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Electrical power security for data centres

04 May 2011

Les Hunt reports on recent UPS developments for data centre operators and installers of mission-critical IT equipment

According to Schneider Electric' specialist UPS subsidiary, APC (http://www.apc.com/), there are a number of key criteria to consider when selecting which type of UPS (uninterruptible power supply) system will provide optimum protection for a data centre at a competitive price. APC's Shri Karve spells them out: criticality, UPS type, configuration/availability, load rating/battery autonomy, maintainability and installation/structural considerations.

Where data centres are concerned, the very first question is one of mission criticality; in other words, what are the consequences for an organisation if a mains disruption or outage crashes its computer network. Continuity of power and quality of power are essential to the reliable operation of IT equipment and the successful delivery of IT services.

Mr Karve says that in certain modes of operation, some 3-phase UPS systems provide no better protection than the cheapest UPS you can find in a local PC store. "What’s more," he warns, "they expose IT equipment to raw mains power – a situation that no serious data centre operator would countenance under normal circumstances.”

Highly complex operational mode switching may only achieve 99% efficiency momentarily. By contrast, Double Conversion (VFI) UPS is a proven technology and up to 97% efficient. APC’s Symmetra PX and Symmentra MW, for example, are modular and scalable and will protect an IT load continuously against mains variations and outages. With no switching risks, these provide peace of mind where IT continuity, resilience and efficiency are concerned.

Emerson Network Power (http://www.chloridepower.com/) recently extended its mid-size, modular Chloride MP-NET UPS series; the 98% efficient system now provides 50% more power for the same footprint size. The new unit, providing up to 120kVA of uninterruptible power for data centres, is a hot-swappable modular UPS system that can be 'grown' to meet changing load requirements.

If loads increase, or additional N+1 parallel redundancy is needed, the Chloride MP-NET allows up to six 20kVA modules to be slotted in, hot swapped or reassigned within each 120kVA UPS. Its parallel capability means that four units can together provide up to a maximum of 480kVA (or 460kVA N+1) of protected power. This modularity also avoids having to run oversized standalone UPS at partial loads, a common practice associated with big throughput losses, even for UPS otherwise capable of achieving high operating efficiencies. The Chloride MP-NET’s modularity allows headline efficiencies to be maintained at off-peak loads of as little as 40% of total load.

For more modest IT installations, Rittal's (http://www.rittal.co.uk/) new range of PMC 12 UPS systems can be used as a tower or in a 19in rack mounted format. Available in 1, 2, 3, 4.5 and 6kVA sizes, both the 4.5kVA and 6kVA versions can be paralleled to give additional power or additional resilience. The PMC 12 features double-conversion topology, which provides the basis for an optimum supply voltage to all connected loads - ideal not just for the IT sector, but also for control and automation applications.

Riello UPS (http://www.riello-ups.co.uk/) recently introduced its Premium Pro series covering the power range from 700VA to 3kVA and satisfying a growing demand for energy-efficient UPS systems. Featuring a digitally controlled inverter to provide true on-line power protection, and coupled with a new rectifier-charger, the Premium Pro operates at 0.99 power factor, saving 10% more energy than its predecessor in full on-line mode. The UPS avoids unnecessary power consumption by sensing circuits and battery chargers, and can also be configured to power-off automatically when no load is present, further reducing energy consumption.

The inverter section is designed for extended runtime applications and additional battery packs can be added, which can run for several hours. A built-in front mimic panel and LCD provides operating status and alarm information, while interface card slots, USB and serial communication ports provide remote monitoring and network connection.

Batteries are, of course, an essential part of the UPS installation. LEM's (http://www.lem.com/) new Sentinel 3+ battery monitoring transducer upgrade continuously monitors the state of health of batteries in UPS systems, and offers easy integration with the latest UPS architectures, including the new transformer-less circuit topologies. The device measures voltage, temperature and impedance of cells and complete batteries in UPS installations of all sizes up to MW-sized systems and reports its measurements (on valve-regulated lead-acid, gel or flooded stationary batteries) to supervisory systems over a dedicated communications bus. It is able to assess the true state-of-health of UPS batteries while they are in service, identifying weak and failing cells without the need to remove them from service for cycling tests.

With the elimination of transformers from modern circuit configurations, battery packs operate in floating voltage mode and thus subject to higher ripple currents. Consequently, the monitoring circuitry must handle high common-mode voltages, with superimposed high-amplitude, fast transients. Sentinel 3+ features upgraded algorithms to take account of this more challenging  measurement environment, delivering voltage measurements over a range of 0.9 to 16V with an accuracy of +/-0.5%, and impedance measurement from 0.05 to 250 mΩ with a repeatability of +/-2%. Sentinel 3+ features Common Mode Transient Immunity up to 20kV/µsec with a common-mode voltage level of +/-600V, and a transient repetition rate of 20kHz.


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