FACILITIES power+cooling
TODAY, data centre and ICT system operators face multiple pressures: Demand for data processing capacity is growing rapidly, while the costs of providing this capacity are increasing sharply. An effective energy strategy will help to ease this scenario, partly by controlling ever-rising energy costs, but also by addressing today’s legislative, social and ultimately financial pressure to ‘go green’.
Space efficiency is also increasingly important, as operators seek to pack expanding volumes of data processing equipment into limited floor area. These pressures are exacerbated as the rate of growth can be difficult to predict. How can you plan effectively for future growth if you’re not sure exactly what its rate will be? Today, uninterruptible power supplies (UPSs) are essential to any significant data centre or ICT facility and they must be large enough to handle all the critical equipment’s power. Any energy or space saving achieved by the UPS system contributes significantly to the efficiency of the entire facility.
With these considerations in mind, it is worth investigating the benefits offered by modern, modular UPS technology. This is because these benefits lead to not only improved energy and space efficiency, but also to powerful scalability and efficient management of rapidly changing capacity demand.
Transformerless technology When static UPS systems began appearing in the seventies, they utilised an internal transformer to step up the output voltage to the critical load – not, as commonly believed, to provide galvanic isolation. Transformer- based systems were very widely produced, especially for the highest power applications.
However, advances in power semiconductor technology, particularly the introduction of the Insulated Gate Bipolar Transistor (IGBT) device have made the transformerless UPS systems a viable proposition. Fig.1 shows a transformerless UPS design in which a fixed rectifier uses a DC converter to boost its DC output to a high level. With this level, the inverter can directly produce an output RMS voltage compatible with the rectifier input supply, and the requirement of the critical load.
If the mains supply fails the battery is instantly connected to the unregulated DC busbar by an electronic switching device such as a fast-acting SCR. The DC boost
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converter continues to supply the regulated DC busbar, so power to the critical load remains uninterrupted. As with transformer- based designs, a long-term power blackout will eventually exhaust the battery and shut down the inverter.
Transformerless topology has largely taken over from traditional transformer-based designs because of the many advantages it offers. Above all, it delivers improved efficiency from a considerably smaller and lighter form factor, while a number of further benefits also apply.
Fig.2 shows the scope of the efficiency benefits, which extend over the entire load spectrum. Overall, efficiency is improved by around 5%, which yields substantial reductions in both direct energy costs and indirect cooling costs.
Transformerless UPS systems also present a higher input power factor than their transformer-based equivalents. The phase- controlled input rectifier used within the transformer-based systems has a lagging input power factor which falls further from unity as the UPS systems load reduces.
By contrast, transformerless topology utilises a free-running rectifier and a DC – DC boost converter with very high- frequency switching; these factors produce an input power factor inherently much closer to unity and less load-dependant than the transformer- based UPS systems. Moving the power factor towards unity reduces input current magnitude; this reduces
cabling, switchgear sizing and possibly electrical energy costs.
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