Do the same exercise as above with the replacement of the earlier transformer by a 40,000-220 V step-down transformer (Neglect, as before, leakage losses though this may not be a good assumption any longer because of the very high voltage transmission involved). Hence, explain why high voltage transmission is preferred?

Explanation

(a) Hence 800 kW power is delivered at town sub station by two way transmission line at 40,000 V. Hence, current through this line is,
`I = ( P )/( V )`
`= ( 800 xx 10^(3))/( 40,000)`
= 20 A
Power loss in the line wires is,
`P. = I^(2) R`
`= ( 20 )^(2) ( 15)`
`:. P . = 6000 W = 6 kW`
(b) Total power supplied by the plain should be,
`P_(t ) = P + P.`
`= 800 kW+ 6kW`
= 806 kW
(c ) Voltage drop across two way line,
= IR
( 20) ( 15)
= 300 V
Voltage across secondary coil of a transformer at power plant should be,
= 300 + 40,000
= 40,300 V
`rArr` Thus, at power station we should use a step-up transformer ( 440V - 40,300V )
Explanation about why power transmission is always preferred at very high voltage.
(i) Power loss in the form of heat in example no. 7.25 is,
`= ( 600)/( 1400) xx 100%`
= 42.86 %
(ii) Power loss in the form of heat in example no. 7.26 is,
`= ( 6)/( 806) xx 100%`
= 0.7444%
Thus, when a given electrical power is transmitted at very high voltage, power loss in the form of heat can be reduced considerbly. Hence, it is always preferred.