UPS topologies explained: online, line-interactive and standby
A UPS topology is how the unit sits between the utility and the load. IEC 62040-3 classifies UPS output by how much it depends on the input: VFD units pass the input through and only take over when it fails, VI units hold the output voltage in a band while the frequency follows the input, and VFI units keep both voltage and frequency unaffected by the input. In practice those are standby, line-interactive and online double-conversion units, and critical loads in data centres run on VFI.
Key facts
- VFD (standby): output voltage and frequency follow the input until the input fails[1]
- VI (line-interactive): output held within a tolerance band against voltage swings, without drawing on the battery[2]
- VFI (online double conversion): output voltage and frequency unaffected by the input[3]
- Typical use: VFD at desktops, VI in network closets, VFI for servers, data centres and critical process loads
How each topology handles a bad input
A standby (VFD) unit passes utility power straight to the load. When the input fails, it switches to its inverter and battery. Anything that is not a full outage, such as a sag, a swell or a frequency drift, reaches the load.
A line-interactive (VI) unit adds a regulating stage on the input path, so it can correct voltage that runs high or low without going to battery. The output frequency still follows the utility or the generator [2].
An online double-conversion (VFI) unit rectifies everything to DC and builds a new AC output with its inverter, all the time. The load never sees the input directly, which is why the output is unaffected by input voltage and frequency variations [3]. When the inverter cannot carry the load, a static bypass switch hands it back to the input.
Efficiency modes on online units
Many current three-phase online units offer a high-efficiency mode that lets the load ride on a filtered bypass while conditions are good, and returns to double conversion when they are not. Eaton, for example, states up to 99% in its Energy Saver System mode on the 93PM 480V and 97% in double conversion [4]. Whether to enable such a mode is a site decision: it saves energy, but for the time it is active the load is not fully isolated from the input.
Choosing a topology
- Servers, storage and network cores in a data centre: VFI, usually with redundancy (see N+1 and 2N).
- Network closets and edge rooms: VI is common, and VFI where a generator’s frequency swings would upset the load.
- Single workstations and small devices: VFD is enough when a short switch-over is acceptable.
Standards cited
- IEC 62040-3 (as quoted in the US DOE UPS test procedure)
Common questions
What is the difference between an online and a line-interactive UPS?
An online (VFI) UPS rebuilds its output continuously, so the load is isolated from input voltage and frequency changes [3]. A line-interactive (VI) UPS corrects voltage without using the battery, but its output frequency follows the input [2].
What does VFI mean on a UPS datasheet?
It means the output voltage and frequency do not depend on the input: the UPS stays in normal mode and keeps its output unaffected by input voltage and frequency variations without depleting the battery [3].
Is an online UPS always in double conversion?
Not always. Many offer an efficiency mode that runs the load on a filtered bypass while the input is clean, such as Eaton's Energy Saver System [4]. Check how your unit is configured.
Which UPS topology do data centres use?
Online double conversion (VFI), because the critical load should not see input disturbances, and usually in a redundant configuration.
Sources
- U.S. Department of Energy, Energy Conservation Program: Test Procedure for Uninterruptible Power Supplies (final rule), footnote 11, section 2.27.1
- U.S. Department of Energy, Energy Conservation Program: Test Procedure for Uninterruptible Power Supplies (final rule), footnote 13, section 2.27.3 of appendices Y and Y1
- U.S. Department of Energy, Energy Conservation Program: Test Procedure for Uninterruptible Power Supplies (final rule), footnote 12, section 2.27.2
- Eaton, Eaton 93PM 480V UPS, overview
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