Harmonics are sinusoidal components at integer multiples of the fundamental, such as 250 Hz and 350 Hz on a 50 Hz system or 300 Hz and 420 Hz on 60 Hz. Rectifiers, variable-speed drives, LED drivers and switch-mode supplies draw current in pulses, injecting harmonic currents that distort the voltage across the system impedance. They overheat transformers and neutrals, trip breakers, resonate with capacitor banks and upset sensitive electronics, so limits apply at the point of common coupling.
Fifth and seventh harmonics are way up on the drive feeder, we need a filter before they turn the PF caps into a resonant circuit.
Triplen harmonics (3rd, 9th, 15th) add in the neutral of a four-wire system instead of cancelling, so a shared neutral feeding office electronics can carry more current than any phase.
Also heard as
- harmonic distortion
Related terms
THD
Total harmonic distortion: the combined size of all harmonics relative to the fundamental, as a percentage.
Power factor
Ratio of active power to apparent power, showing how much of the current drawn does useful work.
True RMS
Describes a meter or reading that computes the real RMS of any waveform, not one scaled from a sine-wave average.