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Maintaining UPS efficiency – even on low loads

While UPSs are valued above all for their availability, their energy efficiency has also become critical. As data centres become larger, more highly populated and more numerous, every percentage point saving is accordingly more significant. And energy savings count, not just commercially, but also for brand reputation and corporate social responsibility.

Below, Alex Emms, Operations Director at Kohler Uninterruptible Power, looks at three methodologies currently available to improve UPS efficiency; transformerless technology, Xtra VFI and Eco-mode – with Xtra VFI being especially beneficial at low UPS loads.

Transformerless technology 
While early double-conversion online UPSs used internal step-up transformers, the industry has generally moved towards transformerless technology, with ever-growing power capacities becoming available.

Transformerless technology has an intrinsic advantage; it significantly improves overall UPS efficiency, increasing it by around 5% to yield a substantial reduction in heat loss and energy costs -see Fig.1.

Fig.1: UPS efficiency curve

However, transformerless technology also improves energy efficiency indirectly. Its considerably reduced size and weight allows transformerless UPSs to be implemented as relatively small modules rather than large, floor-standing units. Among many other benefits, modular technology allows optimum energy efficiency. By adding or removing modules, UPS capacity can be incrementally adjusted to ensure that each module is loaded for best energy efficiency, but not overloaded.

Xtra VFI and low loads
Kohler Uninterruptible Power has further developed and automated this concept – sometimes called ‘right sizing’ – with a smart implementation called Xtra VFI. This automatically adjusts the number of active modules to match the current load requirements. Surplus modules are switched to standby, but remain ready to start up and transfer to active mode immediately if the load increases. The efficiency improvements achieved by this mode of operation are especially significant for loads below 25 percent of full UPS capacity. Fig. 2 shows why this is so.

Fig.2: How Xtra VFI mode enhances energy efficiency for a PowerWAVE 9500DPA modular UPS running on a low load

Example: The figures below describe a PowerWAVE 9500DPA UPS operating in Xtra VFI mode:
• Maximum load: 800 kW
• Redundancy: N+2
• 2 x 500 kW frames (10 x 100 kW modules)
• Load power: 200 kW
• Number of active modules: 4
• UPS active capacity: 400 kW
• UPS standby capacity = 600 kW

The active modules operate in double conversion mode, and share the load equally. When the system calculates the optimal percentage value for maximum efficiency, it allows for the desired redundancy. The redundancy level for active capacity and the highest expected load step are user-configurable to guarantee the highest protection quality.

A mains failure or alarm deactivates Xtra VFI automatically, while all modules are switched to ‘active’ status.

Eco-mode
Most UPSs protecting mission-critical loads are double-conversion, online types. During normal operation, power flows through their rectifier and inverter stages before reaching the load; protecting it from mains aberrations such as spikes, surges, sags and noise. Even with these stages in line, modern UPSs like the PowerWAVE 9250DPA can reach efficiencies of over 97 percent – which for many applications is an optimum result, when combined with the protection also provided.

However, a small minority of applications, whose loads are not particularly sensitive to mains supply issues, opt for UPSs with ‘eco-mode’ capability; incoming power can be switched to bypass a UPS’s conversion stages, removing both their losses and their protection, to deliver efficiencies to 99 percent or more.

Conclusion
Modular technology is a powerful tool for improving energy efficiency, among other benefits. By using automated module switching, Xtra VFI ensures that UPS loading is always enough to achieve optimum energy efficiency. This is particularly effective for loads of 25 percent or less of full UPS capacity.

For more information visit kohler-ups.co.uk, email uksales.ups@kohler.com or call 01256 386700.

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