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Uninterruptible Power Supply Topologies - Line Interactive

The uninterruptible power supply (UPS) inverter only switches on (with a small break in supply) when the mains fails or fluctuates wildly outside the input voltage window of the built-in Automatic Voltage Stabiliser (AVS). An EMI filter will typically help to reduce spikes and electrical noise when mains is present.

The normal output waveform on battery is a step wave or square wave. Some UPS offer sinewave supply such as the Dialog Vision. A line interactive UPS offers an economic alternative to an on-line system where the investment cannot be warranted.

INTERMEDIATE level power protection - from 500VA to 5kVA (single phase only) - Voltage Independent (VI)according to BS EN 62040-3:2001.

The line interactive uninterruptible power supply provides a clean and regulated output voltage. When the mains supply is within a specified input window (voltage or frequency) the output from the Uninterruptible Power Supply is stabilised to within a specific voltage tolerance or window; this is achieved using various voltage regulation techniques known as Automatic Voltage Regulation (AVR) or Buck/Boost.

The size of variation is larger than that achieved with an on-line Uninterruptible Power Supply which will actually regulate to a much tighter tolerance. Once the incoming mains supply exceeds the specified window, the UPS switches on the inverter to continue to supply power to the load. There is a short inverter switch on time of a few milliseconds.

How The Line Interactive Uninterruptible Power Supply Tackles Power Problems:

Output Waveform

When the mains supply is present it is filtered and stabilised by the UPS and passed through to the load. The output waveform from the inverter is normally either a true sinewave or pseudo sinewave (dependent on UPS type). The term pseudo indicates a step-wave or square-wave output. The step-wave or square-wave output may not be suitable for all types of loads.

Sags and surges

The load is supplied by the incoming mains supply which is filtered and stabilised using the automatic voltage regulation device; whilst the inverter remains switched off (provided that the input window is not exceeded). The battery charger continues to convert the mains AC into DC to charge the battery. If the sag or surge exceeds the input window then the inverter is switched on to maintain the required output voltage tolerance.

Transients, spikes and electrical noise

The output to the load is filtered by the Electro Magnetic Interference (EMI) and noise filters within the UPS. These filters attenuate (reduce) the incoming transients, spikes or electrical noise down to an acceptable level. These disturbances will not be completely prevented from passing through to the load and any large voltage spikes or transients that may exceed the input voltage window will cause the UPS to switch on the inverter.


The output from the UPS is maintained to within a specific window; therefore the UPS will automatically adjust the transformer tap settings dependant on the incoming mains supply voltage. Once the incoming mains supply exceeds the specified window, the UPS switches on the inverter to continue to supply power to the load.

Short duration mains failures

During short duration mains supply failures the UPS will supply power to the load using the inverter. However constant short duration supply failures can lead to unnecessary discharging of the battery.

Long duration mains failure - blackouts

The load is supplied by the inverter which is switched on the moment the incoming mains supply fails. Some line interactive UPS have a battery extension pack capability, that enables additional battery packs to be connected. This in turn extends the autonomy of the UPS (battery operation time). The only other alternative is to oversize the UPS to achieve a longer runtime as most line interactive UPS are only provided with a short autonomy of approximately 10 minutes.


The battery charger will generally recharge the battery to 80% within 8 hours to provide sufficient battery autonomy ready for the next supply failure. Where it is possible to extend the battery capacity (typically from 2kVA and above), additional battery chargers can be added to maintain a reasonable recharge time. The internally charger is not normally capable of supporting additional packs as the UPS is built to a tighter price specification than an on-line UPS.

Other advantages

The advantages of a line interactive UPS are size and cost when compared to an on-line power solution. However they offer inferior power quality management capabilities, extended battery runtime options, have a short break in supply when the inverter is activated and do not have a built-in bypass facility for UPS fault or overload scenarios.