Advantages of Adopting High Voltage | Electrical Engineering

There are following advantages of adopting high voltage for transmission and distribution of electrical energy in dc system:

(i) It reduces the size (area of x-section of the core carrying the current) of the feeders and distributors.

Consider first of all a feeder. As no consumer is taped off from a feeder so voltage drops in feeder will not be so important provided that it is not outside the range of compound generators in the generating stations. So the fundamental basis of designing of the size of a feeder is the current-carrying capacity.

Though the permissible current density for a given type of cable is not constant but decreases somewhat with the increase in size due to poor cooling facilities with higher sizes but for approximation we assume it as constant.

Now if supply voltage is increased n times, then for a given power to be delivered the current is reduced to 1/n th so the size of the cable is also reduced approximately to 1/n th.

Consider a distributor in which voltage drop is to be considered mainly while deciding its size. This is because the voltage drop in secondary distribution lines is to be limited to a specified value. Let distributor AB carry a current of I amperes and let permissible voltage drop be of v volts. Then the resistance of the distributor is given by R = v/I.

Now if the supply voltage is increased n times, then for a given power to be delivered the current in the distributor becomes 1/n and permissible voltage drop increases for a given percentage of permissible voltage drop in the distributor and becomes nv volts. The resistance of the distributor with high voltage of distribution is given by R’ = nv/ (1/n) = n²R.

Since for a given length, the area of x-section of a cable is inversely proportional to its resistance, so by increasing the supply voltage to n times, the area of cross-section can be reduced to 1/n² times for supplying a given power with same per cent of voltage drop in feeder.

(ii) It increases the efficiency of transmission.

Let voltage at feeding point = V volts

Current supplied by the feeder = I amps

Resistance of each conductor of feeder = R ohms

Total voltage drop in feeder = 2 IR

Voltage at supply end = V + 2IR

Now let the voltage at feeding point be raised to n V volts then for transmission of same power,

Current in feeder = VI / nV = I/n

Assuming current density constant, the size of the cable of feeder is reduced to 1/n th, hence resistance of each cable of feeder becomes n R,

Voltage drop in feeder = Current × resistance = 1/n × 2nR = 2IR

Voltage at supply end = n V + 2IR

Comparing Eqs. (2.1) and (2.2) we conclude that transmission efficiency is sufficiently increased. For example if I = 50 A; R = 0.4 Ω; V = 100 V and n = 10 then

Thus with the increase in transmission voltage, efficiency of transmission increases and cost of copper in cable decreases. For this reason it is advantageous to transmit power at high voltage.