The purpose of a UPS is to not only provide backup power but also to serve as a source of conditioned power. Power conditioning is a complex subject and different types of UPS condition power to a different extent.
Types of UPS (source: socomec.com)
As the above table illustrates, there are three different types of UPS and they rectify voltage and frequency problems in your raw power or mains supply to different degrees.
VFD or Offline or Standby UPS
VFD stands for voltage and frequency-dependent. The VFD UPS allows connected loads to run on utility power until it detects a power outage and its output is impacted by voltage and frequency variations in utility supply.
Standby UPS (source: apc.com)
When there is a power outage, the UPS switches over to battery power. The inverter only starts when power fails. As such, the UPS is also called Offline or Standby UPS.
The standby UPS is a cost-effective technology but comes with a tradeoff in performance. It does not produce a pure sine wave output on battery. Instead, it produces a modified sine wave that looks like a stepped or square wave.
This output is acceptable to a normal desktop in a typical home or non-critical office environment. However, high-end servers, networking, and storage equipment are best not run on this UPS technology.
The standby UPS is the most energy-efficient type. It has a small form factor and is low cost. It may include a transformer, surge suppressor, or a filter to provide some power conditioning of the utility power supply.
VI or Line Interactive UPS
VI stands for voltage-independent. The output of a VI UPS is frequency-dependent. Like the VFD UPS, it allows connected loads to run on utility power until a power outage. But it employs a buck-boost or tap changing transformer to regulate input utility voltage up or down as necessary before allowing it to pass through to protected equipment.
The output voltage is not dependent on the input voltage directly but is either the same or a set percentage higher or lower. Because of this interaction with the incoming utility line, the VI UPS is also called as Line Interactive (LI) UPS.
Line Interactive UPS (source: apc.com)
Unlike the Standby UPS, the inverter in the LI UPS is always on and connected to the output. This provides additional filtering by reducing switching transients when compared to the Standby UPS type.
The LI UPS technology is usually found in UPS capacities of up to 5 kVA as it has high energy-efficiency, is small in size and low-cost, and has high reliability. Not to mention, it has the ability to correct low or high line voltage conditions.
VFI or Double Conversion On-line UPS
VFI stands for voltage and frequency-independent. Unlike the VFD and VI UPS, this UPS does not connect the utility or mainline to the loads. Instead, the primary power path to the connected loads includes the inverter. Hence, also the name Online UPS.
This UPS converts power twice. First, AC into DC and then back to AC before supplying to connected loads. This double-conversion process isolates critical loads from raw utility power and ensures they receive only clean, reliable electricity.
Double conversion online UPS (source: apc.com)
When input AC power fails, the on-line operation of this UPS results in no transfer time to battery backup. This is because the input AC is charging the backup battery source which provides power to the output inverter.
The Double Conversion On-line UPS provides the best electrical output performance. As such, it is the technology employed in UPS capacities of 10 kVA and more.