THD, total harmonic distortion, is the RMS sum of all harmonic components divided by the fundamental, usually counted up to the 40th or 50th order. Voltage THD on a healthy distribution network typically sits below 5 percent, while current THD at the input of a six-pulse drive can exceed 30 percent without line reactors or filters. Limits such as those in IEEE 519 and EN 50160 are written around it.
Voltage THD at the main switchboard is 7.2 percent with all six drives running, that's over the limit in the connection agreement.
Also heard as
- total harmonic distortion
Do not mix it up with
- Harmonics: Harmonics are the individual frequency components; THD is one number summarising all of them.
Related terms
Harmonics
Voltage or current components at whole multiples of the supply frequency, produced mostly by nonlinear loads.
True RMS
Describes a meter or reading that computes the real RMS of any waveform, not one scaled from a sine-wave average.
Power factor
Ratio of active power to apparent power, showing how much of the current drawn does useful work.