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When to use I²R, V²/R or VI while calculating Electric Power?

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I came across a question that was asked to me quite a couple of time: When to use I²R, V²/R or VI while calculating power? At first, the question looks simple and as if it needs no formal explanation but that's not the case. Use of any of them will give the correct result provided that you apply it correctly. Correct application is possible only when you understand the type of circuit you are dealing with. Another important point is to use the one which is going to take less time and help you calculate easily. All of the three equations tell the same thing but their usage is subject to the type of problem you're trying to solve. Let us understand when, how an what to use while calculating power in an electrical circuit.
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Why transmission is done at Voltages like 11kV, 22kV,..132kV ? Know the exact reason

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Transmission Voltage are 11kV, 22kV, 33kV, 66kV, 132kV etc. Why?

No form factor isn't the reason!

 

Many people sight Form Factor a reason for so. Form Factor is defined as RMS Value to Average value of a given AC voltage and it is different for different waveforms.

Now the commonly used AC waveform we know is Sine Wave. A sine wave AC waveform has form factor of 1.11 (Approx.). So, the reason given is that the transmitted voltage of 10kV, 20kV, 60kV etc. is multiplied to this form factor to obtain such results which you described in the question like

For 10kV → 10 x 1.11 = 11.1 kV (Okay! It is approximately correct)

For 20kV → 20 x 1.11 = 22.2 kV (Okay! approximately 22kV)

For 60kV → 60 x 1.11 = 66.6 kV (Error! it is 66.6kV is considerably greater than 66kV)

Like that

120 x 1.11 = 133.2kV (A big error of about +1.2kV because it is 132kV as used)

So, at each subsequent step an amount of deviation has been seen which differs the actual and calculated result. Moreover, the deviation is not constant and increasing (see the last result, about +1kV deviation). Such results are intolerable and brings to the conclusion that this is the best incorrect way of calculation. Because, Form Factor has nothing to do here. It is utter nonsense to sight it a reason.

The generation companies tends to generate round figure voltages like 10kV, 20kV, 60kV, 120kV etc. But this huge voltage need to be transmitted over huge distance. The overhead lines through which the power will be transmitted has its own impedance which will cause a considerable amount of voltage drop. This drop as being calculated is near about 10% based on all Physical factors. That’s why generation companies add 10% more in their actual target which neutralizes the line losses and the receiving end gets the targeted result. So,

Net Voltage = Target Voltage + 10% of Target Voltage

→ 132kV = 120kV + 12kV (10% of 120kV)

→ 66kV = 60kV + 6kV

→ 11kV = 10kV + 1kV

Hope that helps. Regards

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