IEC 62040 defines UPS topologies:
VFD: Line-interactive; dependent on mains.
VI: Stabilizes voltage but not frequency.
VFI: Double-conversion online; fully isolates output from mains fluctuations.
Data centers require continuous, clean, and stable power. VFI is the only topology that protects against both voltage and frequency disturbances, meeting ANSI/TIA-942 Rated-3/4 requirements.
Thus, VFI is the ideal UPS class.
[References: IEC 62040-3, ANSI/TIA-942-B §6.2., , ]
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