SERIES IMPEDANCE OF
TRANSMISSION LINE
PART-I
Prepared By:
Nuren Zabin Shuchi
Lecturer
Department of Electrical and Electronic Engineering
Shahjalal University of Science and Technology
■ Transmission Line Parameters
Outline
■ Series Impedance of Transmission
Line
■ Resistance of Transmission Lines
■ Factors affecting the Resistance of
Transmission Lines
Transmission Line Parameters
The term Transmission Lines broadly refers to overhead transmission lines and underground cables.
The key function of a transmission line is to transfer bulk power between generation sources and
load centers.
An electric transmission line has four parameters which affect its ability to
fulfill its function as part of a power system:
1)
2)
3)
4)
Series Resistance
Series Inductance
Shunt Capacitance
Shunt Conductance
 The series resistance relies basically on the physical composition of the conductor at a given
temperature.
 The series inductance and shunt capacitance are produced by the presence of magnetic and electric
fields around the conductors, and depend on their geometrical arrangement.
 The shunt conductance is due to leakage currents flowing across insulators and air. As leakage current
is considerably small compared to nominal current, it is usually neglected.
Resistance of Transmission Lines
 The resistance o f transmission-line conductors is the most important cause of power loss in a
transmission line.
 If DC current is flowing along a solid-round cylindrical conductor, the current is uniformly
distributed over its cross-section area and its DC resistance is evaluated by
 Since the current that flows in the transmission line is alternating, the effective resistance
of the transmission line deviates from the DC resistance.
 The following factors affect the resistance of a transmission line:
 Frequency
 Temperature
 Spiraling & Bundling
Resistance of Transmission Lines (cont.)
Frequency Effect
 If AC current is flowing, rather than DC
current, the conductor effective resistance is
higher due to frequency or skin effect.
 The frequency of the AC voltage produces a
skin-effect due to the nonuniform
distribution of the current.
 As frequency increases, the current tends to go
toward the surface (or skin) of the conductor and
the current density decreases at the center.
 Skin effect reduces the effective cross-section area
used by the current, and thus, the effective resistance
increases.
 The skin effect is a non-linear effect.
Resistance of Transmission Lines (cont.)
Temperature Effect
 The resistivity of any conductive material
varies linearly over an operating
temperature, and therefore, the resistance of
any conductor suffers the same variations.
 As temperature rises, the conductor resistance
increases linearly, over normal operating
temperatures, according to the following equation:
Figure: Temperature Vs. Resistance curve
Resistance of Transmission Lines (cont.)
Spiraling & Bundling
Stranded Conductors
 The conductors used for overhead Transmission Line are
usually stranded in order to increase the flexibility and to cope
up with skin effect.
 The strands are electrically parallel.
 Since a stranded conductor is spiraled, each strand is longer
than the finished conductor. This results in a slightly higher
resistance than the DC resistance.
Resistance of Transmission Lines (cont.)
Bundled Conductors
 In high-voltage transmission lines, there may be more than one conductor per phase (bundle configuration) to
increase the current capability and to reduce corona effect discharge.
Figure: Stranded conductors arranged in bundles per phase of (a) two, (b) three, and (c) four.
 The phenomenon of ionization of surrounding air around the conductor due to which luminous glow with
hissing noise is rise is known as the corona effect.
 Corona, however, can be reduced by increasing the total conductor surface.
 By increasing the number of conductors per phase, the total cross-section area increases, the current capacity
increases, and the total AC resistance decreases proportionally to the number of conductors per bundle.
 To maintain the distance between bundle conductors along the line, spacers made of steel or aluminum bars are
used.
Resistance of Transmission Lines (cont.)
 Since the factors affecting the resistance include both linear and non-linear effects
the effective resistance cannot be calculated simply by adding the DC and AC
resistances.
 The effective resistance of a conductor can be calculated using the following
formula:
𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒=
𝑃𝑜𝑤𝑒𝑟 𝑙𝑜𝑠𝑠 𝑖𝑛 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟
2
¿ 𝐼 ∨¿ ( 𝑜h𝑚) ¿
Where, The power is in watts & I is the rms current in the conductor in
amperes.
Any Questions?