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EETE JULAUG 2012

DESIGN & PRODUCTS POwER SUPPlIES 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 inef- ficiencies. 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. The bigger picture overall, operators responsible for data centres and other critical or sensitive loads prefer the processed, ‘no break’ power that only on line upSs can provide. However to make the most of the topology and its potential for protection, the upSs should be installed as part of a power scheme that caters for all eventuali- ties. For example, suppose the UPS rectifier and bypass line are fed from a common power source, which at some time suffers a power failure period exceeding the autonomy of the UPS battery. The upS has no further options, so power to the critical load is Fig. 3: Line interactive UPS with buck/boost transformer. lost. in some applications this may be acceptable, because the to be unsuitable for their applications because of these power upS battery autonomy provides enough time for the critical load quality and changeover break factors. to be shut down systematically and safely. in others, this will not Another disadvantage of a basic off line system is that, unlike be enough; any loss of service is unacceptable. an on line design, it cannot compensate for mains supply volt- One solution is to use a configuration known as a split bypass age limit transgressions. instead it must respond immediately by system, in which the bypass line and UPS rectifier are connected switching between bypass and the inverter for every transgres- to different mains supplies. This means that if the UPS’s supply sion event. This can happen frequently if the mains supply is fails and its battery becomes exhausted, the UPS can switch to unstable or the critical load has a tight voltage tolerance. apart the still-healthy bypass line to deliver continued – although unpro- from subjecting the load to frequent supply interruptions, the tected - power to the load. battery may have its life reduced or be insufficiently charged as another widely used solution is to connect a standby genera- a result. However applications which can tolerate these disad- tor as an alternative feed to the upS. if the upS detects a mains vantages enjoy two key cost benefits from using off line topol- failure which is more than a transient problem, it can send a ogy. Firstly, an off line UPS reduces capital costs compared with start-up signal to the generator. This can then run up to speed an on line equivalent through using lower-rated components and stabilise within the upS battery autonomy, so the load enjoys and dispensing with a power rectifier. Secondly, running costs continued clean power without interruption. are lower because the charger and inverter, with their inevitable as well as protecting the load from power supply problems, the inefficiencies, are not on line during normal operation. upS must also manage problems caused by the load itself. The upS’s response to an overload may be to transfer it to bypass – Line-interactive topology choices but if the bypass supply is not available, this will simply deprive Some UPS manufacturers also offer line-interactive UPS systems. the load of power. a better alternative can be to allow the upS These use off line topology, but address the voltage transgres- to supply the overload for a set period; enough for automatic sion problems of off line systems mentioned earlier. They do so by disconnection by a protection fuse or circuit breaker. during this providing voltage regulation in the raw mains line which supplies period, the upS itself can be protected by being operated in the load during normal operation – see figure 3. This voltage current-limited, reduced voltage mode, which is preferable to a regulation, by reducing the number of ‘out of limit’ voltage events total loss of power. reaching the load, lowers the frequency of mains – battery trans- in summary we can say that for critical applications or those in- fers. apart from subjecting the load to fewer power interruptions, volving sensitive equipment, on line upS topology is the only real this design offers slightly better running cost efficiency and less solution, due to its delivery of conditioned power and seamless battery ‘wear’ than off line systems. transfer to battery. For less critical applications, off line topology Line-interactive systems are usually implemented with buck/ may be suitable, and is attractive due to its reduced capital and boost transformers or ferro-resonant transformers. Buck/boost operating costs. Line interactive systems offer an improvement on transformers will maintain their load voltage over an input voltage off line topology, with better power conditioning and regulation, range of +20% to -30%, however their switching involves tap and reduced battery wear. Ferro-resonant transformer line interac- changes which impose step voltage changes on the load. line tive solutions also offer seamless load transfer. Pressure to reduce interactive implementations based on ferro-resonant transform- puE has increased interest in Eco mode operation, but this shares ers can typically supply a load voltage within 3% of nominal over most of the characteristics and disadvantages of off line topology. a raw supply range of +20% to -40%. They also provide power The best possible power quality and availability results come from conditioning and protection from disturbances such as electrical on line upS systems designed into an overall power protection line noise. However, their key advantage is that they store enough strategy. electrical energy to support most pC loads while the upS inverter 22 Electronic Engineering Times Europe July/August 2012 www.electronics-eetimes.com


EETE JULAUG 2012
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