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ITGlossary · Power quality
Harmonic Distortion
Deviation of the waveform from a pure sine wave, caused mainly by non-linear loads such as power supplies and inverters.
Technical level:Advanced
Computer power supplies, LED lights, variable speed drives and chargers do not draw current as a smooth sine wave but in pulses. This creates currents at multiples of the 50 Hz fundamental (150 Hz, 250 Hz and so on), expressed as total harmonic distortion (THD) in per cent.
Two figures matter for a UPS: input THDi, meaning how much distortion the UPS feeds back into the grid or generator, and output THDv, meaning how clean a sine wave it supplies to the load. A good online UPS typically delivers THDv below 3% with a linear load.
Harmonics heat up cables and transformers and overload the neutral in three-phase installations. If the UPS will run alongside a generator, choose a model with low input THDi.
Related terms
- Pure Sine WaveUPS output with the same waveform as the mains, needed for PFC power supplies, motors and sensitive equipment, unlike a stepped approximation.
- InverterThe component inside a UPS that converts DC power from batteries back to usable AC power.
- Noise (Electrical)Unwanted high-frequency disturbances on the supply voltage, e.g. from motors or power supplies, which UPS systems and line conditioners filter out.
- ATS (Automatic Transfer Switch)A device that automatically moves the load from the grid to a generator during an outage and back again when grid power returns.
- BlackoutA complete and sudden loss of grid power, lasting anywhere from seconds to several hours.
- Input Voltage WindowThe range of mains voltage within which a UPS keeps powering the load from the grid without switching to battery.