UPS Update
Peter Bentley of Uninterruptible Power Supplies Limited looks at the recent advances in UPS technology.
Of the three main Uninterruptible Power Supply (UPS) architectures (or topologies) 'Off-Line', 'Line Interactive' and 'On-Line', only 'On-Line', particularly the three phase, rackmount modular derivative has seen significant technological development over the last 12 months.
'On-Line' UPS topology:
'On-Line' double-conversion UPS offers the greatest degree of supply integrity. The critical load is supplied with conditioned AC power at all times. It isolates the output power from all input anomalies such as low voltage, surges and frequency fluctuations.
Points that might influence the choice of an on-line system:
It offers the highest level of critical load protection - the load is supplied with closely regulated power at all times
No load break when transferring between inverter and bypass (in either direction)
Critical load fully isolated from all types of mains disturbances
Highest capital cost
Highest running cost - system efficiency is lower than the other types of system because the rectifier and inverter are permanently on load, although advances in on-line efficiency have been made.
It is widely accepted that the best way to maximise the availability of clean power is to use independent UPS modules operating in a parallel redundant (known as N+n) configuration.
There are two basic parallel redundant (N+n) configurations:
1) Centralised Parallel Architecture (CPA)
2) De-centralised Parallel Architecture (DPA)
CPA configured UPS systems comprise fewer components than DPA systems and consequently may be a slightly lower cost. However, the “centralised” nature of a CPA system means that potential single points of failure exist in CPA systems that do not exist in DPA systems.
De-centralised Parallel Architecture (DPA) provide the most comprehensive power protection currently available as when this architecture is combined with “hot swappable” rack mounted “on-line” UPS modules, previously unprecedented "six nines" (99.9999%) availability of critical power is achieved.
Decentralised Parallel Architecture (DPA) embraces the concept of removing single points of failure by connecting in parallel, independent, self-reliant, 3-phase UPS modules. DPA is state-of-the art for UPS systems for high end applications like data-centres and computer rooms:
Significant improvements to the reliability of UPS power support can be gained by conveniently operating the UPS system in a parallel-redundant (N+n) mode. In the unlikely event of a UPS module failing, it automatically isolates itself from the array and the remaining healthy paralleled modules continue supporting the critical load up to their collective capacity
The combination of DPA architecture and 'true' UPS module and battery hot-swap capability facilitate extremely low mean-time-to-restore (MTTR) which, in turn, allows "best in class" power availability of "six nines" (99.9999 percent)
In summary DPA is the most advanced UPS topology available for business critical applications offering reduced single points of failure, class leading availability, the smallest footprint in its class, high efficiency (95 to 96 percent efficiency between 30 and 100 percent full load rating), low whole-life costs and very low input current harmonic distortion (2 to 3 percent THDi between 30 and 100 percent full load rating)
Blade-server friendly - capable of supplying critical loads with a power factor up to 0.9 leading without the need for UPS de-rating.
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