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Selecting the right UPS topology depends largely on three factors: your environment, your budget and the level of sensitivity of the equipment you want to protect. Because modern IT power supplies are built to withstand a wide range of power quality issues, a variety of UPS topologies provide some degree of protection against power loss and disturbances in the electrical system.
The most cost-effective topology available, a standby or offline UPS provides surge suppression and battery backup in the event of an input voltage sag or power outage. This type of system allows equipment to run off of utility power while providing continuous surge protection, then switches to battery mode during voltage sags or outages. Standby topology is best suited for small office/home office and point-of-sale applications, providing solid protection for equipment such as LCD monitors, docking stations, modems and routers. Small size, simple design, low cost, ease of use and high efficiency are among the primary benefits of this topology.
With a highly reliable and efficient design, line-interactive UPS topology is the most widely used to support loads under 10 kVA. Ideal for distributed IT equipment such as servers, switches and storage, these UPSs actively regulate voltage by either boosting or decreasing utility voltage as needed before allowing it to pass to the protected load. Available in a wide range of shapes and sizes, line-interactive UPS models boast high energy efficiency, reliability, small size, moderate cost and the ability to correct low or high voltage conditions, making them attractive for many applications.
With the ability to filter even the fastest power quality disturbances, online UPS topology offers optimal protection for highly sensitive loads and are ideal for use in environments with poor power quality. Unlike the other topologies, an online UPS provides zero transfer time to battery. Because its design is more complex, the online UPS price is usually greater than that of standby or line-interactive UPSs. Online UPSs re-create the voltage output from raw utility power by converting it from AC to DC and back to AC, making them best suited for highly sensitive IT equipment, PLC power supplies, ice cube relays and other sensitive electronics.
The difference between online, off-line and line-interactive UPSs is also evident in the topologies’ battery lifespans. It’s important to note that battery lifespan is affected by a number of different factors, including frequency of use and extremely cold or hot environments. Although a lead-acid (VRLA) UPS battery is typically rated for a three- to five-year lifespan, frequent transfers to battery and high ambient temperatures can reduce expected battery life and increase the need for recurrent battery changes.
Because standby UPSs often switch to the battery when installed in environments with poor utility voltage, battery replacement may be required more frequently than if using a line-interactive UPS. The more the voltage fluctuates, the more the offline UPS resorts to the battery.
In line-interactive UPSs, the use of an automatic voltage regulation (AVR) transformer to correct abnormal voltages prevents excessive battery use during undervoltage and overvoltage conditions. Some line-interactive UPSs also feature a more advanced battery charging and management scheme than standby UPSs such as Eaton’s ABM technology, which helps extend battery life and provides early warning of battery failure. In addition, certain line-interactive UPSs are able to provide longer runtime with the addition of external battery packs.
Online UPSs only draw power from the batteries only when the utility voltage is extremely low, high or when a complete power loss occurs, which may decrease the need for costly and frequent battery replacements. Most online models have similar battery management schemes as higher-end line-interactive units, which further extends battery life. Additionally, an online UPS can provide long periods of runtime by adding extra battery packs like some line-interactive designs.
