Question

why even harmonics are absent in electrical systems

Answer 1

To give a simple ans to this question is that, power  mainly deals with AC. And we describe AC is a sinusoidal waveform which have equal positive and negative half cycles (excluding noise in the signal, considering ideal form). A odd harmonic can be simply described as a sinusoidal waveform terminating in odd pi points(for ex-pi, 3 pi, 5 pi) thus it have odd no. of cycles(1 cycle for pi, 3 cycles in 3 pi(2 positive cycles & 1 negative cycle)). So for odd harmonics, there is a positive half cycle still left which does not have a negative half cycle to cancel it.
That does not happen for even harmonics. They have equal number of positive as well as negative half cycles. Actually of the cancellation they cease to exist in power systems.