UPS Basics: How Does A UPS Work?

In our latest ‘UPS Basics’ video, we answer the question ‘How does a UPS work?’ and cover the three main types of power protection: Offline, Line-Interactive & Online UPS.

An uninterruptible power supply (UPS) is the ultimate insurance policy for any environment that’s reliant on a clean and continuous electrical power supply. But not all UPSs are the same. They don’t operate in the same way and they don’t all offer the same level of power protection.

This ‘UPS Basics’ film explains how a UPS operates by exploring the differences between the three most common topologies of uninterruptible power supplies.

First up is Offline UPS, also Voltage and Frequency Dependent (VFD). A simple system where the load is always powered by mains supply and if an outage or failure takes place, the load temporarily switches to the UPS.

The video then highlights Line-Interactive UPS versions, sometimes called Voltage Independent (VI). This type of UPS is similar to an offline unit, but with the additional protection of an automatic voltage stabiliser to reduce any fluctuations.

Finally, we turn to Online UPS. Often described as double conversion and known as Voltage and Frequency Independent (VFI), online systems have a rectifier that converts mains supply from AC to DC, plus an inverter that changes it back again. If there’s a failure or disturbance to the mains, there’s not even a few milliseconds gap before the power switches to the UPS’s back-up supply.


Video Transcript

Hi I’m Steve. So far we’ve looked at power generation and distribution in the UK, as well as the main power problems that we experience.

Now let’s take a look at the three main types of static UPS. All UPS, regardless of the technology involved, perform the same fundamental task, to protect any equipment connected to it from a total blackout. There are three main types or topologies of UPS, each with varying levels of mains protection.

The most basic and cheapest UPS is known as Offline or Voltage and Frequency Dependent (VFD). An offline UPS is the most cost-effective way of providing protection against power outages.

In an offline UPS design the load is always supplied by the mains, whilst in normal operation an internal battery is charged and ready for use.

Only when a blackout occurs does the UPS choose the battery to provide power to the load. This switching from mains supply to the battery is typically less than 25 milliseconds. Although this sounds a very short period of time, it is enough of a break in power for sensitive modern equipment to notice.

Due to its design, an offline UPS does not protect against other mains-borne disturbances. Therefore this type should only be used for non-critical equipment.

The next step up the protection is a Line-Interactive or Voltage Independent (VI) UPS. The line-interactive design of a UPS works largely the same as an offline, but with the addition of an automatic voltage stabiliser or AVS.

With the AVS connected to the load, it is able to provide protection against voltage instability by increasing or decreasing the voltage if the main supply falls out of tolerance. The switchover period to batteries still exists in this design. However, it is typically reduced to around 5 milliseconds.

Line-interactive systems, with their greater power protection features and their larger power capacities, are well-suited and often used for larger non-critical loads such as telephone switches, servers, and smaller motor applications.

And now for the ultimate in power protection and availability. The Online UPS, sometimes referred to as double conversion or Voltage and Frequency Independent (VFI).

Different in design to the offline and line-interactive varieties, the load in an online UPS is not directly connected to the main supply.

Mains power enters the UPS and travels through a rectifier, which converts the power from AC to DC. This is then put through an inverter to convert back from DC to AC, before powering the load.

If a mains power failure or a disturbance occurs there is no break in power while the system switches over. Simply the rectifier switches off and allows the inverter to take power from the battery set automatically. The batteries in an online UPS design are always connected to the inverter ready for any power event.

As a result of the conversion to DC power, any disturbances on the incoming supply are stopped in their tracks.

Online UPSs can typically range from 700 VA to multi-MVA systems. This coupled with their total power protection makes them and the obvious choice for all critical loads.


Watch the complete UPS Basics series: