The Different Types of UPS Systems

Overview

The paper explains that IEC 62040-3 classifies static UPSs along three axes: how dependent the output is on mains voltage and frequency (VFD, VI, VFI), the output waveform in normal and stored-energy mode (S, X, Y), and dynamic output performance (class 1, 2, 3). The result is a code in AAA-BB-CC form, which lets units from different manufacturers be compared in the same terms.

The four common market topologies are then mapped onto those classes: standby, line-interactive, double-conversion on-line and the delta/hybrid variants. For each, the paper tabulates which power problems it can correct and which it cannot. Only VFI-class units cover all seven power-problem types, including frequency variation and harmonics.

An important nuance is the high-efficiency (eco) mode found on many double-conversion units. These modes run the load through bypass and change the IEC class: a unit that is VFI-SS-11 in normal operation becomes VFD-SS-31 in eco mode. A catalogue that quotes only one class can therefore mislead the reader.

In scope, the paper describes topologies; it does not select products or compare costs. As a rule of thumb it states that double conversion dominates above about 10 kVA and line-interactive dominates between 0.5 and 5 kVA. For a data-centre designer the main value is reading datasheet codes correctly and noticing the reduced protection that eco mode brings.

Key points

1. The IEC 62040-3 code format is AAA-BB-CC: input dependency (VFD/VI/VFI), waveform (S/X/Y) and dynamic class (1/2/3).
2. Dynamic class 1: no interruption, ±20% regulation within 10 ms; class 2: up to 1 ms interruption; class 3: up to 10 ms interruption with +10/-20% within 100 ms.
3. Typical results: standby VFD-SY-33; line-interactive VI-SS-31; double-conversion on-line VFI-SS-11 in normal mode and VFD-SS-31 in bypass/eco mode.
4. UPSs of 2 kVA and above generally have SS (sinusoidal) output; many consumer units below 2 kVA are SY.
5. Only VFI-class UPSs address all seven power-problem types.
6. Double conversion dominates above roughly 10 kVA; line-interactive dominates 0.5-5 kVA.
7. Standby and line-interactive units need almost 10 ms to detect a failure and transfer.

Using it in design

• Store the IEC class code (for example VFI-SS-111) and the topology type on every UPS model, and compare models through those fields.
• For data-hall critical load (above 10 kVA, Tier II and up), require VFI class in normal mode.
• If a design relies on eco mode, state the VFD fallback class explicitly and check it against the load's sensitivity.
• Show a tooltip that decodes the IEC code so novice users can read it.

Who should read it

Electrical designers selecting UPSs, engineers writing specifications, and teams building catalogue or data models.

Related standards

• IEC 62040-3
• IEC 62040-1
• IEEE 1100

Publication details

Authors

Jim Spitaels; Linda Zhang; Paul Lin

Publisher

Schneider Electric

Year

2024

Document number

WP 1

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation