When a sine wave voltage is applied to a nonlinear load, the current becomes non-sine wave. The non-sine wave current generates a voltage drop across the grid impedance, which also causes the voltage waveform to become non-sine wave. When the non-sine wave is decomposed using Fourier series, the component with the same frequency as the power frequency is called the fundamental wave, and the component with a frequency higher than the power frequency is called the harmonic wave.