Uninterruptible Power Supply Technologies Explained

Offline, Online and Line-interactive

What's the difference, and which technology is right for you?
There a three different types of UPS all suitable for different tasks, and supporting different types of load. Here are explanations of the different technologies, along with their advantages and disadvantages. Hopefully it will help you decide which type of UPS is suitable for you:

Offline-Standby Technology

During normal operation, the power flows straight through the unit and hence only RFI filtering is usually provided. When the input voltage fails or fluctuates outside of a pre-set tolerance window, the UPS detects this and a relay will close, allowing the UPS to start feeding battery power via the inverter.

The inverter is then switched on and either a square, step or sinewave form output is supplied. Upon the return of mains power, the output is switched back onto mains and the inverter is turned off. Typically there will be a break of between 4-10 ms during the transfer to and from the battery mode.

Advantages:
Low cost
Silent operation (when in standby)
Efficient

Disadvantages:
Minimal power protection – only protects against a small percentage of problems
Poor output voltage regulation – fluctuations such as sags and surges will be passed straight to the load
Break transfer to battery mode
No failsafe – UPS will drop the load if there is a high start-up current, overload or inverter failure.

Line-Interactive Technology

A line-interactive UPS operates in a very similar fashion to an offline UPS, except with the advantage of better filtering and output voltage boost/reduce features. Whilst not eliminating mains-borne interference, line-interactive technologies reduce the impact of spikes, surges and sags by ‘clipping’ the peaks and valleys, boosting power or switching to battery back-up.

As with offline UPS, when the input voltage fails or fluctuates outside of a pre-set tolerance window, the UPS detects this and a relay will close allowing the UPS to start feeding battery power via the inverter. The inverter, in a good line-interactive UPS, will supply a sinewave output. Upon the return of mains power, the output is switched back onto mains and the inverter is turned off.
As with offline UPS, typically there will be a break on the transfer to and from battery mode, though usually this will be shorter than with an offline UPS.

Some manufacturers will try to pass their line-interactive UPS off as online models by calling them ‘digital online’, ‘inline’ or ‘online interactive’ – make sure you know what technology the UPS you are buying actually uses.

Advantages:
Lower cost than online
Gives better protection than offline
Silent operation when in standby
Efficient

Disadvantages:
Fluctuations, such as spikes, can still be passed straight to the load
Break on transfer to battery mode.
No failsafe – UPS will drop the load if there is a high start-up current, overload or inverter failure.

Online Double Conversion Technology

An uninterruptible power supply using true online double conversion technology provides the highest level of power protection available. The UPS converts the 230V input AC mains supply to DC power which is then used to charge the battery. The DC current flow is then fed through an inverter stage which reconstructs the 230V AC mains output. Because the AC output is completely regenerated, it will be completely free from any mains-borne interference such as spikes and voltage variations.

The output voltage and frequency is controlled precisely, thus ensuring a clean and stable sinewave power output. Online UPS are able to withstand large fluctuations on the input voltage before transferring to battery power (typically 276V-184V) thus eliminating unnecessary battery discharges. Upon mains failure, transfer to battery power is seamless – no break. Online UPS also have various failsafe and self-diagnostic features that will instantly transfer the load onto mains power if there is a failure within the UPS hardware, or if the UPS is overloaded.

Advantages: Continuous & total power conditioning
Failsafe/overload protection with static bypass facility
No break on mains failure
Wide input voltage tolerance

Disadvantages: Because of the technology used, online usually costs more than other types of UPS technology

AEC Uninterruptible Power Supply Products

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