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NCERT Class XII Chapter
Alternating Current
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Question : 26
Total: 26
Repeat the same exercise as in the previous question with the replacement of the earlier transformer by a 40,000-220 V step down transformer. (Neglect, as before, leakage losses through 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?
Solution:  

Virtual a.c in the time
Ip =
Poutput
Ep
 =
800×103
40000
 = 20 A
(a) Line power loss, Ploss = lv2R = (20)2 × 15 = 6 kW
(b) Power supplied by the plant
PTotal = PLoss+Poutput = 6 kW + 800 kW = 806 kW
(c) Voltage drop in the line, V = IvR = 20 × 15 = 300 V.
Voltage output of step-up transformer at power station = 40,000 + 300 = 40,300 V
So, the step up transformer at the power plant is 220 V-40,300 V.
Power loss in earlier arrangement, P1 =
600×103
1400×103
 × 100 = 43 %
Power loss in this arrangement, P2 =
6×103
806×103
 × 100 = 0.74 %
So, by supply of electricity at higher voltage, 40,000 V instead by 4000 V the power loss is reduced greatly that is why the electric power is always transmitted at very high voltage.
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