The bigger picture
Overall, operators responsible for data centers 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 eventualities. 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 lost. In some applications this may be acceptable, because the UPS battery autonomy provides enough time for the critical load to be shut down systematically and safely. In others, this will not be enough; any loss of service is unacceptable. One solution is to use a configuration known as a split bypass system, in which the bypass line and UPS rectifier are connected to different mains supplies. This means that if the UPS’s supply fails and its battery becomes exhausted, the UPS can switch to the still-healthy bypass line to deliver continued – although unprotected - power to the load.
Another widely used solution is to connect a standby generator as an alternative feed to the UPS. If the UPS detects a mains failure which is more than a transient problem, it can send a start-up signal to the generator. This can then run up to speed and stabilize within the UPS battery autonomy, so the load enjoys continued clean power without interruption.
As well as protecting the load from power supply problems, the UPS must also manage problems caused by the load itself. The UPS’s response to an overload may be to transfer it to bypass – but if the bypass supply is not available, this will simply deprive the load of power. A better alternative can be to allow the UPS to supply the overload for a set period; enough for automatic disconnection by a protection fuse or circuit breaker. During this period, the UPS itself can be protected by being operated in current-limited, reduced voltage mode, which is preferable to a total loss of power.
In summary, we can say that for critical applications or those involving sensitive equipment, on line UPS topology is the only real solution, due to its delivery of conditioned power and seamless transfer to battery. For less critical applications, off line topology may be suitable, and is attractive due to its reduced capital and operating costs. Line interactive systems offer an improvement on off line topology, with better power conditioning and regulation, and reduced battery wear. Ferro-resonant transformer line interactive solutions also offer seamless load transfer. Pressure to reduce PUE has increased interest in Eco mode operation, but this shares most of the characteristics and disadvantages of off line topology. The best possible power quality and availability results come from on line UPS systems designed into an overall power protection strategy.
About the author:
Mike Elms is Technical Sales Manager for Uninterruptible PowerSupplies Limited - www.upspower.co.uk
– he can be reached at email@example.com
Courtesy of EETimes Europe
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