Why we use only sinusoidal alternating voltage waveform in power system?

it has special properties, one of which is the following. No other periodic wave will pass through a linear network containing energy storage elements, (capacitors and inductors) and leave with the same frequency and shape except for phase and amplitude changes. All those other waves will not have the same output shape as the input shape. That is because the derivative and integral of a sinusoidal wave are the same except for phase differences.
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Mukesh Sharma
AskIITians Faculty