Considerations for choosing a UPS

An uninterruptible power supply provides essential safeguards against equipment malfunction caused by failures in the mains power supply. But the variety of options available require careful consideration to match the best choice of UPS to specific needs. Neil Rasmussen, senior VP of innovation at Schneider Electric, explains:

Several factors must be considered when choosing an uninterruptible power supply (UPS) to provide emergency power to vital electrical systems in the event of a disruption to mains supply.

These include the electrical load of the equipment to be safeguarded, the quality of electrical signal required by such equipment and the speed of response of the UPS itself. Underpinning all selection choices are the cost considerations necessary to match the most appropriate protection for the equipment under load.

The basic function of a UPS is to switch over seamlessly to a backup power supply in the event that the utility supply is impaired. Typically this means a battery which provides temporary power while an alternative power source, such as a standby generator, is brought online. UPS systems come in a variety of power ratings and different topologies applicable to different requirements.

The simplest design of UPS, most commonly used for safeguarding a desktop computer, is the standby model. It is essentially a battery which supplies power to an inverter that converts the battery’s DC output to AC power. In the event that the primary power fails or falls below a preset level, a transfer switch automatically switches the load over to receive power from the inverter. This takes place in a matter of milliseconds, and the battery only becomes operable in the event of a power disruption, hence the standby name.

Standby UPS systems tend to be small, low cost and highly efficient. They are typically used to power loads up to 0.5kVA and are most suitable when used with standalone PCs.

The next most common type of UPS is the line-interactive model. In this design, the battery-to-AC converter (inverter) is always connected to the output of the UPS so that in normal operation, the battery is being charged by the mains supply.

The line-interactive design also includes a tap-changing transformer, which provides a measure of voltage regulation as the mains input voltage varies. This makes a line-interactive UPS particularly effective in situations where the primary supply is subject to frequent drops or spikes in voltage causing “brownouts” or surges as it automatically adjusts the output to the required level without switching over completely to battery mode.

As well as prolonging battery life, which is reduced by frequent use, the inverter of a line-interactive UPS can be designed in such a way that its failure will still permit power flow from the AC input to the output, thereby eliminating the potential of a single point of failure and effectively providing for two independent power paths.

Line interactive UPS systems typically provide backup power in the 0.5 to 5kVA range making them suitable for departmental servers, Web servers or networks of PCs in small businesses. They are highly efficient and have a medium cost per VA compared with other types of UPS. Although

the initial cost is typically higher, in the long term total cost of ownership is reduced because of the prolonged battery life.

For loads above 10kVA the most common type of UPS is the double conversion on-line model. In this configuration the primary power source feeds an AC-DC rectifier which charges the battery which is connected in turn to a DC-AC inverter. The main power path therefore always passes through the inverter so that, in the event that the primary power fails there is no delay in switching over to battery backup because the battery is already feeding the inverter. Double conversion therefore provides nearly ideal electrical output performance.

Choice of a UPS then depends primarily on the nature of the equipment that forms the load and to a lesser extent on the quality of the primary mains supply. For standalone equipment up to 0.5kVA, a standby UPS is perhaps the best option. For larger loads, such as departmental or web servers or small racks, or in locations with erratic mains supply, a line-interactive model provides high reliability and good voltage conditioning. For the largest installations a double-conversion online model should be considered.