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Do the same exercise as above with the replacement of the earlier transformer.

Q7.26 Do the same exercise as above with the replacement of the earlier transformer by a 40,000-220V  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?

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Power required 

P=800kW=800*10^3kW

The total resistance of the two-wire line

R=2*15*0.5=15\Omega

Input Voltage 

V_{input}=40000V

Output Voltage:

V_{output}=220V

RMS current in the wireline

I=\frac{P}{V_{input}}=\frac{800*10^3}{40000}=20A

Now,

a) power loss in the line

P_{loss}=I^2R=20^2*15=6kW

b)

Power supplied by plant = 800 kW + 6 kW = 806kW.

c)

Voltage drop in the power line = IR=20*15=300V

Total voltage transmitted from the plant = 300+40000=40300

as power is generated at 440V, The rating of the power plant is 440V-40300V.

We prefer high voltage transmission because power loss is a lot lesser than low voltage transmission. 

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