By contrast an off line UPS, during normal operation, feeds raw mains to the critical load through a bypass line and static switch - see figure 2. 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. Many operators of data centers and other sensitive equipment consider off line UPS systems to be unsuitable for their applications because of these power quality
and changeover break factors. Another disadvantage of a basic off line
system is that, unlike an on line design, it cannot compensate for mains
supply voltage limit transgressions. Instead it must respond
immediately by switching between bypass and the inverter for every
transgression event. This can happen frequently if the mains supply is
unstable or the critical load has a tight voltage tolerance. Apart from
subjecting the load to frequent supply interruptions, the battery may
have its life reduced or be insufficiently charged as a result. However
applications which can tolerate these disadvantages enjoy two key cost
benefits from using off line topology.
Figure 3: Line interactive UPS with buck/boost transformer.
Firstly, an off line UPS reduces capital costs compared with an on line equivalent through using lower-rated components and dispensing with a power rectifier. Secondly, running costs are lower because the charger and inverter, with their inevitable inefficiencies, are not on line during normal operation.
Line-interactive topology choices
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 – see figure 3. 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 percent to -40 percent. 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 trade-off, especially as they can choose to operate the UPS in on line rather than Eco mode during periods of increased utility grid disturbances.