What is the best UPS technology to use?

Offline UPS, Line-interactive UPS, or Online double-conversion UPS; which one should you choose and how does an Online double-conversion UPS work?

There are three types of UPS system designs. Primarily each one represents an advancement in UPS technology.

1. Offline UPS

Offline UPS technology means that the mains supply feeds the connected UPS load. Only when the mains supply fails will the UPS inverter startup and supply modified Sinewave power to the load from its internal lead-acid minimum runtime battery.

Offline UPS systems typically only have an inverter that can supply load for very short periods of a few minutes. If the inverter supplies load for an extended period, it will typically overheat.

When the utility supply returns, the UPS inverter switches off, and the load is resupplied by mains.

During normal operation, the only working part of this UPS is the battery charger.

This type of UPS technology is normally only available up to 3000 VA and involves a switching time for the inverter to turn on. It is thus commonly only used for supplying loads that have an internal AC to DC power supply, like a PC.

This is the cheapest entry-level backup UPS system that you can buy and is not suitable for very sensitive loads or for extended run times.

This can also be referred to as a single conversion UPS, i.e. one conversion in power from mains to load. The battery charger is not big enough to supply the inverter with full load power; it only charges the internal backup battery.

2. Line Interactive UPS

A line-interactive UPS is an improvement on the offline UPS technology in that the UPS inverter is always on, but the load is still fed by mains via the UPS bypass switch.

As the inverter is always on, the switching time from ‘load on bypass’, to ‘load on inverter’, is much quicker. The momentary break in power is much less than in an offline UPS.

The line-interactive UPS is usually supplied with an inverter that can run on its battery backup for extended periods, but the inverter is not typically rated to supply load continuously.

The inverters used in this type of UPS system are generally not pure sinewave but rather modified sinewave.

Once mains return after a power failure the inverter switches off and the load is fed by the bypass switch again using the normal raw mains.

This type of technology UPS is thus not suitable for loads that are highly sensitive to any short break in supply or that are easily affected by transients (surges and dips) in the supply.

This can also be referred to as a single conversion UPS, i.e. one conversion in power from mains to load. The battery charger is not big enough to supply the inverter with full load power; it only charges the internal backup battery.

Line-interactive UPS systems are more expensive than offline UPS systems as they use much better and more advanced UPS technology and electronic controls.

3. Online double-conversion UPS systems

The best way to ensure that your connected critical load is 100% protected and isolated from any irregularity in the utility supply; be it spikes, dips, brownouts (short power failures of seconds) or complete blackouts; is to use online double-conversion UPS technology.

This UPS has a rectifier that converts the 220 Volts AC mains supply from its AC power to DC power (1st conversion) by means of its rectifier.

The DC power is then used to charge the parallel-connected internal short runtime battery backup. This is also available for use with a larger than normal rectifier for a larger, long runtime battery.

The rectifier will, under healthy mains supply, charge the battery and feed the full inverter load with the power flow direction as shown below.


Diagram 1: Rectifier and Battery – AC to DC

The DC power supplied to the inverter input by the rectifier is then used in a conversion process whereby the load AC pure Sinewave power is electronically recreated by taking the DC power and changing it back into 220 volts AC power (2nd Conversion).

The 100% pure Sinewave inverter then feeds its connected load with clean, stable power and ensures maximum load uptime and reliability.

With these two power conversion processes, it is easy to see that in an online double-conversion UPS system, the load is completely isolated from the dirty and unreliable mains supply grid.


Diagram 2: Inverter – DC to AC

The power flow is shown with the arrows in the diagrams; there is never any switching as every component in the UPS is on, working all the time, and rated to supply a full UPS load continuously.

If the mains power supply to the UPS rectifier should fail, the power flow into the battery is reversed instantaneously.

The inverter never sees this power outage, as its input DC power is common to both battery and rectifier also referred to as the DC link.


Diagram 3: UPS System when supply fails

The online double conversion UPS technology is by far the best way to ensure your critical loads are 100% protected 100% of the time from any bad power in the utility supply grid or power outages.

Of course, this online double-conversion UPS technology is highly advanced, with all components rated for a full load, continuously operating, and thus the most expensive technology available in today’s modern UPS market place.