Business and organisational processes rely on information technology and this in turn requires a clean source of stable and reliable electrical power. The increasing demands for power on the UK electricity grid are placing unprecedented pressure on an aging infrastructure and this is becoming even more evident in heavily populated areas such as London’s Canary Wharf. Here demand for electrical power is greater at times than can be supplied to power the banking industries data centres and IT operations.

Defining Uninterruptible Power Supplies

UPS Systems, uninterruptible power supplies, back-up power systems all have one fundamental characteristic; they provide an instantaneous or almost instantaneous source of battery power when their primary AC supply fails – be this the mains power supply or some alternative source such as a standby generator.

An uninterruptible power supply sits between the mains power supply and critical load to be protected – the loading being any application, system or complete data center site that needs a continuous source of electrical power. When mains power is present a UPS provides filtering and conditioning surpressing, clamping or quite simply filtering to zero the impact from harmful power problems such as spikes, electrical noise, harmonics, sags, surges and brownouts. When the mains power supply fails (a blackout or power outage), UPS systems use their battery sets to provide back-up power as an alternative energy source. Running an uninterruptible power supply can quite simply ensure ‘business as usual’ – IT systems, phone systems, services, estores, data centres and operations simply continue and ride through the power failure.

Selecting An Uninterruptible Power Supply

There are several types of uninterruptible power supply or UPS topology.

On-line double-conversion UPS: with a constantly running inverter providing a conditioned, regulated AC supply. The inverter draws a DC supply from its rectifier which is powered from the mains or batteries.
Line interactive UPS: with a built-in automatic voltage stabiliser providing voltage stabilisation when mains power is present and an inverter that switches in when the mains power supply fails.
Standby UPS (Off-line): with a basic filter when mains power is present and an inverter than turns-on when the mains power supply fails.

On-line UPS systems are also known as ‘double-conversion UPS’ because of their operation, converting mains AC power to DC (rectification) to charge a battery set and then converting this DC back to AC (inverter) to power the load. An online UPS has a constantly running digitally controlled inverter which will provide a stable, voltage and frequency independent (VFI) output waveform (sinewave). When mains power is present, the rectification process and additional built-in filters providing filtering and conditioning.

Apart from the quality if its output waveform, On-line UPS also include an automatic bypass system. This monitors the output waveform of the inverter and will transfer the load to the mains supply, automatically should the inverter output wave star to collapse – either due to an internal fault or overload condition.

On-line UPS systems run from 700VA typically up to 800kVA with options for parallel/redundant N+X configurations (for additional system resilience, higher MTBF (mean time between failure) and availability) from 10kVA upwards.

There are also typically two types of on-line UPS system: transformerless and transformer-based systems. Transformer-based UPS systems are most commonly used to protected large applications such as datacentres, entire operating sites, hospital and other critical applications. They tend to have a slightly lower operating efficiency and heat output than a transformerless system. Transformerless systems offer smaller footprints, size and weight, higher operating efficiency, silent operation (except for their fans) and lower heat outputs. On-line UPS tend to be tower formats with rackmount options. They are ideal for critical applications.

A Line Interactive UPS system is voltage independent (VI) only. The UPS uses built-in passive electronic filters to surpress spikes and electrical noise. An automatic voltage stabiliser (AVS – also known as an automatic voltage regulator AVR) stabilises any fluctuations in voltage (sags, surges and brownouts). The output frequency of a line interactive UPS therefore follows the input frqeuency. The UPS tracks the frequency and switches on its inverter when this falls outside pre-set operating limits. The same function applies to voltage fluctuations and mains power breaks. The line interactive UPS typically runs from 300VA to 3kVA. However, some on-line UPS also include line interactive as an optional operating mode – known as an ‘ECO Mode’ because of its higher operating efficiency. The output waveform may be a sinewave, step-sinewave or squarewave and care should be taken when selecting a line interactive UPS as its waveform may not be suitable for the identified loads. Line Interactive UPS tend to be tower formats with rackmount options. They are ideal for larger PCs, file servers and other such applications.

Standby or off-line UPS are passive UPS systems. They typically run from 300VA to 1kVA and their output waveform simply tracks the input mains voltage and frequency. The UPS will include some form of basic filtering to provide protection from spikes and electrical noise but will resort to battery power and an inverter set-up to switch on when the mains power voltage fluctuates wildly or fails. Standby UPS tend to be desktop sized and are ideal for the protection of small PCs, EPoS terminals and office peripherals.

Battery Runtime and Extended Runtime Options

Battery runtime for uninterruptible power supplies is load and battery size dependent. Most UPS have an internal battery pack and the lower the load placed on the UPS, the longer the battery will run for on mains power supply failure. Most standby and line interactive UPS rely on UPS oversizing to provide longer runtimes. The larger UPS systems tend to offer an extended runtime pack option but for a further limited operating period to protect their inverter systems.

On-line UPS systems on the other hand have a constantly running inverter. This can again be ‘under-loaded’ to increase battery back-up time but on-line uninterruptible power supplies also offer the option to connect battery extension packs. Off course for larger UPS systems rather than increase battery packs, standby generators and even fuel cells are used.

Maintenance Bypasses

For critical installations, an external maintenance bypass is always recommended. This allows the UPS system to be maintained during normal working-hours without disruption to the connected loads. The UPS can be safely isolated and disconnected for removal, service, swap-out or upgrade without interruption.