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8/3/2019 Power System Engineering Lecture 3
1/18
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Distribution system
Overhead vs. underground system
DC vs. AC systems
Choice of working voltage
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Transmission Cost as Function of
Voltage Level.
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Choice of Working Voltage
100
3
6.1
5.5
1506.15.5
PLV
kVALV
Vis the line voltage in kV
L is the line distance in km
P is the estimated
maximum power in kW
per phase
Using these empirical relations preliminary estimate is
made. Then voltage level is chosen by doing complete
economic study of existing interconnected system.
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Line Parameters
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Electrical Parameters
Resistance
Inductance
Capacitance
Conductance
Conductance is due to leakage over line insulators. It isvery small and can be always neglected.
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Types of Conductor
Copper
Aluminum: Cheaper, lighter, but less conductive and less
tensile strength than copper
ACSR (Aluminum Conductor Steel Reinforced)
AAC (All Aluminum Conductor)
AAAC (All Aluminum Alloy Conductor)
ACAR (Aluminum Conductor Alloy Reinforced)
Expanded ACSR
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ACSR Conductor
Internal steel strands increase the tensile strength
Outer aluminum strands carry the current
Stranded conductor with twisted wires for strength and
flexibility of mechanical handling.
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Resistance
The DC resistance of conductor at specified temperatureis:
T is the resistivity of the conductor at temperature T
l is the length of the conductor in m
A is the cross-sectional area of conductor in m2
A
lR
T
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Resistance
Temperature dependence of resistance:
R0 is the resistance at 00C
0 is the temperature coefficient of the resistance of conductor
at 00C
Thus, if the resistance at temperature T1 is known, the
resistance at any other temperature T2 can be computedusing:
)1( 00 TRRt
)1(
)1(
10
20
1
2
T
T
R
R
t
t
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AC Resistance:
Skin and Proximity Effect
Currents in same direction Currents in opposite direction
Skin Effect
Proximity Effect
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Resistance
Resistance depends on:
Temperature
Dimensions
Frequency
Material % Conductivity Resistivity (200
C)m
Copper 100% 1.72
Aluminum 61% 2.83
Iron 17.2% 10
Silver 108% 1.59
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Inductance
Most dominant line parameter
Therefore
In linear magnetic systems
Mutual inductance is:
dt
diL
dt
di
di
d
dt
de
di
dL
iL
2
12
12
IM
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Steps in Calculating Inductance
Magnetic field intensity (H) using Amperes law
Magnetic flux density B (=H)
Flux linkages ()
Inductance from flux linkage per ampere (L=/I)
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Internal Inductance
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/power-system/ui/Course_home-1.htm
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Internal Inductance
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/power-system/ui/Course_home-1.htm
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External Inductance
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/power-system/ui/Course_home-1.htm
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Inductance of Single-Phase Line
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/power-system/ui/Course_home-1.htm
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Inductance of Single-Phase Line
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/power-system/ui/Course_home-1.htm
If r1 = r2 = r , then