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
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