Selecting the Right UPS for Today’s Facilities

April 3, 2006
Equipment damage, data loss, and spurious errors are usually caused by more common power quality issues

When most people think about poor power quality, blackouts and brownouts come to mind. But, these are some of the least-common power quality issues facing facilities. The most typical power quality problems are under-voltage (sags), spikes and surges, over-voltage, and noise. While outages are the most visible power problems, equipment damage, data loss, and spurious errors are usually caused by these more common power quality issues.

Non-Essential vs. Critical-Load Applications
The consequence of equipment failure is the single most important factor determining the type of UPS to purchase. The first step in selecting the right UPS involves performing an “electronic triage” assessment of the importance or value of the processes and hardware to be protected.

For small office PCs, less-expensive, single-phase UPSs can be an adequate solution. Typically, off-line or line-interactive topologies create a brief interruption when transferring to and from battery power, which may be acceptable for standard PCs and small non-critical servers. Because PCs, workstations, and peripherals are often located in a decentralized manner throughout offices, a dedicated line-interactive UPS can be an inexpensive source of very basic power protection.

If the equipment to be protected is critical, an online double-conversion UPS is the best choice; these systems regenerate pure power of significantly higher quality than line-interactive units. Fully isolating and protecting against all power disturbances, this topology transfers between battery and internal bypass without power interruption - providing a truly seamless transfer to battery power.

For individual or aggregate loads above 10 kVA supporting business-critical applications, the most practical and comprehensive solution is a three-phase, double-conversion online UPS that offers the advantage of providing centralized protection using a single UPS. This simplifies maintenance and battery replacement while supplying high-quality uninterruptible power to critical loads. Server farms, data centers, 7x24 offices, and critical process plants with higher loads are best served with single or parallel three-phase UPSs, often with a redundant UPS backing up the primary UPS.

An online UPS uses similar components to other UPS technologies with a few key differences - most notably, a rectifier that takes the input voltage and changes it from AC to DC voltage to charge batteries and provide DC power to the inverter. In most online UPSs, the rectifier or filtering is used to make sure that neither the load nor the UPS will feed unnecessary noise and harmonics back into a building’s power. Because online UPSs continually create power via the rectifier and inverter, there are no transfer interruptions. Make sure that the UPS you choose is a true online system; some UPS technologies cannot provide important filtering or genset compatibility, which is imperative for mission-critical applications. True online UPSs incorporate additional features - desirable for highly critical applications - such as sophisticated monitoring and diagnostics that include the ability to transfer to a bypass line in the event of a UPS internal fault or during maintenance. Today’s UPSs can be integrated with additional battery banks, flywheel power systems, and generators for even longer back-up times.

Brad Amano is market segment manager at Costa Mesa, CA-based MGE UPS Systems Inc. (

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