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