In the world of parallel-redundant uninterruptible power supply architecture, modularity means combining two or more UPSs to create a UPS system that is fault-tolerant, flexible and scaleable. Modularity and fault tolerance are crucial to the delivery of "six nines" availability – that is, 99.9999 percent up-time.
System availability or up-time is a measure of the time, usually expressed as a percentage, that the system is available for use. Highly available systems have a high value of MTBF (mean time between failures) and a low MTTR (mean time to repair).
Powerful modular UPSs are able to deliver unprecedented six-nines level of availability by combining short MTTR (a typical value of 15 minutes), and hot-swappability (the ability of UPS systems to tolerate the insertion and removal of UPS modules whilst still running, rather than having to be by-passing or shutdown, thereby exposing the critical load to the raw AC mains supply).
Power availability can also be expressed as downtime per year:
90 percent (one nine) availability equates to 36.5 days' down-time per year
99 percent (two nines) 3.65 days
99.9 percent (three nines) 8.76 hours
99.99 percent (four nines) 52 minutes
99.999 percent (five nines) 5 minutes
99.9999 percent (six nines) 31 seconds!
A fault-tolerant UPS system must be capable of supplying significantly more power than that demanded by the critical load which it supports. A modular system has the capability that in the unlikely event that one of the UPS modules develops a fault, then the remaining module or modules will instantly and seamlessly increase their output power contribution, evenly sharing the task of supplying the critical load. The load therefore continues to be supported with power.
A Modular UPS can take several forms: printed circuit board (PCB) assembly, rack-mountable unit, or free-standing (floor or desktop) unit.
Early parallel-redundant UPS systems required the system manager to assign each UPS module to either a 'capacity' or a 'redundancy' role. Such systems were known as 'N+n' parallel-redundant systems, where N is the coefficient of capacity, and n is the coefficient of redundancy
A modern modular parallel-redundant UPS, adjusts capacity and redundancy automatically in response to the prevailing critical load, by a system whose objective can be described as: "Redundancy if it can be, capacity if it must be!"
Scaleability is a key benefit of modular UPSs making them future proof with significant commercial benefits. Modularity gives the flexibility to right-size the system to the critical load, thereby reducing the total cost of ownership.
Anticipating the expansion of the critical load, users can purchase a flexible modular UPS system like the PowerWAVE9000DPA Series. With this system, a free-standing cabinet accommodates up to five rack-mountable UPS modules. Initially, if the load is less than the power rating of one UPS module, then the cabinet could be populated by two UPS modules, to provide 1+1 parallel redundancy.
Later, if the size of the critical load required it, then the cabinet could be populated by five modules. If the critical load were to demand, for example, between 60 and 80 percent of the total UPS capacity, then the system would be described as operating in 4+1 parallel redundant mode. If the critical load were to demand between 80 and 100 percent of the total UPS capacity then it would be described as 5+0. Populating slots with modules in single cabinets is commonly referred to as vertical scaleability.
If the size of the critical load were to increase beyond the capacity of a single cabinet then additional cabinets, partly or fully populated by UPS modules, could be connected in parallel to the first. Increasing the capacity of a UPS system in this way is commonly referred to as horizontal scalability.
As the requirements for data processing continue to increase, so will the demands on UPS Systems to keep pace with the requirements of IT systems. The combined features of modular UPSs will be critical to deliver the highest levels of power availability and efficiency whilst offering the flexibility to re-configure systems to match frequently changing loads.
For further information contact Peter Bentley, Uninterruptible Power Supplies Limited, Bacchus House, Calleva Park, Aldermaston, Berkshire RG7 8EN.
Tel: +44 118 981 5151. Fax: +44 118 981 5152. Email:sales@upspower.co.uk Or visit: http://www.upspower.co.uk/
Posted on November 13th 2008 in bomisasia.com edit
