Criterion For Choice Of Voltage For Transmission And Distribution:
We know that the power P =VI, so for the same power if voltage is increased then current will decrease. If suppose power required is 500 W and voltage is 250V then current I will be equal to 500/250 =2A.
Now if voltage is increased from 250V to 500V and the power required is same then I = 500/500 =1 A
Now, if the voltage is still increased to say 1000V for the same power
then I = 500/1000 =0.5 A
From the above examples, we see that if the voltage is doubled, the current will be halved and when it is quadrupled then current is reduced to 1/4th of its initial value.
Thus we conclude that if voltage is increased to n times then the current will be reduced to 1/nth times for the same power.With the reduction of the current to 1/nth times, the conductor area will also be reduced to 1/nth times of its original area for the same current density, Hence less material is required when the voltage is increased.
We also know that when the current passes through any conductor, there is loss of power in that particular conductor according to the relation, I^2R. As the loss is proportional to the square of the current. So if the current is reduced to ½ value , then the loss will be reduced to 1/4th its original value. Hence the efficiency of the transmission line and all others equipments associated with the line will increase and more power will be available for use.
When current is passing through a conductor there will be a voltage drop according to the relation V=IR. So, when the current is reduced the drop of the voltage is less in the line, of course with the same cross sectional area of the conductor.
With the reduction of cross sectional area, considered the main advantage of transmitting electrical energy at very high voltage viz 132kV , 220kV or even 400kV. But in case of distribution system such high voltage is dangerous, so distribution voltage is generally 400/230V.
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