Power is usually transmitted over long distances as alternating current (AC). Th

Question

Power is usually transmitted over long distances as alternating current (AC). The reason for this is that AC transformers can easily be used to "step-up" the voltage for transmission, and then "step-down" to supply to residences, whereas DC transformers are complicated and costly. Why is it preferable to transmit at high voltage (typically the frequency used is 60 Hz)? For very long distances the conducting cables actually start to gain a significant capacitance (for example, a typical copper 5 mile long cable would have a capacitance of 2.4 microFarad), which adds to the total impedance. In order to minimize the reactance of the above transmission cable, what would you have to do?

Explanation / Answer

Cable is considered as long charged plate with high voltage and ground is other plate with low voltage ( zero voltage). By this way, cable makes several small capacitors with air gap along its length that are connected in parallel situation.When capacitors are in parallel situtation, then, equivalent capacitance ( net capacitance) increases. As the result of it, voltage across the length of cable increases. It means voltage at the receiving terminal will be greater than the sending terminal. So, this is capacitive nature of cable, it is added capacitive reactance to get the total impadance.

(note- cable also has the inductive nature, so, inductive reactance is also added to get the total impadence but in our question, we are restricted to discuss with capacitance only.)

So, to minimise the reactance of transmission cable, Shut reactors are used ( it is a device to control the voltage.)

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