Question

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?

Exercise:

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A small town with a demand of 800 kW of electric power at 220 V is situated 15 km away from an electric plant generating power at 440 V.The resistance of the two wire line carrying power is 0.5 $\Omega$per km.The town gets power from the line through a 4000-220 V step-down transformer at a sub-station in the town.

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Answer 1

Current is $\frac{800\times 100}{40000}=20A$

(a) line power loss = ${20}^2\times 15=6kW$

(b) Power supply by the plant =$800+6=806kW$

(c) Voltage drop on the line is $20\times 15=300V$

It is clear that th epercentage power loss is greatly reduced by high voltage transmission. The power loss is $(6\times 806)\times 100=0.74\%$.