Data centers require power, in a stable and continuous supply, and it must be controlled carefully. Incoming fluctuations such as voltage dips or surges can damage sensitive electronic equipment, and power outages result in costly downtime. These hazards can be prevented with the use of uninterruptible power supply (UPS) systems. A UPS regulates the flow of power and, in the event of an outage, provides battery backup so operations can continue with little to no disruption. Battery power will last long enough to either shut down equipment safely or activate an alternative source such as a generator.
There are three main types of uninterruptible power supply systems, defined by how power moves through the unit:
In a standby or off-line system, the AC input is filtered through the unit and past the transfer switch at the output point. When an input failure occurs, the battery and the inverter (which converts the battery’s DC power to AC) are activated and feed the output. These uninterruptible power supply systems are relatively low-cost and efficient, work best in applications under 2kVA, and are frequently used for personal computers and workstations.
There are some subcategories. Standby-Ferro systems add a special “Ferro-resonant” transformer to help regulate voltage. This once-popular design for mid-range UPSs has fallen out of favor because of low efficiency and instability issues. Many uninterruptible power supply systems below 10kVA that are designated as on-line are actually standby on-line hybrids, since the inverter is on-line but the DC/DC converter from the battery is in standby.
With line-interactive uninterruptible power supply systems, the inverter becomes part of the output and is always on. The inverter can operate in reverse to charge the battery while AC input is normal, and switch to battery power when input fails. This design provides increased filtering and voltage regulation. This highly reliable and efficient system is well-suited for small business, Web, and departmental servers, though it becomes impractical in applications over 5kVA.
For higher voltage applications, an online UPS is best. In an online system, the inverter is the primary power path instead of the AC input. There are two major types of online uninterruptible power supply systems:
- Double Conversion: In this approach, often used above 10kVA, the AC input is converted to DC power to charge the battery, and then flows through the inverter, which converts it back to AC for output. This double conversion process virtually eliminates any spikes or dips from the input, but lowers efficiency and increases heat generation as well as wear on components.
- Delta Conversion: For applications between 5kVA and 1MV, this relatively new model was designed to counter some of the drawbacks of double conversion. Its function is similar to the double conversion system, but it adds a delta transformer that routes some of the input power directly to the inverter output after regulating it, saving energy by shortening the path between input and output. The delta conversion online uninterruptible power supply system is the only one with patent-protected technology, which means it is not likely to be widely available except from industry leaders such as APC.
As the development of the delta conversion approach shows, UPS technology continues to evolve. This 2016 white paper from APC by Schneider Electric discusses the growing advantages of lithium-ion batteries over lead acid in UPSs. There is no “one size fits all” UPS solution, but a range of types for different applications. Your VAR can help you navigate the available choices and select the best uninterruptible power supply system for your data center.