Modelling and Performance of transmission lines.pptx
1. UNIT - II
Modeling and Performance of
Performance of Transmission lines - short line,
medium line and long line – equivalent circuits,
phasor diagram, attenuation constant, phase
constant, surge impedance - transmission efficiency
and voltage regulation, real and reactive power flow
in lines – Power Circle diagrams – Ferranti effect -
Formation of Corona – Critical Voltages – Effect on
3. Classification of Overhead
• Short transmission line: upto about 50 km, <20kV
• Medium transmission line: about 50 km to 150
km, (>20 kV < 100 kV)
• Long transmission line: more than 150 km (> 100
4. Important Terms
• Voltage regulation:
• The difference in voltage at the receiving end of a
transmission line between conditions of no load
and full load is called voltage regulation and is
expressed as a percentage of the receiving end
• Obviously, it is desirable that the voltage
regulation of a transmission line should be low i.e.
the increase in load current should make very little
difference in the receiving end voltage
5. Important Terms
• Transmission efficiency:
• The ratio of receiving end power to the sending
end power of a transmission line is known as the
transmission efficiency of the line.
39. Surge Impedance
• Surge Impedance is the characteristic
impedance of a lossless transmission line. It is
also called Natural Impedance because this
impedance has nothing to do with load
impedance. Since line is assumed to be
lossless, this means that series resistance and
shunt conductance is negligible i.e. zero for
power lines. Z =
• Characteristics impedance is defined as the
square root of the ratio of series impedance to
40. Surge Impedance
• The surge impedance is defined as the ratio of
the amplitudes of voltage and current of a single
wave propagating along the line; that is, a wave
travelling in one direction in the absence of
reflections in the other direction. Its SI unit is
ohm. It is purely real with no reactive
• The approximate value of surge impedance of
overhead lines is 400 Ω while typically it is in the
range of 400 Ω to 600 Ω.
41. Surge Impedance Loading (SIL)
• SIL of a line is the power delivered by a line to
a purely resistive load equal to its surge
impedance. The line is assumed to have no
• SIL = VR
• SIL is also called natural power of line.
• Sometimes it is convenient to express the
power transmitted by a line in terms of per
unit of SIL which is the ratio of the power
transmitted to the surge impedance loading.
42. Surge Impedance Loading (SIL)
• The permissible loading of a transmission line
may be expressed as a fraction of its SIL and SIL
provides a comparison of load carrying
capabilities of lines.
43. Ferranti Effect
• It can be seen that for a long transmission line
under no load conditions, the voltage at
receiving end is more than at sending end
because of the effect of the line capacitance. This
is called Ferranti effect
• When an alternating potential difference is
applied across two conductors whose spacing is
large as compared to their diameters there is no
apparent change in the condition of atmospheric
air surrounding the wires if the applied voltage is
low. However, when the applied voltage exceeds
a certain value, called critical disruptive voltage,
the conductors are surrounded by a faint violet
glow called corona.
• The phenomenon of corona is accomplished by a
hissing sound, production of ozone, power loss
and radio interference. The higher the voltage is
raised, the larger and higher the luminous
envelope becomes, and greater are the sound,
the power loss and the radio noise. If the applied
voltage is increased to breakdown value, a flash-
over will occur between the conductors due to
breakdown of air insulation.
46. CORONA Discharges
• The phenomenon of violet glow, hissing noise
and production of ozone gas in an overhead
transmission line is known as corona.
• If the conductors are polished and smooth, the
corona glow will be uniform throughout the
length of the conductors, otherwise the rough
points will appear brighter.
47. CORONA Discharges
Factors Affecting Corona:
• Atmosphere.(stormy weather)
• Conductor size. (shape and condition, rough and
• Spacing between conductors.
• Line voltage.
55. CORONA Discharges
• It has been seen that intense corona effects are
observed at a working voltage of 33 kV or above.
• Therefore, careful design should be made to
avoid corona on the sub-stations or bus-bars
rated for 33 kV and higher voltages otherwise
highly ionized air may cause flash-over in the
insulators or between the phases, causing
considerable damage to the equipment.