Instructions

A thermal power plant produces electric power of 600 kW at 4000 V, which is to be transported to a place 20 km away from the powerplant for consumers’ usage. It can be transported either directly with a cable of large current carrying capacity or by using a combination of step-up and step-downtransformers at the two ends. The drawbackof the direct transmission is the large energy dissipation. In the method using transformers, the dissipation is much smaller. In this method, a step-up transformeris used at the plant side so that the current is reduced to a smaller value. At the consumers’ end, a step-down transformer is used to supply powerto the consumersat the specified lower voltage. It is reasonable to assume that the powercable is purely resistive and the transformers are ideal with a powerfactor unity. All the currents and voltages mentioned are rms values.

Question 15

If the direct transmission method with a cable of resistance $$0.4Ωkm^{-1}$$ is used, the power dissipation (in %) during transmission is


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