How operating in Xtra VFI mode improves UPS energy efficiency

The advent of transformerless technology not only improved double-conversion UPS energy efficiency; it also maintained this high efficiency over a much wider load range. Nevertheless, if the load drops to about 25% or below, then efficiency levels start to fall away significantly. Xtra VFI is a smart way to minimise the resulting losses and improve uninterruptible power supply efficiency when operating considerably under capacity.

Xtra VFI is based on a concept that has always been inherent in modular UPS topology. Suppose, for example, that a significant number of a data centre’s servers are decommissioned for an extended period, or even permanently. Modular UPS power systems supporting the data centre could easily be kept adequately loaded with the remaining servers by switching off or removing modules that have become excess to requirement. They can just as easily be restored later, if necessary. Easy adaptation to required load capacity is a natural benefit of modularity, sometimes referred to as ‘rightsizing’.

Xtra VFI mode is available on KUP’s Kohler PW 9500DPA and PW 9250DPA uninterruptible power supplies. This automates the process by detecting the current load level and adjusting the number of active modules accordingly. No operator intervention is required in performing the optimal efficiency calculation as the UPS system allows for any desired redundancy level. Redundancy levels for active capacity and highest expected load steps can be user-configured to guarantee the highest protection level.

Excess modules are switched to standby but remain ready to start up, become active and share the load again if it increases. If a mains power failure or alarm event occurs, the Xtra VFI mode is deactivated, and all modules are switched to ‘active’ status.

The UPS efficiency improvements achieved by this mode of operation are especially significant when the load is less than 25 percent of full UPS system capacity.

A typical UPS implementation could comprise a maximum load of 800 kW, with a required redundancy level of N+2. This could be entirely supported by a pair of 500 kW UPS frames, each containing five 100 kW modules. If the current load demand drops to 200 kW, the number of active modules will be reduced to four to provide 400 kW active capacity. The 200 kW load is being supported with N+2 redundancy, as required. All modules share the load equally, so each is 50% loaded and operating efficiently. The UPS standby capacity becomes 600 kW; some or all of this is available to bring back on-line immediately if the load increases again.

Even in the most carefully managed data centre environments, loads can vary unexpectedly for operational or business reasons. The Xtra VFI mode ensures that uninterruptible power supplies can accommodate these changes without loss of efficiency.


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