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Energy Efficient Uninterruptible Power Supplies

Published by Jason Koffler on 14 February 2010

Datacentre managers are under increasing pressure to manage and reduce their energy consumption. Whatever the hardware they are installing, its green credentials can be a key deciding factor, and this is becoming more so with Uninterruptible Power Supplies. It is not just greater environmental drivers or awareness of Corporate Social Responsibility (CSR) which is driving the upgrade to energy efficient UPS but the cost of electrical power and the need to manage operational budgets.

Green and ECO UPS Technologies

The traditional misconception when it came to Uninterruptible Power Supplies was a manufacturer lead argument between On-line and Line Interactive UPS. These operating modes affect system resilience with on-line being the most secure. The Line Interactive operating topology has been the de facto standard for UPS below 2kVA for many years. This is partly related to the investment in the IT systems protected being relatively small and therefore requiring a cheaper form of UPS. Whilst On-line UPS are available below 2kVA they tend to be higher cost due to their more complex electronics which are designed for continuous rather than intermittent running (namely the inverter section) and their built-in automatic bypass.

However, UPS manufacturers in their bid to raise operating efficiencies are starting to blur the distinction between the two technologies. The automatic bypass in an on-line UPS is there to route the load safely to the raw (or a filtered) mains supply, should the UPS inverter fail. UPS manufacturers are using this feature as one way to provide higher operating efficiencies.

For some on-line UPS it is possible to instruct the UPS to actually operate in a line interactive mode. Typically this feature is available on UPS from around 5kVA upwards (with some manufacturers offering this even as high as 800kVA). The inverter is powered but does not power the load. Instead the load is routed via the automatic bypass through built-in filters. The operation is principally that of a line interactive UPS with manufacturers calling this an ‘ECO Mode’ and quoting UPS system operational efficiencies of 98% or greater. However, when run in full on-line mode the UPS efficiency drops to around 95% or less.

Apart from ‘specmanship’ what can UPS manufacturers do to raise the operating efficiency of their UPS systems?

Transformerless and Transformer-based UPS Technologies

The main approach for on-line UPS manufacturers is to focus on how their UPS interact with the mains power supply in terms of design: how they draw power from the mains supply, their internal rectification, conversion and inversion processes and how this is delivered either break-free or with a minimal break to the load.

Transformerless UPS designs have been made more compact and efficient through the use of more sophisticated inverter assemblies, moving from double to triple conversion. Transformer-based UPS tend to focus on the use of IGBT-based rectifiers and more efficient transformer design. This is especially evident for large MVA-rated installations but even at around 100kVA the savings can contribute towards a higher PUE figure when determining datacentre efficiency.

The upside for datacentre managers is operating efficiencies of 96% or greater at full UPS load. Even a 1% rise in operating efficiency can save a datacentre manager thousands in electricity costs over the operational life of a UPS and a faster capital payback.

The Role of The Critical Power Specialist

Understanding UPS specifications has become slightly more involved as UPS manufacturers update their UPS systems to the very latest energy efficient technologies. Their specifications can require more thorough examination if datacentre and other critical IT systems users are to install the most efficient, resilient and secure uninterruptible power supplies and power solutions.

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