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1. EE-504.2 1
DEPARTMENT OF TECHNICAL EDUCATION
ANDHRA PRADESH
Name : G.Venkateswara Rao
Designation : Lecturer
Branch : Electrical & Electronic Engg.
Institute :G.P.T,Hyderabad
Year/semester :v Semester
Subject :Power systems-II
( Transmission & Distribution)
Subject code :EE-504
Topic :Transmission lines
Sub topic :Conductor material required for
supply systems in head lines & under
ground tables
Duration :50 min
Teaching Aids :ppt, diagrams.

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RECAP
• The need of Transmission lines
• Choice of supply systems
• relative advantages of D.C and A.C systems.
In the previous class, we have discussed about

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OBJECTIVES
On completion of this period, you would be able
to know
• The Conductor material required by the supply
systems for Over head lines and Under
ground cables.

4. EE-504.2 4
The following assumptions are made for
calculating conducting material
• Equal amounts of power transmitted.
• Equal lengths of line
• Equal line loss.
• The max. voltage between two conductors
is same (Vm).

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CONDUCTOR MATERIAL REQUIRED FOR
OVER HEAD LINE FOR VARIOUS
SYSTEMS
D.C system:
1.D.C two wire system with one conductor earthed
Fig.1

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Vm is the max.potential difference between
conductor and earth in volts.
I is current in amperes, R is the resistance of the
conductor in ohms.
Line voltage = Vm
Power transmitted = P = Vm I watts.
Line loss or power loss = 2 I2 R watts.

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2. D.C Two wire system with mid point earthed:
Fig.2

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Vm is the max.potential difference between
conductor and earth in volts.
I1 is current in amperes, R1 is the resistance of the
conductor in ohms.
Line voltage = 2 Vm
Power transmitted = P = 2 Vm I1 watts.
Line loss or power loss = 2 I1
2 R1 watts.

9. EE-504.2 9
Equating for equal power
2 Vm I1= Vm I
I1 = I/2
Equating for equal power loss
2 I1
2 R1 = 2 I2 R
R / R1 = I1
2 / I2
= (I / 2)2 / I2
= 1/4 = a1 / a

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Amt. of cond. Mat. Req. for D.C 2wire sys. with mid point earthed
Amt. of cond. Mat. Req. for D.C 2wire sys. with one end earthed
=1/4

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A.C SYSTEM
1.Single phase two wire system with one conductor
earthed:
Fig.3

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Vm is the max.potential difference between
conductor and earth in volts.
I2 is current in amperes, R2 is the resistance of the
conductor in ohms and power factor =
Rms voltage =
Power transmitted = P = I2 watts.
Line loss or power loss = 2 I2
2 R2 watts.

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Equating for equal power, I2 = Vm I
I2 = I /
Equating for equal power loss, 2 I2
2 R2 = 2 I2 R
2 ( I / )2 R2 = 2 I2 R
R / R2 = 2 / = a2 / a

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Amt. of cond. Mat.Req.for A.C 2wire sys.with one cond.earthed
Amt. of cond. Mat.Req. for D.C 2wire sys. with one end earthed

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Fig.4
2. Single phase two wire system with mid point
earthed:

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Vm is the max.potential difference between
conductor and earth in volts.
I3 is current in amperes, R3 is the resistance of the conductor in
ohms and power factor =
Rms voltage = 2 =
Power transmitted = P = I3 watts
Line loss or power loss = 2 I3
2 R3 watts.

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Equating for equal power, I3 = Vm I
I3 = I /
Equating for equal power loss, 2 I3
2 R3 = 2 I2 R
R / R3 = I3
2 / I2 = 1 / 2 = a3 / a

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Amt.of cond. Mat. Req.for A.C 2wire sys.with mid point earthed
Amt.of cond. Mat. Req. for D.C 2wire sys. with one end earthed

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3. Three phase three wire system:
Fig.6

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Vm is the max.potential difference between
conductor and earth in volts.
I5 is current in amperes, R5 is the resistance of the
conductor in ohms and power factor =
Rms voltage =
Power transmitted = P = 3 I5 watts.
Line loss or power loss = 3 I5
2 R5 watts.

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Equating for equal power, 3 I5 = Vm I
I5 = (I / 3)
Equating for equal power loss, 3 I5
2 R5 = 2 I2 R
R / R5 = 3 I5
2 / 2 I2 = 1 / 3 = a5 / a

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Amt.of cond. Mat.Req.for A.C 3- 3wire system
Amt.of cond Mat. Req.for D.C 2wire sys.with one end earthed


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CONDUCTOR MATERIAL REQUIRED FOR
UNDERGROUND CABLES FOR VARIOUS SYSTEMS
Fig.7
D.C system:
1.D.C two wire system

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Vm is the max.potential difference between
conductor and earth in volts.
I is current in amperes, R is the resistance of the conductor in
ohms.
Line voltage = Vm
Power transmitted = P = Vm I watts.
Line loss or power loss = 2 I2 R watts.

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2.D.C Two wire system with mid point earthed:
Fig.8

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Vm is the max.potential difference between
conductors.
I1 is current in amperes, R1 is the resistance of the
conductor in ohms.
Line voltage = Vm
Power transmitted = P = Vm I1 watts.
Line loss or power loss = 2 I1
2 R1 watts.

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Equating for equal power
Vm I1= Vm I
I1 = I
Equating for equal power loss
2 I1
2 R1 = 2 I2 R
R / R1 = I1
2 / I2
= (I )2 / I2
= 1 = a1 / a

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Amt.of cond.Mat.Req.for D.C 2wire sys. with mid point earthed
Amt.of cond.Mat. Req. for D.C 2wire system.

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A.C SYSTEM
1. Single phase two wire system:
Fig.9

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Vm is the max.potential difference between
conductors.
I2 is current in amperes, R2 is the resistance of the conductor
in ohms and power factor =
Rms voltage =
Power transmitted = P = I2 watts.
Line loss or power loss = 2 I2
2 R2 watts.

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Equating for equal power, I2 = Vm I
I2 = I /
Equating for equal power loss, 2 I2
2 R2 = 2 I2 R
2 ( I / )2 R2 = 2 I2 R
R / R2 = 2 / = a2 / a

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Amt.of cond.Mat.Req.for A.C 2wire system.
Amt.of cond.Mat. Req. for D.C 2wire system.

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Fig.10
2. Single phase two wire system with mid point
earthed:

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Vm is the max.potential difference between
conductors.
I3 is current in amperes, R3 is the resistance of the conductor
in ohms and power factor =
Rms voltage =
Power transmitted = P = I3 watts.
Line loss or power loss = 2 I3
2 R3 watts.

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Equating for equal power, I3 = Vm I
I3 = I /
Equating for equal power loss, 2 I3
2 R3 = 2 I2 R
2 ( I / )2 R3 = 2 I2 R
R / R3 = 2 / = a3 / a

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Amt.of cond.Mat.Req.for 1 phase A.C 2wire system.
Amt.of cond.Mat. Req. for D.C 2wire system.

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Fig.11
3. Three phase three wire system:

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Vm is the max. potential difference between
conductors.
I4 is current in amperes, R4 is the resistance of the conductor
in ohms and power factor =
Max voltage between conductor to neutral =

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Rms voltage = =
Power transmitted = P = 3 I4 watts.
Line loss or power loss = 3 I4
2 R4 watts.

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Equating for equal power, 3 I4 = Vm I
I4 = I / 3
Equating for equal power loss, 3 I4
2 R4 = 2 I2 R
3 ( I / 3 )2 R4 = 2 I2 R
R / R4 = 1 / = a4 / a

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Amt.of cond.Mat.Req.for A.C 3 phase 3 wire system.
Amt.of cond. Mat. Req. for D.C 2wire system.

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COMPARISON OF VARIOUS SYSTEMS OF
TRANSMISSION
(The ratio of conductor material required in any system
compared with that in the corresponding 2 wire D.C system)
S.no System max.voltage to earth max.voltage bet.cond.
D.C
1. Two wire 1 1
2. Two wire mid point earthed 0.25 1
A.C
1. Single phase two wire
2. Single phase two wire
mid point earthed
3. Three phase three wire

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SUMMARY
• Conductor material required for over head lines for
different systems.
• Conductor material required for under ground cables
for different systems.
We have discussed about

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a) Conductors
b) Conductor and earth
c) Tower to earth
d) Tower to conductor
QUIZ
1. In the over system the comparison of various system
is made on the basis of max. voltage between.

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2. Transmission of electrical power is normally done by
a) Two wire D.C
b) Three wire D.C
c) Three phase three wire A.C
d) Single phase A.C
QUIZ

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a) Unity
b) 1.25 units
c) 2/
d) 1.5/
QUIZ
3. On the basis of max. P.D between any conductor and
earth, the conductor material required for single phase
A.C two wire system over D.C two wire system is

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FREQUENTLY ASKED QUESTIONS
1. Compare the volume of the conductor material required
in D.C two and A.C single phase two wire system?
2. Compare the volume of the conductor material required
in D.C two and A.C three phase three wire system?
3. Compare the various systems of transmission on the
basis of max.voltage between conductors for over head
lines?

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