Download voltage increase

Choice of transmission
voltage and economical
conductor size
CHOICE OF SYSTEM VOLTAGE
In both AC and DC systems, the actual power
transmitted is proportional to the system voltage
For given power, the current decreases as
voltage increase
This possibility of reduction in current for an
increase in voltage has an important economic
aspect of power transmission
If current is reduced using HV, resistance can be
increased without incurring additional losses by
decreasing cross-section of conductor. Hence we
can use smaller conductors i.e lesser cost
R
l
A
CHOICE OF SYSTEM VOLTAGE: DC 2 WIRE SYSTEM
If voltage is raised by n times, current in line is
reduced by n times for same power transfer. So
losses I2 R is reduced by n2 times. Hence efficiency
is increased
output IVR VR



input
IVS VS
and
VR  VS  IR
Where R = resistance of both go
and return conductors, Vs and
VR are sending and receiving
end voltages respectively, I is
current delivered
CHOICE OF SYSTEM VOLTAGE: DC 2 WIRE SYSTEM
 IR  100 
VS  IR
% 
 100  100  

VS
V
S


 IR  100 
whre 
 is % voltage drop
VS


in both conductors
As percentage voltage increases, efficiency increase
for given voltage drop
Due to increase of voltage by n times, current in
line is reduced n times, so line loss is reduced n2
times. Hence to transmit power with same losses,
resistance of line can be increase by n2 times i.e.
amount of material can be reduced by n2 times.
So great saving in copper is grabbed.
CHOICE OF SYSTEM VOLTAGE: AC 3 WIRE SYSTEM
P
P 
3 VI cos   I 
3 V cos 
I
Now J  Current demsity 
A
P
1
 A

J
3 V cos 
CHOICE OF SYSTEM VOLTAGE: AC 3 WIRE SYSTEM
P  3 VI cos   I 
P
3 V cos 
Now J  Current demsity 
 A
I
A
P
1

3 V cos  J
 3 V cos   J
R
  l  
A
P

l



2
 3 V cos   J


P
 PL  Loss  3I R  3  
   l  
P
 3 V cos  

2
 PL 
3 V  lJP
V cos 
1



CHOICE OF SYSTEM VOLTAGE: AC 3 WIRE SYSTEM
Pinput  Poutput  losses
 Pinput

3 V  lJ
 P 1 
V cos 

3 V  lJP
 P
V cos 



2

3 V  lJ 


 1 

Pinput
V cos  

3 V  lJ  
P 1 

3
V
cos



 3 V cos   J 
P
Voltage drop : IR 
  l  
   lJ
P
3 V cos 


Poutput
P
CHOICE OF SYSTEM VOLTAGE: AC 3 WIRE SYSTEM
Volume of conductor    3 Al

   3 

P
1
3 Pl
 l 
JV cos 
3 V cos  J 
Assume P, l , J and ρ are constant
► Line loss is inversely proportional to V and cosΦ
(equ. -1)
► Line efficiency increases with V and cosΦ (equ. -2)
► As IR drop is constant, percentage voltage drop is
decreased when V is increased (equ -3)
► Volume required for the conductors is inversely
proportional to V and cosΦ (equ. -4)
4