Today, 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 fail. However there are still many applications with less critical requirements; these can benefit from the lower costs of an off line UPS solution.
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. 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. This seamless power continuity is a key attraction for critical applications that may not be capable of riding out a switching delay.
Off Line Topology
By contrast an off line UPS, during normal operation, feeds raw mains to the critical load through a bypass line and static switch. The load is therefore exposed to poor voltage regulation and possibly noise and transients from the mains. If the mains transgresses preset limits, the static switch transfers the load to the UPS’s inverter and battery a changeover break that can last from 2ms to 10ms.
Line-interactive Topology
Some UPS manufacturers also offer line-interactive UPS systems. These use off line topology, but address the voltage transgression problems of off line systems mentioned earlier. They do so by providing voltage regulation in the raw mains line which supplies the load during normal operation. This voltage regulation, by reducing the number of ‘out of limit’ voltage events reaching the load, lowers the frequency of mains – battery transfers. Apart from subjecting the load to fewer power interruptions, this design offers slightly better running cost efficiency and less battery ‘wear’ than off line systems. Line-interactive systems are usually implemented with buck/boost transformers or Ferro resonant transformers. Buck/boost transformers will maintain their load voltage over an input voltage range of +20% to -30%, however their switching involves tap changes which impose step voltage changes on the load. Line interactive implementations based on Ferro resonant transformers can typically supply a load voltage within 3% of nominal over a raw supply range of +20% to -40%. They also provide power conditioning and protection from disturbances such as electrical line noise. However, their key advantage is that they store enough electrical energy to support most PC loads while the UPS inverter switches on, effectively creating an on line UPS solution. With today’s rising energy prices, pressure to improve energy efficiency and data centre PUE has renewed interest in eco mode operation of on line UPSs. This mode allows users to route raw mains directly to their critical load during normal operation to save the energy that would otherwise be lost through the UPS’s inefficiencies. However, in exchange for this energy gain of perhaps 3%, the critical load is being exposed to mains disturbances and transfer events, as with an off line UPS. Some users will accept this tradeoff, especially as they can choose to operate the UPS in on line rather than Eco mode during periods of increased utility grid disturbances.