Designing Uninterrupted Power Supplies For Data Centres

Today’s modern data centres face an uphill struggle. Before even considering power protection, many are facing difficulties even securing enough supply for their sprawling facilities. Research organisation Gartner predicted that by this year (2008) 50% of currently established data centres would have insufficient power and cooling capacity to meet demands. Quocirca found that 43% of data centres are aware of an approaching power constraint with 14% (19% in the USA) having already reached their limit.

This is due, in part, to server sprawl as well as a lack of consolidation between procurement, application management and energy use. But even with a complete change around in management data centres still need to elevate power generation and protection further up the boardroom agenda as their businesses, and those of their customers, rely so heavily on power continuity.

Data centre power protection must focus on availability, redundancy, resilience and serviceability with uninterruptible power supplies at the centre and as the bridge between mains power and standby power (whether a diesel generator, fuel cell or other source).

The most important first step towards uninterrupted power is to categorise loads into critical, essential and non-essential and then size those that warrant UPS protection in terms of their energy use. Critical loads (IT infrastructure, servers, networks, routers and so forth) are those that the business simply cannot function without. They will require UPS protection and redundancy and may even warrant extended runtime. Essential loads (heating and emergency lighting, for example) are those that do not necessarily directly affect business continuity but which may be required for health & safety reasons. They may need UPS protection to ensure their continuity until generator start-up but may not require redundancy. Non-essential loads (printers, canteen facilities) can be temporarily lost in a power failure and do not require any form of UPS protection.

Sizing an uninterruptible power supply system can be tricky. If it is significantly oversized it will run inefficiently and cost more to install. Conversely, ‘undersizing’ will introduce the risk of system overloads. Whilst an on-line uninterruptible power supply has a built-in automatic bypass for emergencies, running close to design limits with regular overloads is bad practice.

UPS Sizing: understanding the importance of ‘real power’ is crucial for power protection sizing. Kilowatts (kW) are a measure of the real power drawn by the load whereas kilovolt-amps (kVA) are a measure of apparent power. The difference between the two is the power factor (pf) and its size presents challenges when specifying UPS.

The greatest efficiency comes from operating at a power factor of 1.0 or ‘unity’. An uninterruptible power supply, with as high an output power factor as possible, should be specified. A power factor of 0.9 is the standard set by reputable UPS manufacturers like Riello UPS.

Power protection systems should consist of units offering an input power factor of not less than 99%, enabling users to cut energy wastage. Today’s UPS should offer a small footprint so as not to take up too much valuable revenue-earning rack space.

An online or double conversion UPS is recommended for the protection of critical telecom or data centre loads. The inverter (which is powered when mains supply is present from a rectified mains supply) continuously powers the load from the UPS battery when mains power fails. The transfer from mains to battery is seamless with no break in supply, which is critical for this type of installation.

Various UPS configurations are available including single, parallel and series-redundant with each resulting in a different level of resilience, MTBF (meantime-between-failure) and availability.

Resilience is built into the UPS in the form of an automatic static transfer switch. A sensor monitors the output waveform of the inverter. Should the inverter fail due to short-circuit, overload or fault condition, the static switch transfers the load to mains without a break.

Planning for the Future: data centre loads are not static and will be under constant change and adaptation throughout the life of the facility. A graduated ‘growth model’ needs to be conceptualized at the outset to allow for future expansion of power protection equipment.

UPS Monitoring and Maintenance: the advent of Html capability in the software arena has enabled modern UPS manufacturers to integrate sophisticated onsite and remote monitoring capability within their hardware. In fact, modern systems enable remote 24/7 monitoring (either by the client or at the manufacturer’s facility) of all critical UPS, generator, air-conditioning and fire suppression equipment.

Nowadays, modern UPS systems, attached to mission critical equipment, have an internal or external maintenance bypass, which allows servicing to be carried out without having to shut down the whole network and lose vital productivity. Proper maintenance, carried out in a timely and effective manner, can prolong the life of the UPS, increase its effectiveness and achieve a better return on investment.

Power protection is vital for today’s data centres and UPS providers like Riello are specifically designing products to suit the needs and constraints of this type of installation. For more information about designing, installing and operating power protection systems read The Power Protection Guide.

Alicia D. Walker

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