What are the key differences between online, offline and line-interactive UPSs?

The UPSs used in most applications today use solid-state technology to manage their power control and communications functions, and are therefore described as static UPS systems. This is in contrast to the earlier rotary systems which they have largely superseded.

Static UPS sytems are available in various power topologies, but all are designed around a stored energy source – usually a lead/acid battery – that accumulates power from the electrical mains supply during normal power availability, and then dispenses it to the critical load during mains supply disturbances or blackouts. This means that they use common building blocks; a rectifier/charger to convert incoming AC into a DC charging current for the battery; an inverter to generate AC current for the critical load; and a static switch to connect either the inverter output or the mains supply to the critical load as circumstances demand.

These components interact in protecting the load within either an online, offline or line-interactive topology. As we shall see, these modes represent different trade-offs between capital, operating costs, and levels of protection. It is up to each user to decide which approach is most appropriate to the needs of his communications and data processing load, and the business that it supports.

Off-line UPS systems are so-called because the load is fed directly from the raw mains during normal operation, rather than from the inverter output. To that extent, the energy storage components – charger, battery and inverter – are off-line as far as the load is concerned, although the charger and battery remain connected to the mains to ensure the battery is always fully charged. If the mains voltage fails, or exceeds acceptable limits, the static switch immediately connects the inverter output to the critical load. During this changeover there is an inevitable break in power to the load of typically 2 to 10 ms. In practice, however, most loads can ride through this period without any problems.

A more serious objection to offline systems is that the load is continuously exposed to spikes, transients and any other aberrations coming down the power line, creating a risk of loss or damage to sensitive equipment and data. However in many systems this risk is mitigated, although not eliminated, by spike suppression and radio frequency filtering.

The advantages of offline UPS systems are that they are cheaper to purchase, and without the charger and inverter being permanently on load, they are more economical and energy efficient.

By contrast, online UPS systems draw power through the power conditioning and charging components during normal operation, so the load always receives conditioned power rather than raw mains. Online UPSs are often called ‘double conversion’ types because incoming power is converted once to DC, for the battery, and then back to AC before reaching the load – which is therefore well-insulated from the mains. Additionally, there is no power interruption if the battery is called upon to take over from the mains.

Line-interactive UPS systems are basically offline topology enhanced by the addition of voltage regulation features to the raw mains supply. These typically use either a ferroresonant transformer or a buck-boost transformer implementation. Both approaches reduce the frequency of transfers to battery, slightly improving efficiency and reducing battery ‘wear’. Ferroresonant designs also offer power conditioning and tight voltage regulation, as well as an energy store that can maintain uninterrupted power supply output while the inverter switches on.

Today, most users with highly-critical loads will inevitably choose online UPS topology; its ability to protect sensitive equipment and data from mains problems at all times far outweighs any extra cost consideration – especially as modern UPS hardware has become highly efficient in operation, significantly reducing excess operating costs.

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