit of ACSR:
a) Easier to manufactured
b) Easier to handle
c) More flexible
d) High strength-to-weight ratio (no insulating cover)
16. • EHV have more than one conductor per phase; these are called
bundle
• Bundle conductors have a lower electric field strength at the surfaces,
in controlling corona.
• Smaller series reactance.
Electrical Factors
Mechanical Factors
Environmental Factors
Economic Factors
17. Factors affecting line parameters
The main factors are
1. Voltage level.
2. Line length.
3. Type of conductor used.
4. Number of conductors.
5. The spacing of the conductors as they are mounted on the
supporting structure.
18. TRANSMISSION LINE PARAMETERS
Three parameters that effects power transmission line:
1. Resistance
2. Inductance (magnetic field)
3. Capacitance (electrical field)
20. • Conductor resistance depends on the following factors:
a) Spiraling
b) Temperature
c) Frequency (skin effect)
d) Current magnitude – magnetic conductors
21. RESISTANCE
• Spiraling – Transmission conductors are usually made in stranded
(pintalan) to hold the strands together. Make it stronger
• Spiraling makes the strands 1% to 2% longer than actual length.
• As the result, Rdc is 1% to 2% larger than calculated.
• Temperature – Resistivity of conductor metals varies linearly over
normal operating temperatures according to:
25. • Figure above is the cross sectional of a conductor single phase two
wire line
• If a current go through conductor 1 and went back through conductor
2, the current will induce the magnetic field linked each other. Voltage
will be induced due to the current in the conductor.
26. • The total inductance of a wire per unit length in this transmission line
is a sum of the internal inductance and the external inductance
between the conductor surface (r) and the separation distance (D):
By symmetry, the total inductance of the other wire is the same,
therefore, the total inductance of a two-wire transmission line is
Where r is the radius of each conductor and D is the distance
between conductors.
28. Inductive Reactance of a Line
• The series inductive reactance of a transmission line depends on both
the inductance of the line and the frequency of the power system.
Denoting the inductance per unit length as l, the inductive reactance
per unit length will be
where f is the power system frequency. Therefore, the total series
inductive reactance of a transmission line can be found as
29. Capacitance and capacitive reactance
• Since a voltage V is applied to a pair of conductors separated by a
dielectric (air), charges of equal magnitude but opposite sign will
accumulate on the conductors:
• Where C is the capacitance between the pair of conductors
• In AC power systems, a transmission line carries a time-varying voltage
different in each phase. This time-varying voltage causes the changes in
charges stored on conductors. Changing charges produce a changing
current, which will increase the current through the transmission line
and affect the power factor and voltage drop of the line. This changing
current will flow in a transmission line even if it is open circuited.
q CV
30. Capacitance of a single phase two-wire
transmission line
• The potential difference due to the charge on conductor a can be
found as
, ln
2
a
ab a
q D
V
r
31. • Similarly, the potential difference due to the charge on conductor b is
, ln
2
b
ba b
q D
V
r
or
, ln
2
b
ab b
q D
V
r
32. Thus,
ln
ab
c
D
r
Which is the capacitance per unit length of a single-phase two-wire transmission line.
The potential difference between each conductor and the ground (or neutral) is one
half of the potential difference between the two conductors. Therefore, the
capacitance to ground of this single-phase transmission line will be
2
ln
n an bn
c c c
D
r
33. Shunt capacitive admittance
• The shunt capacitive admittance of a transmission line depends on
both the capacitance of the line and the frequency of the power
system. Denoting the capacitance per unit length as c, the shunt
admittance per unit length will be
2
C
y j c j fc
The total shunt capacitive admittance therefore is
2
C C
Y y d j fcd
34. • where d is the length of the line. The corresponding capacitive
reactance is the reciprocal to the admittance:
1 1
2
C
C
Z j
Y fcd
35. EXAMPLE
An 8000 V, 60 Hz, single-phase, transmission line consists of two hard-
drawn aluminum conductors with a radius of 2 cm spaced 1.2 m apart.
If the transmission line is 30 km long and the temperature of the
conductors is 200C,
1. What is the series resistance per kilometer of this line?
2. What is the series inductance per kilometer of this line?
3. What is the shunt capacitance per kilometer of this line?
4. What is the total series reactance of this line?
5. What is the total shunt admittance of this line?
36. 1. The series resistance of the transmission line is
l
R
A
8
2
2.83 10 1000
0.0225
0.02
l
r km
A
37. 2. The series inductance per kilometer of the transmission line is
3
1 1 1.2
ln 1000 ln 1000 1.738 10
4 4 0.02
D
l H km
r
3. The shunt capacitance per kilometer of the transmission line is
12
9
8.854 10
1000 1000 6.794 10
1.2
ln ln
0.02
ab
c F km
D
r
4. The series impedance per kilometer of the transmission line is
3
2 0.0225 2 60 1.738 10 0.0225 0.655
se
z r jx r j fl j j km
38. Then the total series impedance of the line is
0.0225 0.655 30 0.675 19.7
se
Z j j
5. The shunt admittance per kilometer of the transmission line is
9 6
2 2 60 6.794 10 2.561 10
C
y j fc j j S m
The total shunt admittance will be
6 5
2.561 10 30 7.684 10
se
Y j j S
39. • The corresponding shunt capacitive reactance is
5
1 1
13.0
7.684 10
sh
sh
Z j k
Y j
40. EXERCISE
1. Calculate the dc resistance in ohms per kilometer for an aluminum
with a 3cm diameter.
2. A single phase, 8kv, 50Hz, 50 km-long transmission line consisting of
two aluminum conductors with a 3cm diameter separated by a spacing
of 2 meters.
a) Calculate the inductive reactance of this line in ohms
b) Assume that the 50Hz ac resistance of the line is 5% greater than its
dc resistance, and calculate the series impedance of the line in
ohms per km
c) Calculate the shunt admittance of the line in siemens per km.
