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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