Choosing an uninterruptible power supply is a major decision for many organisations. And it is not a question of simply comparing features and prices—because it is seldom possible to compare ‘apples with apples’.
Key to the decision is the drawing up of essential requirements. In order to guide potential acquirers of a UPS system or multiple systems, a recent report by the international survey firm of Frost & Sullivan, entitled “Centralised versus Decentralised Uninterruptible Power Supplies (UPS)—Pros and Cons” (October, 2006) is now available.
Customer requirements will vary but there are some that fall in a ‘universal’ area. Amongst these are ‘redundancy’ and ability to work with stand-by generators as well as causing minimum additional loading on the internal electrical power distribution system through poor power quality.
Redundancy provides a high degree of assurance as to continuity of supply in the event of failure of a part or parts of the UPS system. Stand-by generators are often chosen to have a rating of double that of the UPS system in order to function with a high degree of frequency and voltage stability, thus adding significantly to the cost of the overall system.
Additional loading of the electrical distribution network in the client’s premises is caused by the distortion of input current by the input stage charging the battery or battery bank (poor power quality). The Frost & Sullivan Report clearly sets out the various considerations necessary in order to come to an informed purchase specification and the means of evaluation of salient specifications.
For example, redundancy is a theoretical concept in that a guaranty of 100% availability of power is not possible. The Frost & Sullivan Report indicates that so-called ‘single points of failure’ whereby in practice the concept of redundancy is defeated, must not exist for critical applications.
The distortion of input current particularly for large UPS loads can cause overheating problems in sub-station transformers, voltage distortion within the internal power supply system, and react badly with energy cost-saving power factor correction equipment.
The Report is available from Power Parameters Pty Ltd as a service to the Australian Business Community. Power Parameters Pty Ltd, through its Power Protection business unit sells, installs, maintains and services the range of Socomec UPS systems.