A 6-pulse thyristor rectifier typically produces 25-35% THDi at nominal load. To reach #3% THDi, an active harmonic filter (or a 12/18-pulse or IGBT rectifier UPS) is required. Passive filters are load-specific and less effective over a wide operating range-undesirable with frequent changes. References: IEEE 519-2014 (harmonic current limits), UPS vendor application notes (6-pulse vs IGBT rectifier THDi), ANSI/TIA-942-B Annex (power quality).
