Xtra VFI – Maintaining UPS Efficiency for Diminished Loads

Although modern transformerless UPSs offer a flat efficiency/load curve for loads of about 25% or more, efficiency will drop significantly at lower levels. However, a solution exists, in the form of Xtra VFI; an operating mode for UPSs that mitigates this low-load efficiency loss.

Xtra VFI is a smart technique that exploits the modular topology of UPSs such as the KOHLER PW 9500DPA. When enabled, it automatically adjusts the number of active modules to meet changing load requirements. By being continuously and dynamically resized in this way, the UPS’s capacity can be kept more fully-loaded, and therefore optimally efficient in operation.


Modules that are not needed are switched to standby but remain in a state of readiness, primed to start up and transfer to active mode if the load increases. The efficiency improvements achieved by this mode of operation are especially significant when the load is less than 25 percent of full UPS system capacity. Another benefit is that Xtra VFI involves rotating the modules between active and standby, therefore extending the service life of the UPS.

Fig.1 below shows how the Xtra VFI operating mode can enhance efficiency when running at a low load level for a KOHLER PW 9500DPA modular UPS system, with 10 UPS modules (100kW) and total available capacity of 1MW.

How Xtra VFI improves UPS energy efficiency at low loads


A typical system could be supporting an 800-kW maximum load with N+2 redundancy. It could be implemented in two fully-populated 500kW UPS frames together containing ten 100kW modules. If the load demand drops to 200kW, the Xtra VFI control would reduce the number of active modules to four, to match the reduced load requirement while maintaining N+2 redundancy. Each module would be 50% loaded and operating within its optimal efficiency/load range. The other six modules would be passive.

The active modules would be working in double-conversion mode; the load is shared equally between all active modules. The passive modules would be on standby, with their inverter switched off, ready to transfer to active double conversion operation when the load increases.

User interface and set-up

The redundancy level for active capacity and the highest expected load step can be user-configured to guarantee the highest protection level, through an ergonomic user interface. Xtra VFI also calculates and displays the energy savings it has made compared with normal operation. If a mains failure or alarm occurs, Xtra VFI is automatically deactivated, immediately switching all modules to ‘active’ status.


Xtra VFI allows data centre operators to optimise efficiency in situations where loading on the UPS system is minimal, or is unexpectedly reduced due to operational or business circumstances.

(Some information obtained from a video on ABB’s website http://new.abb.com/ups/systems/three-phase-ups/conceptpower-dpa-500/xtra-vfi)

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