A: THD stands for Total Harmonic Distortion. The term “harmonics” originated in the field of acoustics, where it was related to the vibration of a string or an air column at a frequency that is a multiple of the base frequency. A harmonic component in an AC power system is defined as a sinusoidal component of a periodic waveform that has a frequency equal to an integer multiple of the fundamental frequency of the system, i.e.
fh = (h) x fundamental frequency
where h is an integer. For example, the frequency of the fifth harmonic of a 50 Hz power supply can be calculated as:
f5 = (5) x 50Hz = 250Hz
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. It is used as a measure of the power quality of electric power systems.