An electrical signal can be broken down into harmonic components using a very popular mathematical calculation based on Fourier’s theorem.  Fouriers theorem says that any periodic signal can be represented as a sum of sinusoidal components with frequencies equal to multiples of basic frequency of such signal.  This signal can be subjected to Fast Fourier Transform (FFT) to receive amplitudes and phases of harmonic components in the frequency range.

In a perfect situation, the voltage generated is output as a pure sinusoidal 50/60 Hz waveform with an absence of any higher harmonics.  If the electrical load is a linear system, then the current in this situation is also a pure sinusoidal waveform. In real systems, voltage and current waveforms can be distorted, hence in addition to the fundamental component there must be harmonics of higher orders.