Today, uninterruptible power supply (UPS) systems are typically employed to support data centers, communications hubs and other applications using sensitive ICT equipment. These installations’ continuous availability is usually critical to their owners’ survival, so a clean, uninterrupted power supply becomes a business-critical requirement. Under these circumstances, on line UPSs are understandably the most popular choice. They deliver processed, clean power under all conditions and transfer to battery without power interruption if the mains fails. However there are still many applications with less critical requirements; these can benefit from the lower costs of an off line UPS solution.
This article therefore compares on line and off line UPS topologies, together with a third variant – line interactive topology. As rising energy costs sustain the demand for better energy efficiency, this article also reviews Eco mode UPS operation and its contribution to improved PUE. Applications that use UPSs vary greatly in size as well as nature. Small desk top or instrumentation devices may run from a portable unit with a capacity of up to 250 VA, whereas a large data centre serving a significant enterprise could call for multiple units delivering up to 5 MVA or more.
On line UPS topology and advantages Irrespective of their size or topology, all UPSs depend on a battery, a means of charging it and a method of converting its DC output to AC for the critical load during a mains failure. An on line UPS performs these functions with a series of power blocks between the mains supply and the critical load - see figure 1. The first of these is a rectifier/charger which float charges the battery while the mains is present. It also supplies a stable DC voltage to the second block - an inverter which feeds the critical load. During a mains failure the floating battery seamlessly takes over the task of supplying DC to the inverter, to which it is permanently connected. Therefore, the event remains truly invisible to the critical load unless the battery becomes discharged. This seamless power continuity is a key attraction for critical applications that may not be capable of riding out a switching delay. Another major advantage arises from the on line UPS’s dual conversion topology. The input rectifier removes all the incoming mains spikes, transients and noise by converting it into DC.
Figure 1: On line UPS operation
Figure 2: Off line UPS topology
The inverter then feeds the load with a clean new AC waveform which is tightly regulated by the UPS. The rectifier also provides power factor correction, which reduces high frequency harmonics and the current drawn from the mains. The on line UPS acts as a true power firewall, protecting sensitive instrumentation and ICT equipment from mains-borne problems while improving the load presented to the mains.
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