UPS Power Supply System, Uninterruptible
Total Harmonic Distortion (THD) is expressed as THD% = (rms value of all harmonics) / (rms value of the fundamental). There are two common variations: THDI for current and THDU for voltage. To eliminate harmonic currents, UPS systems utilize filters such as passive LC filters, double-bridge or phase-shifting filters, and active filters, which can reduce distortion from ~30% to <5%. A clean rectifier with Power Factor Correction can further reduce THDI to under 3%. Compliance with IEC / EN 61000-3-4 standards necessitates THDI <5% at the UPS input, THDU = 5% on upstream busbars, and no individual harmonic orders exceeding 5%. Most true on-line UPS systems operate at less than 5% THDI.
THD is Total Harmonic Distortion (THD):
THD% = (rms value of all harmoics) / (rms value of the fundamental) in %
- THDI where I stands for current.
- THDU where U stands for voltage
To eliminate the harmonic currents, UPS’s are equipped with either an input filter, which can be:
- a passive LC filter, compensated or non-compensated, with or without a contactor
- a double-bridge or phase-shifting filter (harmonic recombination)
- an active filter, derived from active harmonic conditioners.
These filters reduce distortion from approximately 30% to less that 5%.
A “clean” rectifier of the PFC (Power Factor Correction using control electronics) type that does not draw harmonics. In this case (without a filter), THDI drops to under 3%. Both solutions meet the requirements of standard IEC / EN 61000-3-4 for low-voltage devices drawing less than 16 A, i.e.:
- THDI < 5% at the UPS input
- THDU = 5% on the upstream busbars
- no individual harmonic order higher than 5%.
NOTE – MOST TRUE ON-LINE UPS’s ARE BUILT TO OPERATE WITH LESS THAN 5% THDi
